STRUCTURAL SYSTEMS
RESEARCH PROJECT
Report No.
SSRP– 06/ 15
HYGROTHERMAL EFFECTS ON
DURABILITY AND MOISTURE
KINETICS OF FIBER- REINFORCED
POLYMER COMPOSITES
by
PADMAVATHI SURATHI
VISTASP M. KARBHARI
Interim Report Submitted to the California Department of
Transportation Under Contract No. 59A0309.
June 2006
Department of Structural Engineering
University of California, San Diego
La Jolla, California 92093- 0085
University of California, San Diego
Department of Structural Engineering
Structural Systems Research Project
Report No. SSRP– 06/ 15
Hygrothermal Effects on Durability and moisture Kinetics
of Fiber- Reinforced Polymer Composites
by
Padmavathi Surathi
Graduate Student Researcher
Vistasp M. Karbhari
Professor of Structural Engineering
Interim Report Submitted to the California Department of Transportation
Under Contract No. 59A0309
Department of Structural Engineering
University of California, San Diego
La Jolla, California 92093- 0085
June 2006
Technical Report Documentation Page
1. Report No.
FHWA/ CA/ ES- 07/ 01
2. Government Accession No.
3. Recipient’s Catalog No.
4. Title and Subtitle
Hygrothermal Effects on Durability and Moisture Kinetics of Fiber- Reinforced Polymer
Composites
5. Report Date
June 2006
6. Performing Organization Code
7. Author( s)
Padmavathi Surathi and Vistasp M. Karbhari
8. Performing Organization Report No.
UCSD / SSRP- 06/ 15
9. Performing Organization Name and Address
Department of Structural Engineering
School of Engineering
10. Work Unit No. ( TRAIS)
University of California, San Diego
La Jolla, California 92093- 0085
11. Contract or Grant No.
59A0309
12. Sponsoring Agency Name and Address
California Department of Transportation
13. Type of Report and Period Covered
Interim Report
Division of Engineering Services
1801 30th St., West Building MS- 9- 2/ 5I
Sacramento, California 95807
14. Sponsoring Agency Code
15. Supplementary Notes
Prepared in cooperation with the State of California Department of Transportation. This report is one of a series of reports
16. Abstract
Fiber- Reinforced Polymer ( FRP) composites offer many advantages over conventional materials for
applications in the marine and civil infrastructure areas. Their increasing widespread use emphasizes the
need to predict their performance over long periods of time after being subjected to exposure to different
environmental conditions. The kinetics of fluid sorption E- glass/ vinylester composites is studied widely using
the Fickian and Langmuir diffusion models. The time and temperature dependence of the rate of diffusion and
maximum moisture content are analyzed and moisture kinetics data is assessed is assessed for use in
performance predictions.
It is seen that various processes of degradation, both reversible and irreversible, are induced in the
composite materials on exposure to moisture. The durability characteristics of unidirectional E- glass- Vinylester
composites under the influence of relative humidity and immersion in water at different temperatures are
investigated. The correlation between tensile and flexural strength data is investigated using statistical models.
This research attempts to analyze the behavior of FRP composites exposed to the aforementioned
environments and theoretically model their effects on the mechanical properties ( tensile strength, tensile
modulus, flexural strength and short beam shear strength) of the FRP composites, for purposes of long- term
prediction. This study attempts to develop an initial correlation between effects due to immersion in deionized
water with those due to exposure to humidity to further develop techniques for prediction of durability of these
materials under field conditions.
17. Key Words
Durability; E- Glass; Moisture; Temperature; Humidity; Mechanical
Properties; Deterioration
18. Distribution Statement
No restrictions. This document is available to the public
through the National Technical Information Service,
Springfield, Virginia 22161
19. Security Classification ( of this report)
Unclassified
20. Security Classification ( of this page)
Unclassified
21. No. of Pages
291
22. Price
Form DOT F 1700.7 ( 8- 72) Reproduction of completed page authorized
Disclaimer
The contents of this report reflect the views of the authors who are responsible for
the facts and the accuracy of the data presented herein. The contents do not
necessarily reflect the official views or policies of the State of California or the
Federal Highway Administration. This report does not constitute a standard,
specification, or regulation.
The United States Government does not endorse products or manufacturers.
Trade and manufacturers’ names appear in this report only because they are
considered essential to the object of the document.
Table of Contents
Table of Contents.......................................................................................................... ii
List of Figures............................................................................................................ viii
List of Tables .............................................................................................................. xv
Abstract.................................................................................................................... xxiii
Chapter 1 Introduction........................................................................................... 1
1.1 Background................................................................................................... 1
1.2 Objectives of the Research............................................................................ 3
1.3 Overview of the Thesis ................................................................................. 3
1.4 References..................................................................................................... 6
Chapter 2 Literature Review ................................................................................. 8
2.1 Moisture Absorption in Polymeric Composites............................................ 8
2.1.1 Classical Fickian Diffusion............................................................................ 9
2.1.2 Non- Fickian Diffusion ................................................................................ 10
2.1.3 Factors Affecting the Diffusion Coefficient ................................................ 13
2.1.4 Factors Affecting Equilibrium Moisture Content ........................................ 14
2.2 Hygrothermal Ageing of Composites ......................................................... 14
2.2.1 Hygrothermal Effects on Polymer Matrices ................................................ 15
2.2.2 Hygrothermal Effects on Fibers................................................................... 18
2.2.3 Hygrothermal Effects on the Interfacial Region.......................................... 19
2.2.4 Effect of Humidity on Composites .............................................................. 21
2.2.5 Summary of Previous Research ................................................................... 23
2.3 Performance Prediction Models.................................................................. 28
2.3.1 Arrhenius Prediction Model......................................................................... 28
2.3.2 Phani and Bose Prediction Model................................................................ 29
2.3.3 Time and Temperature Superposition Model .............................................. 30
2.3.4 Pritchard and Speake Prediction Model....................................................... 32
2.3.5 Phillips Prediction Model ............................................................................ 33
2.4 References................................................................................................... 35
Chapter 3 Materials and Test Procedures ................................................................... 41
3.1 Material Constituents.................................................................................. 41
3.1.1. Glass Fiber Properties.................................................................................. 41
3.1.2 Vinylester Matrix Properties........................................................................ 41
3.1.3 Fabrication Method...................................................................................... 42
3.2 Environmental Conditions .......................................................................... 42
3.3 Test Procedures........................................................................................... 44
3.3.1 Moisture Sorption ........................................................................................ 44
3.3.2 Tensile Characterization .............................................................................. 45
3.3.3 Flexure Characterization.............................................................................. 45
3.3.4 Short Beam Shear Characterization ............................................................. 46
3.3.5 Dynamic Mechanical Thermal Analysis...................................................... 47
3.4 References................................................................................................... 48
Chapter Experimental Results .............................................................................. 49
4.1 Moisture Uptake Results............................................................................. 49
4.1.1 Immersion in Water ..................................................................................... 49
4.1.2 Exposure to Humid Air................................................................................ 49
4.2 Tensile Strength ........................................................................................... 55
4.3 Tensile Modulus.......................................................................................... 55
4.4 Flexural Strength......................................................................................... 55
4.5 Short Beam Shear Strength......................................................................... 55
4.6 Glass Transition Temperature..................................................................... 56
Chapter 5 Moisture Absorption........................................................................... 71
5.1 Fickian Diffusion Model............................................................................. 71
5.2 Langmuir Diffusion Model ......................................................................... 73
5.3 Correction for Edge Effects ........................................................................ 74
5.4 Immersion in deionized water..................................................................... 75
5.4.1 Full Model.................................................................................................... 77
5.4.2 Long- term Approximation ........................................................................... 81
5.4.3 Short- term Approximation........................................................................... 84
5.5 Exposure to Relative Humidity at 23 oC..................................................... 87
5.5.1 Full Model.................................................................................................... 87
5.5.2 Long- term Approximation ........................................................................... 92
5.5.3 Short- term Approximation........................................................................... 94
5.6 Exposure to Relative Humidity at 95 oC..................................................... 95
5.6.1 Full Model.................................................................................................... 95
5.6.2 Long- term Approximation ......................................................................... 100
5.6.3 Short- term Approximation......................................................................... 102
5.7 Summary of Results.................................................................................. 102
5.8 Discussion.................................................................................................. 107
5.8.1 Comparison between Fickian and Langmuir Models ............................... 109
5.8.2 Comparison with previously published data.............................................. 109
5.9 Diffusion Coefficients............................................................................... 115
5.9.1 Comparison I.............................................................................................. 115
5.9.2 Comparison II ............................................................................................ 115
5.9.3 Comparison III........................................................................................... 115
5.10 Activation Energy ..................................................................................... 119
5.11 References................................................................................................. 125
Chapter 6 Correlation between Tension and Flexure Results ........................... 127
6.1 Introduction............................................................................................... 127
6.2 Weibull Statistical Strength Model........................................................... 128
6.3 Prediction of Flexural Strength from Tensile Tests.................................. 130
6.4 Prediction of Tensile Strength from Flexural Tests.................................. 135
6.5 Discussion................................................................................................. 140
6.6 References................................................................................................. 141
Chapter 7 Performance Prediction: Immersion in Deionized Water................. 142
7.1 Introduction............................................................................................... 142
7.2 Arrhenius Prediction Model...................................................................... 142
7.2.1 Analysis Procedure .................................................................................... 144
7.2.2 Tensile Strength Prediction........................................................................ 153
7.2.3 Tensile Modulus......................................................................................... 155
7.2.4 Flexural Strength........................................................................................ 157
7.2.5 Short- Beam Shear Strength........................................................................ 159
7.2.6 Summary – Arrhenius Prediction Model ................................................... 161
7.3 Phani and Bose Model .............................................................................. 167
7.3.1 Analysis Procedure .................................................................................... 168
7.3.2 Flexural Strength........................................................................................ 169
7.3.3. Tensile Strength ......................................................................................... 179
7.3.4 Tensile Modulus......................................................................................... 184
7.3.5 Short- Beam Shear Strength........................................................................ 189
7.3.6 Summary – Phani and Bose Prediction...................................................... 194
7.4 Comparison of Predictive Models ............................................................ 194
7.5 References.................................................................................................. 200
Chapter 8 Prediction of Life under Varying Conditions of Humidity Exposure201
8.1 Introduction............................................................................................... 201
8.2 Predictions for exposure conditions of humid air at 23 0C....................... 203
8.3 Predictions for exposure conditions of humid air at 95 0C....................... 212
8.4 Conclusions................................................................................................ 220
8.5 References................................................................................................. 221
Chapter 9 Summary and Conclusions ............................................................... 222
9.1 Overview................................................................................................... 222
9.2 Restatement of Goals and Rationale ......................................................... 222
9.3 Summary and Conclusions ....................................................................... 224
9.4 Implementation ......................................................................................... 226
9.5 Future Research ........................................................................................ 227
APPENDIX A........................................................................................................... 228
APPENDIX B........................................................................................................... 260
List of Figures
Figure 2.1: Shape of a typical Fickian diffusion curve…………………..…...………… 10
Figure 2.2: Schematic curves representing different types of anomalous diffusion in
Polymeric composites………………………………………………..…...... 11
Figure 2.3: Modulus E as a function of temperature for a typical amorphous
polymer………………………..………………………………………..…... 16
Figure 2.4 Time Temperature Superposition Principle……...……….…………...….... 31
Figure 4.1 Moisture sorption profiles of E- glass/ Vinylester specimens immersed in
deionized water at temperatures of 23 oC, 40 oC, 60 oC, 80 oC and 95
oC…….……...……………………………………………………...………. 56
Figure 4.2 Moisture sorption profiles of E- glass/ Vinylester specimens exposed to
relative humidity levels of 0- 5 %, 45 %, 60 %, 80 % and 98 % at a constant
temperature of 23 oC……..……………………………………………….... 59
Figure 4.3 Moisture sorption profiles of E- glass/ Vinylester specimens exposed to
relative humidity levels of 0- 5 %, 45 %, 60 %, 80 % and 98 % at a constant
temperature of 95 oC…………….………………………………...……..… 59
Figure 4.4 Tensile strength profiles of E- glass/ Vinylester specimens immersed in
deionized water at temperatures of 23 oC, 40 oC, 60 oC, 80 oC and 95 oC and
“ control” specimens………………..…………………………..…………... 63
Figure 4.5 Tensile modulus profiles of E- glass/ Vinylester composite specimens
immersed in deionized water at temperatures of 23 oC, 40 oC, 60 oC, 80 oC
and 95 oC and “ control” specimens……………………….………..……… 65
Figure 4.6 Flexural strength profiles of E- glass/ Vinylester composite specimens
exposed to immersion in deionized water at temperatures of 23 oC, 40 oC, 60
oC, 80 oC and 95 oC and “ control” specimens…………..………..……..…. 67
Figure 4.7 Short- beam shear strength profiles of E- glass/ Vinylester composite
specimens exposed to immersion in deionized water at temperatures of 23 oC,
40 oC, 60 oC, 80 oC and 95 oC and “ control” specimens…………………… 69
Figure 4.8 Short- beam shear strength profiles of E- glass/ Vinylester composite
specimens exposed to relative humidity levels of 0- 5 %, 45 %, 60 %, 80 %
and 98 % at a constant temperature 23 oC and “ control” specimens....….… 71
Figure 4.9 Short- beam shear strength profiles of E- glass/ Vinylester composite
specimens exposed to relative humidity levels of 0- 5 %, 45 %, 60 %, 80 %
and 98 % at a constant temperature 95 oC and “ control” specimens………. 73
Figure 4.10 Changes in the glass transition temperature of the E- glass/ Vinylester
composite specimens immersed in deionized water at temperatures of 23 oC,
40 oC, 60oC, 80 oC and 95 oC and “ control” specimens……………………. 75
Figure 5.1 Geometry of the specimen……………………….…….…………………… 79
Figure 5.2 Moisture sorption profile of E- glass vinylester composite specimens
immersed in deionized water at 23 oC ( Fickian Model)…………….…...…. 84
Figure 5.3 Moisture sorption profile of E- glass vinylester composite specimens
immersed in deionized water at 23 oC ( Langmuir Model)……….………… 85
Figure 5.4 Prediction of moisture sorption profile of E- glass vinylester composite
specimens immersed in deionized water at 23 oC with long- term
approximation terms…….………………………………………………..… 88
Figure 5.5 Schematic of Classical Fickian Diffusion Process……………………....…. 89
Figure 5.6 Prediction of Moisture sorption profile of E- glass vinylester composite
specimens exposed to Relative Humidity of 45 % at 23 oC with Fickian
Model………………………………………………………………….…… 95
Figure 5.7 Prediction of Moisture sorption profile of E- glass vinylester composite
specimens exposed to Relative Humidity of 45 % at 23 oC with Langmuir
Model………………….…………………………………………………… 96
Figure 5.8 Prediction of Moisture sorption profile of E- glass vinylester composite
specimens exposed to Relative Humidity of 45 % at 23 oC with long- term
approximation terms…………………………………..………………..….. 98
Figure 5.9 Prediction of Moisture sorption profile of E- glass vinylester composite
specimen exposed to a Relative Humidity of 45 % at 95 oC with Fickian
Diffusion Model. ………………………………………..………………... 103
Figure 5.10 Prediction of Moisture sorption profile of E- glass vinylester composite
specimens exposed to a Relative Humidity of 45 % at 95 oC with Langmuir
Diffusion Model.. ……………………………………………………….... 104
Figure 5.11 Prediction of Moisture sorption profile of E- glass vinylester composite
specimens exposed toRelative Humidity of 45 % at 95 oC with long- term
approximation terms…………………………………..…………………... 106
Figure 5.12 Activation energy for specimens immersed in deionized water – Fickian
diffusion model…………..……………………………………………….. 126
Figure5.13 Activation energy for specimens immersed in deionized water - Langmuir
Diffusion Model…………………………………….…………………..… 126
Figure 5.14 Activation Energy ( Relative Humidity 0- 5%)……………….…………… 127
Figure 5.15 Activation Energy ( Relative Humidity 45%)………………..………….… 128
Figure 5.16 Activation Energy ( Relative Humidity 60%)………………………...…… 128
Figure 5.17 Activation Energy ( Relative Humidity 75%)..……………………….....… 129
Figure 5.18 Activation Energy ( Relative Humidity 98%)………………………...…… 129
Figure 6.1 Values of flexural strength predicted from tensile test data……………..... 138
Figure 6.2 Values of tensile strength predicted from flexural test data………………. 144
Figure 7.1 Percent retention of tensile strength for E- glass/ Vinylester composite
specimens…………………………………………………………………. 151
Figure 7.2 Arrhenius plot for decrease in percent retention of tensile strength for E-glass/
Vinylester composite specimens……………………………………. 152
Figure 7.3 Percent retention of tensile strength Vs. Inverse of temperature…………. 154
Figure 7.4 Comparison between the experimental and predicted values of tensile
strength for specimens immersed in deionized water at 23 oC…………… 157
Figure 7.5 Comparison between the experimental and predicted values of tensile
modulus for specimens immersed in deionized water at 230C……….…… 162
Figure 7.6 Comparison between the experimental and predicted values of flexural
strength for specimens immersed in deionized water at 230C…………….. 164
Figure 7.7 Comparison between the experimental and predicted values of short- beam
shear strength for specimens immersed in deionized water at 230C……… 166
Figure 7.8 Predicted values of tensile strength immersed in deionized water at different
temperatures – Arrhenius Rate Method…………………………………... 168
Figure 7 .9 Predicted values of tensile modulus immersed in deionized water at different
temperatures – Arrhenius Rate Method……………………….………….. 169
Figure 7.10 Predicted values of flexural strength immersed in deionized water at different
temperatures – Arrhenius Rate Method…………………………………... 170
Figure 7.11 Predicted values of short- beam shear strength immersed in deionized water at
different temperatures – Arrhenius Rate Method…………………………. 171
Figure 7.12 ( 1/ τ) Vs. ( 1/ T) – Phani and Bose predictions………………………….….. 178
Figure 7.13 Comparison of experimental values of flexural strength for specimens
immersed in deionized water at 23 oC with the predicted values using the
Phani and Bose equations………………………………………………… 180
Figure 7.14 Comparison of experimental values of flexural strength for specimens
immersed in deionized water at 40 oC with the predicted values using the
Phani and Bose equations…………………………………………………. 180
Figure 7.15 Comparison of experimental values of flexural strength for specimens
immersed in deionized water at 60 oC with the predicted values using the
Phani and Bose equations…………………………………………………. 181
Figure 7.16 Comparison of experimental values of flexural strength for specimens
immersed in deionized water at 80 oC with the predicted values using the
Phani and Bose equations……………………………………………….… 181
Figure 7.17 Comparison of experimental values of flexural strength for specimens
immersed in deionized water at 95 oC with the predicted values using the
Phani and Bose equations…………………………………………………. 182
Figure 7.18 TTSP – Master curve for long term predictions of flexural strength using the
Phani and Bose method…………………………………………………… 183
Figure 7.19 Comparison of experimental values of tensile strength for specimens
immersed in deionized water at 23 oC with the predicted values using the
Phani and Bose equations…………………………………………………. 186
Figure 7.20 Comparison of experimental values of tensile strength for specimens
immersed in deionized water at 40 oC with the predicted values using the
Phani and Bose equations……………………………………………….… 186
Figure 7.21 Comparison of experimental values of tensile strength for specimens
immersed in deionized water at 60 oC with the predicted values using the
Phani and Bose equations…………………………………………….…… 187
Figure 7.22 Comparison of experimental values of tensile strength for specimens
immersed in deionized water at 80 oC with the predicted values using the
Phani and Bose equations…………………………………………………. 187
Figure 7.23 Comparison of experimental values of tensile strength for specimens
immersed in deionized water at 95 oC with the predicted values using the
Phani and Bose equations…………………………………………….…… 188
Figure 7.24 TTSP – Master curve for long term predictions of tensile strength using the
Phani and Bose method…………………………………………………… 188
.
Figure 7.25 Comparison of experimental values of tensile modulus for specimens
immersed in deionized water at 23 oC with the predicted values using the
Phani and Bose equations…………………………………………….…… 191
Figure 7.26 Comparison of experimental values of tensile modulus for specimens
immersed in deionized water at 40 oC with the predicted values using the
Phani and Bose equations…………………………………………………. 191
Figure 7.27 Comparison of experimental values of tensile modulus for specimens
immersed in deionized water at 60 oC with the predicted values using the
Phani and Bose equations…………………………………………………. 192
Figure 7.28 Comparison of experimental values of tensile modulus for specimens
mmersed in deionized water at 80 oC with the predicted values using the
Phani and Bose equations…………………………………………………. 192
Figure 7.29 Comparison of experimental values of tensile modulus forspecimens
immersed in deionized water at 95 oC with the predicted values using the
Phani and Bose equations ………………………………………………… 193
Figure 7.30 TTSP – Master curve for long term predictions of tensile modulus using the
Phani and Bose method…………………………………………………… 193
Figure 7.31 Comparison of experimental values of short- beam shear strength for
specimens immersed in deionized water at 23 oC with the predicted values
using the Phani and Bose equations…………………………………… .. 196
Figure 7.32 Comparison of experimental values of short- beam shear strength for
specimens immersed in deionized water at 40 oC with the predicted values
using the Phani and Bose equations………………………………………. 196
Figure 7.33 Comparison of experimental values of short- beam shear strength for
specimens immersed in deionized water at 60 oC with the predicted values
using the Phani and Bose equations………………………………………. 197
Figure 7.34 Comparison of experimental values of short- beam shear strength for
specimens immersed in deionized water at 80 oC with the predicted values
using the Phani and Bose Equations……………………………………... 197
Figure 7.35 Comparison of experimental values of short- beam shear strength for
specimens immersed in deionized water at 95 oC with the predicted values
using the Phani and Bose equations…………………………………….... 198
Figure 7.36 TTSP – Master curve for long term predictions of short- beam shear strength
using the Phani and Bose method………………………………………… 198
Figure 7.37 Comparison of predictions for tensile strength retention for specimens
immersed in 23 0C deionized water……………………………………….. 201
Figure 7.38 Comparison of predictions for tensile modulus retention for specimens
immersed in 23 0C deionized water……………………………………….. 202
Figure 7.39 Comparison of predictions for flexural strength retention for specimens
immersed in 23 0C deionized water……………………………………….. 203
Figure 7.40 Comparison of predictions for short- beam shear strength Retention for
specimens immersed in 23 0C deionized water…………………………… 204
Figure 8.1 Comparison between the experimental and predicted values of short- beam
shear strength for specimens exposed to 45% relative humidity at 23 oC... 210
Figure 8.2 Comparison between the experimental and predicted values of short- beam
shear strength for specimens exposed to 60% relative humidity at 23 oC.. 212
Figure 8.3 Comparison between the experimental and predicted values of short- beam
shear strength for specimens exposed to 75% relative humidity at 23 oC.. 214
Figure 8.4 Comparison between the experimental and predicted values of short- beam
shear strength for specimens exposed to 98% relative humidity at 23 oC.. 216
Figure 8.5 Comparison between the experimental and predicted values of short- beam
shear strength for specimens exposed to 45% relative humidity at 95 oC.. 218
Figure 8.6 Comparison between the experimental and predicted values of short- beam
shear strength for specimens exposed to 60% relative humidity at 95 oC.. 220
Figure 8.7 Comparison between the experimental and predicted values of short- beam
shear strength for specimens exposed to 75% relative humidity at 95 oC.. 222
Figure 8.8 Comparison between the experimental and predicted values of short- beam
shear strength for specimens exposed to 98% relative humidity at 95 oC.. 224
List of Tables
Table 2.1 Resin systems subjected to environmental ageing .……...……………….... 23
Table 2.2 Kevlar fiber- reinforced polymer composites subjected to environmental
ageing…………..…………………………………………………………... 23
Table 2.3 Glass fiber- reinforced polymer composites subjected to environmental
ageing…………………………………………………………………….… 24
Table 2.4 Carbon/ Graphite fiber- reinforced polymer composites subjected to
environmental ageing………………………………………………….…… 26
Table 3.1 Properties of E- glass Fibers………………………..……………………….. 47
Table 3.2 Typical Liquid Resin Properties of Dow Derakane 411- 350 Vinylester
Resin………………………………………………………………………... 47
Table 3.3 Typical Properties of Clear Resin Castings……..………………………….. 48
Table 4.1 Percentage weight gain for E- glass/ Vinylester specimens immersed in
deionized water at temperatures of 23 oC, 40 oC, 60 oC, 80 oC and 95
oC…………………………………………………………………………… 55
Table 4.2 Percentage weight gain for E- glass/ Vinylester specimens exposed to relative
humidity levels of 0- 5 %, 45 %, 60 %, 80 % and 98 % at a constant
temperature of 23 oC……….………………………………………..……... 57
Table 4.3 Percentage Weight Gain for E- glass/ Vinylester specimens exposed to
relative humidity levels of 0- 5 %, 45 %, 60 %, 80 % and 98 % at a constant
temperature of 95 oC………………………………………………………. 58
Table 4.4 Tensile strength data for E- glass/ Vinylester specimens immersed in
deionized water at temperatures of 23 oC, 40 oC, 60 oC, 80 oC and 95 oC and
under “ control” conditions of 30 % RH at 23 0C………..…..……….……. 62
Table 4.5 Tensile modulus ( GPa) data for E- glass/ Vinylester specimens immersed in
deionized water at temperatures of 23 oC, 40 oC, 60 oC, 80 oC and 95 oC and
under “ control” conditions of 30 % RH at 23 0C…………………….……. 64
Table 4.6 Flexural strength data for E- glass/ Vinylester composite specimens immersed
in deionized water at temperatures of 23 oC, 40 oC, 60 oC, 80 oC and 95 oC
and under “ control” conditions of 30 % RH at 23 0C……………..………. 66
Table 4.7 Short beam shear strength data for E- glass/ Vinylester composite specimens
immersed in deionized water at temperatures of 23 oC, 40 oC, 60 oC, 80 oC
and 95 oC and under “ control” conditions of 30 % RH at 23 0C……..…… 68
Table 4.8 Short- beam shear strength data for E- glass/ Vinylester composite specimens
exposed to relative humidity levels of 0- 5 %, 45 %, 60 %, 80 % and 98 % at
a constant temperature 23 oC and under “ control” conditions of 30 % RH at
23 0C ..…………………………………………….………………………... 70
Table 4.9 Short beam shear strength data for E- glass/ Vinylester composite specimens
exposed to relative humidity levels of 0- 5 %, 45 %, 60 %, 80 % and 98 % at
a constant temperature 95 oC and under “ control” conditions of 30 % RH at
23 0C…………………………………………….………………………..… 74
Table 4.10 Glass Transition Temperature data for E- glass/ Vinylester composite
specimens immersed in deionized water at temperatures of 23 oC, 40 oC,
60oC, 80 oC and 95 oC and under “ control” conditions of 30 % RH at 23
0C…………………………………………………………………………… 81
Table 5.1 Moisture sorption (%) for E- glass/ Vinylester specimens immersed in
deionized water at temperatures 23 oC, 40 oC, 60 oC, 80 oC and 95
oC………………………………………………………………...…………. 83
Table 5.2 Maximum moisture contents and diffusion coefficients obtainedfrom Fickian
Diffusion Model ( FULL MODEL) for specimens immersed in
water………………………………………………………………..………. 83
Table 5.3 Maximum moisture contents and diffusion coefficients obtained from
Langmuir Diffusion Model ( FULL MODEL) for specimens immersed in
water…………………………………………………………………..……. 87
Table 5.4 Maximum moisture contents and diffusion coefficients obtained from
Fickian Diffusion Model ( LONG- TERM APPROXIMATION) for
specimens immersed in water……………………...………………………. 87
Table 5.5 Maximum moisture contents and diffusion coefficients obtained from
Langmuir Diffusion Model ( LONG- TERM APPROXIMATION) for
specimens immersed in water……………………………………...……..... 87
Table 5.6 Maximum moisture contents and diffusion coefficients obtained from
Fickian Diffusion Model ( SHORT- TERM APPROXIMATION) for
specimens immersed in water……………………………………..……. … 91
Table 5.7 Maximum moisture contents and diffusion coefficients obtained from
Langmuir Diffusion Model ( SHORT- TERM APPROXIMATION) for
specimens immersed in water………………………………...……………. 91
Table 5.8 Moisture sorption data of E- glass/ Vinylester specimens exposed to relative
humidity levels of 0- 5 %, 45 %, 60 %, 80 % and 98 % at a constant
temperature of 23 oC…………………………………………….……….… 93
Table 5.9 Maximum moisture contents and diffusion coefficients obtained from
Fickian Diffusion Model ( FULL MODEL) for specimens exposed to
different relative humidity levels at 23 0C…………………..……………... 94
Table 5.10 Maximum moisture contents and diffusion coefficients obtained from
Langmuir Diffusion Model ( FULL MODEL) for specimens exposed to
different relative humidity levels at 23 0C……………………….………… 94
Table 5.11 Maximum moisture contents and Diffusion coefficients obtained from
Fickian Diffusion Model ( LONG- TERM APPROXIMATION) for
specimens exposed to different relative humidity levels at 23 0C……..…… 97
Table 5.12 Maximum moisture contents and diffusion coefficients obtained from
Langmuir Diffusion Model ( LONG- TERM APPROXIMATION) for
specimens exposed to different relative humidity levels at 23 0C………..… 97
Table 5.13 Maximum moisture contents and diffusion coefficients obtained from
Fickian Diffusion Model ( SHORT- TERM APPROXIMATION) for
specimens exposed to different relative humidity levels at 23 0C………..... 99
Table 5.14 Maximum moisture contents and diffusion coefficients obtained from
Langmuir Diffusion Model ( SHORT- TERM APPROXIMATION) for
specimens exposed to different relative humidity levels at 23 0C……..…… 99
Table 5.15 Moisture sorption data of E- glass/ Vinylester specimens exposed to relative
humidity levels of 0- 5 %, 45 %, 60 %, 80 % and 98 % at a constant
temperature of 95 oC…………………………………………….………... 101
Table 5.16 Maximum moisture contents and diffusion coefficients obtained from
Fickian Diffusion Model ( FULL MODEL) for specimens exposed to
different relative humidity levels at 95 0C……………………………...… 102
Table 5.17 Maximum moisture contents and diffusion coefficients obtained from
Langmuir Diffusion Model ( FULL MODEL) for specimens exposed to
different relative humidity levels at 95 0C………………………….…….. 102
Table 5.18 Maximum moisture contents and diffusion coefficients obtained from
Fickian diffusion Model ( LONG- TERM APPROXIMATION) for specimens
exposed to different relative humidity levels at 95 0C………………...….. 105
Table 5.19 Maximum moisture contents and Diffusion coefficients obtained from
Langmuir Diffusion Model ( LONG- TERM APPROXIMATION) for
specimens exposed to different relative humidity levels at 95 0C……...…. 105
Table 5.20 Maximum moisture contents and diffusion coefficients obtained from
Fickian Diffusion Model ( SHORT- TERM APPROXIMATION) for
specimens exposed to different relative humidity levels at 95 0C….….…. 107
Table 5.21 Maximum moisture contents and diffusion coefficients obtained from
Langmuir Diffusion Model ( SHORT- TERM APPROXIMATION) for
specimens exposed to different relative humidity levels at 95 0C………... 107
Table 5.22 Maximum moisture contents and Diffusion Coefficients of E- glass vinylester
composite specimens immersed in deionized water at various temperatures,
obtained using Fickian diffusion theory……………………………....….. 108
Table 5.23 Maximum moisture contents and Diffusion Coefficients of E- glass vinylester
composite specimens immersed in deionized water at various temperatures,
obtained using Langmuir diffusion theory…………………………..……. 109
Table 5.24 Maximum moisture contents and Diffusion Coefficients of E- glass vinylester
composite specimens exposed to relative humidity levels at 23 oC, obtained
using Langmuir diffusion theory…………………….………………...….. 110
Table 5.25 Maximum moisture contents and Diffusion Coefficients of E- glass vinylester
composite specimens exposed to relative humidity levels at 95 oC, obtained
using Langmuir diffusion theory………………………………………..… 111
Table 5.26 Comparison of diffusion coefficients with previously published data.....… 116
Table 5.27 Diffusion Coefficients – Comparison I…………………….……………... 121
Table 5.28 Diffusion Coefficients - Comparison II………………….……………….. 122
Table 5.29 Diffusion Coefficients - Comparison III………….………………………. 123
Table 5.30 Activation Energy ( Immersion in Water)………………………….…....… 126
Table 5.31 Activation Energy ( Relative Humidity Exposure)………………...……… 129
Table 6.1 Values of shape parameters for the different exposure conditions……...... 136
Table 6.2 Values of flexural strength predicted from tensile tests of composite
specimens exposed to air at 230C……………………………………..…... 137
Table 6.3 Values of flexural strength predicted from tensile tests of composite
specimens immersed in deionized water at 230C…………………………. 137
Table 6.4 Values of flexural strength predicted from tensile tests of composite
specimens immersed in deionized water at 400C…………………………. 137
Table 6.5 Values of flexural strength predicted from tensile tests of composite
specimens immersed in deionized water at 600C…………………………. 138
Table 6.6 Values of flexural strength predicted from tensile tests of composite
specimens immersed in deionized water at 800C……………….…….…... 138
Table 6.7 Values of flexural strength predicted from tensile tests of composite
specimens immersed in deionized water at 950C………..........……….…. 138
Table 6.8 Values of shape parameters for the different exposure conditions calculated
from flexural tests…………………………………………………...……. 141
Table 6.9 Values of tensile strength predicted from flexural tests of composite
specimens exposed to air at 230C………………………….……………… 141
Table 6.10 Values of tensile strength predicted from flexural tests of composite
specimens immersed in deionized water at 230C……………….……….... 142
Table 6.11 Values of tensile strength predicted from flexural tests of composite
specimens immersed in deionized water at 400C………….……………… 142
Table 6.12 Values of tensile strength predicted from flexural tests of composite
specimens immersed in deionized water at 600C…………………………. 142
Table 6.13 Values of tensile strength predicted from flexural tests of composite
specimens immersed in deionized water at 800C. ………………………... 143
Table 6.14 Values of tensile strength predicted from flexural tests of composite
specimens immersed in deionized water at 950C…………………………. 143
Table 7.1 Tensile Strength data for E- glass/ Vinylester composite specimens immersed
in deionized water and “ control” specimens at 23 oC and 30 % RH……… 150
Table 7.2 Linear relationship between tensile strength and time for E- glass/ Vinylester
composite specimens immersed in deionized water…………………….... 152
Table 7.3 Linear Relationship between percent retention of tensile strength and the
inverse of temperature……………………….……………………………. 155
Table 7.4 Predicted values of tensile strength in comparison with experimentally
obtained values for specimens immersed in deionized water at 23 oC….... 156
Table 7.5 Predicted values of tensile modulus in comparison with experimentally
obtained values for specimens immersed in deionized water at 230C……. 161
Table 7.6 Predicted values of flexural strength in comparison with experimentally
obtained values for specimens immersed in deionized water at 230C……. 163
Table 7.7 Predicted values of short- beam shear strength in comparison with
experimentally obtained values for specimens immersed in deionized water
at 230C……………….……………………………………………………. 165
Table 7.8 Arrhenius equations for prediction of properties of E- glass/ Vinylester
composites immersed in deionized water………………………………… 167
Table 7.9 Flexural strength data for E- glass/ Vinylester composite specimens immersed
in deionized water at temperatures of 23 oC, 40 oC, 60 oC, 80 oC and 95 oC
and under “ control” conditions of 30 % RH at 23 oC……………….…… 175
Table 7.10 Values of the characteristic time and the corresponding temperatures…… 178
Table 7.11 Comparison of experimental values of flexural strength with the predicted
values using the Phani and Bose equations……………………………….. 179
Table 7.12 TTSP shift factors for flexural strength predictions………………………. 183
Table 7.13 Phani and Bose equations for tensile strength predictions at different
temperatures………………….……………….………………...………… 184
Table 7.14 Comparison of experimental values of tensile strength with the predicted
values using the Phani and Bose equations………………………….……. 185
Table 7.15 Phani and Bose equations for tensile modulus predictions at different
temperatures……………….……………….…………………………..…. 189
Table 7.16 Comparison of experimental values of tensile modulus with the predicted
values using the Phani and Bose equations……………….…………...…. 190
Table 7.17 Phani and Bose equations for short- beam shear strength predictions at
different temperatures……………….……………………………………. 194
Table 7.18 Comparison of experimental values of short- beam shear strength with the
predicted values using the Phani and Bose equations……………………. 195
Table 7.19 Comparison of predictions for tensile strength retention for specimens
immersed in 23 0C deionized water………………………………………. 201
Table 7.20 Comparison of predictions for tensile modulus retention for specimens
immersed in 23 0C deionized water……….……………….………...…… 202
Table 7.21 Comparison of predictions for flexural strength retention for specimens
immersed in 23 0C deionized water……….……………….………...…… 203
Table 7.22 Comparison of predictions for short- beam shear strength retention for
specimens immersed in 23 0C deionized water……….………….…..…… 204
Table 8.1 Predicted values of short- beam shear strength in comparison with
experimentally obtained values for specimens exposed to 45% relative
humidity at 23 oC……….……………….……..……..……..……..……… 210
Table 8.2 Predicted values of short- beam shear strength in comparison with
experimentally obtained values for specimens exposed to 60% relative
humidity at 23 oC……….……………….……………….……..………… 212
Table 8.3 Predicted values of short- beam shear strength in comparison with
experimentally obtained values for specimens exposed to 75% relative
humidity at 23 oC……….……………….……………………...….……… 214
Table 8.4 Predicted values of short- beam shear strength in comparison with
experimentally obtained values for specimens exposed to 98% relative
humidity at 23 oC……….……………….……………….………………... 216
Table 8.5 Predicted values of short- beam shear strength in comparison with
experimentally obtained values for specimens exposed to 45% relative
humidity at 95 oC……….……………….……………….……….……….. 218
Table 8.6 Predicted values of short- beam shear strength in comparison with
experimentally obtained values for specimens exposed to 60% relative
humidity at 95 oC……….……………….……………….…………..…… 220
Table 8.7 Predicted values of short- beam shear strength in comparison with
experimentally obtained values for specimens exposed to 75% relative
humidity at 95 oC……….……………….……………….…………...…… 222
Table 8.8 Predicted values of short- beam shear strength in comparison with
experimentally obtained values for specimens exposed to 98% relative
humidity at 95 oC……….……………….……………….……………..… 224
Table 9.1 Summary of predictions……….……………….……………….………… 232
ABSTRACT
Fiber- Reinforced Polymer ( FRP) composites offer many advantages over
conventional materials for applications in the marine and civil infrastructure areas.
Their increasing widespread use emphasizes the need to predict their performance
over long periods of time after being subjected to exposure to different environmental
conditions. The kinetics of fluid sorption E- glass/ vinylester composites is studied
widely using the Fickian and Langmuir diffusion models. The time and temperature
dependence of the rate of diffusion and maximum moisture content are analyzed and
moisture kinetics data is assessed is assessed for use in performance predictions.
It is seen that various processes of degradation, both reversible and irreversible,
are induced in the composite materials on exposure to moisture. The durability
characteristics of unidirectional E- glass- Vinylester composites under the influence of
relative humidity and immersion in water at different temperatures are investigated.
The correlation between tensile and flexural strength data is investigated using
statistical models. This research attempts to analyze the behavior of FRP composites
exposed to the aforementioned environments and theoretically model their effects on
the mechanical properties ( tensile strength, tensile modulus, flexural strength and
short beam shear strength) of the FRP composites, for purposes of long- term
prediction. This study attempts to develop an initial correlation between effects due to
immersion in deionized water with those due to exposure to humidity to further
develop techniques for prediction of durability of these materials under field
conditions.
1
Chapter 1
Introduction
1.1 Background
Recent years have witnessed a substantial increase in the use of Fiber- Reinforced
Polymer ( FRP) Composites in place of conventional construction materials. Engineers around
the world are leaning towards FRP composites because of their high specific strength and
stiffness characteristics, lightweight, tailorability, endurance to fatigue loading and the ease
of fabrication.
FRP composites have found a wide variety of applications in both new construction
and rehabilitation projects alike. Pre- stressing tendons and reinforcing bars made from FRP
[ 1] are now being used in new construction projects. Repair and rehabilitation of existing
structures is also being carried out using FRP composites. FRP is being extensively used in
the seismic retrofit [ 2] of concrete and steel bridge columns and slabs. In addition to these,
FRP composites are being utilized for architectural applications like roofing and partition
walls.
However, the use of FRP to its fullest potential has been hampered by the fact that
there is concern about their reliability and performance over long periods of time. Exposure
to humidity, water, alkalis, elevated temperatures and other harsh environments can induce
physical and chemical changes in polymer composites. On exposure to water or moisture,
FRP composites have been reported to show reduction in strength [ 3,4], plasticization of the
2
matrix [ 5,6,7] and also degradation of the fiber/ matrix interface [ 8,9]. Environmental
exposure can induce various chemical and physical processes of degradation
in FRP composites. The relative rates of these degradation processes depend on the chemistry
of the fiber and matrix, temperature, length of exposure and the stress state [ 10]. Therefore, a
better understanding of the behavior of the FRP composites under these environments is
absolutely essential to aid in the optimal design and the prediction of service- life of structural
components constructed from these materials.
The most noticeable effect of exposure to moisture is the plasticization of the matrix
due to the interruption of Van Der Waals bonds between the polymer chains [ 11]. This in turn
reduces the glass transition temperature of the polymer matrix, and can lead to a decrease in
the matrix dominated strength and stiffness properties. In some cases, moisture introduces
micro- cracks in the fiber/ matrix interface thereby interfering with the transfer of loads from
the matrix to the fibers. Some fibers like glass and Kevlar are also susceptible to moisture
induced degradation. Polymer composites are invariably exposed in civil infrastructure to
moisture or humid air in their applications. By far, moisture in combination with elevated
temperatures is one of the most widely studied exposures. The “ hot- wet” environment is
generally considered to be a very severe exposure condition and is hence used widely for
materials screening.
Measurement of moisture uptake is a common method used to characterize the
hygrothemal behavior of polymer composites, since deterioration is most often initiated by
moisture. Theoretical models based on either classical Fickian diffusion or non- classical
diffusion are used to determine the maximum moisture content and the rate of diffusion [ 12,
13].
3
Moisture sorption in FRP composites not only affects the dimensional stability but
also affects the mechanical properties of the composites. As a result of this, determination of
the rate of degradation of mechanical properties and the resulting effect on service life is of
utmost importance to engineers. This study attempts to investigate the effects of immersion in
water and exposure to humidity at different temperatures on the mechanical properties of
unidirectional E- glass Vinylester composites. Durability characteristics of E- glass Vinylester
composites in these environments are studied by employing life prediction models.
1.2 Objectives of the Research
The primary objective of the research reported in this report is to develop a
fundamental understanding of the effects of hygrothermal exposure ( related to both
immersion and humidity based conditioning) on durability of E- glass/ Vinylester composites.
In addition to the goals of developing an understanding of moisture kinetics and deteriorative
mechanisms, this study attempts to develop an initial correlation between effects due to
immersion in deionized water with those due to exposure to humidity in an attempt to further
develop techniques for prediction of longer- term durability of these materials under field
conditions. It must be noted that although complete immersion is often used as a means of
characterization of durability, and as a method of acceleration, data cannot directly be applied
to prediction of service- life under field environments which intrinsically consists not of
immersion, but rather varying periods at different levels of temperature and humidity.
1.3 Overview of the Investigation
The Literature Review consists of a brief description of the findings of studies
dealing with sorption kinetics and hygrothermal ageing of polymer composites. It also
4
reviews the effects of moisture sorption on glass fiber- reinforced composites. Results of
previous experiments on glass fiber- reinforced composites and the validity of various life
predictive models are also discussed.
In the next chapter, material specifications and the test methods employed to measure
the moisture uptake and to determine the loss of mechanical properties are discussed. The
next chapter presents the results of the moisture sorption tests and tensile strength, flexural
strength and short beam shear strength experiments.
In chapter 5, data from the moisture uptake experiments are analyzed using Fickian
and Langmuir diffusion models and the kinetics of moisture sorption are studied. The
equilibrium moisture contents and diffusion coefficients for different temperatures are
determined. The results from the analyses of data using the two diffusion models are
compared and the suitability of diffusion models for the data is assessed. The results are also
compared with previously published data.
Chapter 6 discusses the correlation between tensile strength and flexural strength and
the ability to predict one from the other. A two parameter Weibull distribution model is used
to predict values of tensile and flexural strength, which are then compared to the experimental
values.
In chapter 7, prediction of mechanical strength characteristics of the E- glass
Vinylester composite specimen is discussed. The experimental data is analyzed using two
models, namely the Arrhenius Rate Model and the Phani- Bose Model. The results from the
analyses of Tensile, Flexural and Short- Beam Shear data are presented and the life prediction
models are utilized to determine the remaining life of the polymer composite. The advantages
and shortcomings of the two models are discussed and the results from the two models are
compared.
5
Chapter 8 discusses the prediction of service- life of the polymer composite exposed
to humidity and the correlation between moisture and humidity is discussed. The last chapter
focuses on the conclusions drawn from the research and discusses further research needs.
Since this investigation is concerned with durability of materials it is important that
the term, durability, be defined as it relates to this investigation. Durability, in the current
context, is defined as the ability of a materil to resist physical, mechanical and/ or chemical
degradation for a specified period of time under specified environmental conditions and load
regimes.
6
1.4 References
1. Malvar L. J., “ Durability of Composites in Reinforced Concrete”, Proceedings of the
First International Conference on Durability of Composites for Construction,
Sherbrooke, Quebec, Canada, August 1998, pp. 361- 372.
2. Reay J. T., Pantelides C. P., Reaveley L. D., Ring T. A., “ Long Term Durability of
Carbon FRP Composites Applied to RC Bridges: State Street Bridge on Interstate
80”, Report No. CVEEN- 04/ 1, University of Utah, Salt Lake City Utah, 2004.
3. Bank L. C., Gentry T. R., and Barkatt A., “ Accelerated Test Methods to Determine the
Long- Term Behavior of FRP Composite Structures: Environmental Effects”, Journal
of Reinforced Plastics and Composites, 1995, Vol. 14, pp. 559- 587.
4. Williams C. J., “ The Effect of Moisture Absorption on Room Temperature
Mechanical Properties of Reinforced Polymer Composites”, Research and
Development Report, Ship Materials Engineering Department, David Taylor
Research Center, Report DTRC- SME- 91/ 35, January 1991.
5. Allred R. E., “ The Effect of Temperature and Moisture Content on the Flexural
Response of Kevlar/ Epoxy Laminates I ( 0/ 90) Filament Orientation” Journal of
Composite Materials, 1981, Vol. 15, pp. 100- 116.
6. Allred R. E., “ The Effect of Temperature and Moisture Content on the Flexural
Response of Kevlar/ Epoxy Laminates II (+ or- 45, 0/ 90) Filament Orientation”
Journal of Composite Materials, 1981, Vol. 15, pp. 117- 132.
7. Chateauminois A., Chabert B., Soulier J. P., Vincent L., “ Hygrothermal Ageing
Effects on the Static Fatigue of Glass/ Epoxy Composites”, Composites, 1993, Vol.
24, Issue 7, pp. 547- 555.
8. Liao Y. T., “ A Study of Glass Fiber- Epoxy Composite Interfaces”, Polymer
Composites, December 1989, Vol. 10, Issue 6, pp. 424- 428.
9. Gautier L., Mortaigne B., and Bellenger V., “ Interface Damage Study of
Hydrothermally Aged Glass- Fiber Reinforced Polyester Composites”, Composites
Science and Technology, 1999, Vol. 59, pp. 2329- 2337.
10. Schutte C. L., “ Environmental Durability of Glass- fiber Composites”, Materials
Science and Engineering, 1994, Vol. R13, pp. 265- 324.
7
11. Wolff E. G., “ Moisture Effects on Polymer Matrix Composites”, SAMPE Journal,
1993, Vol. 29, Issue 3, pp. 11- 19.
12. Shen C. and Springer G. S., “ Moisture Absorption and Desorption of Composite
Materials”, Environmental effects on Composite Materials, 1988, Ed., Springer G. S.,
Vol. 3, pp. 15- 34.
13. Ghorbel I. and Valentin D., “ Hydrothermal Effects on the Physico- Chemical
Properties of Pure and Glass Fiber Reinforced Polyester and Vinylester Resins”,
Polymer Composites, 1993, Vol. 14, Issue 4, pp. 324- 334.
8
Chapter 2
Literature Review
2.1 Moisture Absorption in Polymeric Composites
Polymeric Composites exposed to moisture undergo a wide variety of physico-chemical
changes. Experiments have revealed that plasticization and hydrolysis are the two
main causes of degradation of polymeric matrices and polymeric composites during the
hygrothermal aging process [ 1]. Before delving into the processes of degradation, it is very
important to understand the kinetics of transport and moisture diffusion processes in
polymeric composites. Water molecules dissolve on the polymer surface and diffuse through
the bulk by a series of activated steps under the driving force of concentration gradients. Both
solubility and diffusivity are involved in the process. Diffusion is the process by which matter
is transported from one part to another as a result of random molecular motion [ 2]. Classical
Diffusion behavior in polymer matrices can be classified as follows [ 3]:
( i) Case I or Fickian Diffusion: Rate of the diffusion is much less than that of
polymer segment mobility.
( ii) Case II: Rate of diffusion is much greater than the polymer segment mobility
and is strongly dependent on swelling kinetics.
( iii) Anomalous or Non- Fickian Diffusion: Rate of diffusion and polymer
segment mobility are comparable. Anomalous behavior can be considered as
intermediate between the case I and case II types of diffusion.
9
In this study, the moisture sorption data is analyzed with Fickian and Non- Fickian diffusion
( Langmuir) models.
2.1.1 Classical Fickian Diffusion
Fickian diffusion is characterized by the following features ( Fig. 2.1) [ 2]:
( i) Both sorption and desorption curves are functions of the square root of time and
are linear in the initial stage and the linear region extends to at least Mt/ Mm = 0.6,
where Mt is the moisture absorbed by the composite specimen at time t and Mm is
the maximum moisture content absorbed by the specimen.
( ii) Reflective symmetry between weight gain of initially dry specimens and weight
loss data of saturated coupons, when the diffusion coefficient is constant.
( iii) Above the linear portion, the rate of diffusion decreases until an equilibrium
moisture content is reached.
( iv) The sorption behavior obeys the film thickness scaling law: the uptake curves
obtained by plotting Mt/ Mm vs. t/ h ( reduced sorption curves) coincide
regardless of the thickness of the specimen ( t is the time and h is the thickness of
the specimen)
( v) The Diffusion coefficient, D, is a function of temperature T ( in degrees Kelvin),
and can be expressed as
D D0 exp Ea
RT
= ⎛⎜− ⎞⎟
⎝ ⎠
where D0 is a constant, Ea is the activation energy of the diffusion process
and R is the universal gas constant ( 8.3144 J mol- 1 K- 1).
10
It must, however be noted that the Fickian model does not hold good for all
temperatures and moisture contents. Fickian diffusion theory also assumes that during the
process of moisture sorption only reversible physical reactions occur in the polymer matrix
[ 2, 4].
Fig 2.1 Shape of a typical Fickian diffusion curve
2.1.2 Non- Fickian Diffusion
It has been observed that in many cases Fickian diffusion behavior is not observed
[ 5,6]. Figure 2.2 shows the departures from the Fickian diffusion as postulated by Weitsman
[ 6]. Curve A in Figure 2.2, classified as Pseudo- Fickian, depicts a continuous gradual
increase in the moisture content, with equilibrium never being attained. Curve B in Figure 2.2
represents Two- Stage sorption behavior, wherein the initial uptake is rapid and a linear
11
function of the square root of time. The sorption curve then approaches a quasi- equilibrium
followed by a slow approach towards a true equilibrium.
Curve S represents a sigmoid behavior – the sorption curves are sigmoid in shape
with a single inflection point. Curve C corresponds to rapidly increasing moisture content,
usually accompanied by large deformations and mechanical failure. Lastly Curve D in
Figure 2.2 represents weight loss that is attributed to irreversible chemical or physical
degradation of the material.
Fig 2.2 Schematic curves representing different types of anomalous diffusion in
polymeric composites ( After Weitsman [ 6])
12
A number of models have been proposed to describe anomalous diffusion in
polymeric composites, but there is still lack of a single general theory for anomalous
diffusion in polymeric composites. Roy et al. [ 7] utilized moisture gain data for an epoxy
resin immersed in salt solution at different temperatures, to propose a methodology, which
enables the characterization of non- Fickian diffusion coefficients. These diffusion
coefficients can be used subsequently to predict the moisture concentration profiles through
the thickness of the polymer.
The departure from the classical diffusion is attributed to the time- dependent
response of the polymer analogous to viscoelastic mechanical response. Cai and Weitsman
[ 8] proposed a model correlating the non- Fickian moisture gain data with a set of time-dependent
boundary conditions, as motivated by the viscoelastic mechanical response. This
procedure allows the reduction of non- Fickian moisture gain data in a way that enables the
evaluation of the diffusion coefficients and through- thickness moisture concentration profiles.
More information on non- Fickian diffusion models can be found in [ 9 – 14].
The Langmuir Diffusion Model, which is often used to describe non- Fickian
response, is a dual mode sorption model, which assumes that the penetrant molecules are
divided into two populations, one that is dissolved in the polymer and is hence able to diffuse,
and another that is absorbed in the micro- voids and is therefore locally immobilized [ 15].
Bonniau and Bunsell [ 16] compared the Fickian and Langmuir diffusion theories by
applying the diffusion models to water sorption data of Glass Epoxy composites. A review of
experimentally observed anomalous diffusion behavior in polymers has been made by
Hopfenberg and Stannett [ 17].
13
2.1.3 Factors Affecting the Diffusion Coefficient
Diffusion can be defined as the process by which matter is transported from one part
of a system to another as a result of random molecular motion [ 6]. The diffusion coefficient
describes the rate of diffusion of particles, depending on the particle size, viscosity and
temperature. Diffusion coefficient is a function of absolute temperature and has been shown
to increase with increase in temperature. Diffusion coefficient is related to temperature as
follows:
D D0 exp Ea
RT
= ⎛⎜− ⎞⎟
⎝ ⎠
( Equation 2.1)
where D0 is a constant, Ea is the activation energy of the diffusion process and R is the
universal gas constant. This equation was defined by Arrhenius in 1899 and is applicable to
determination of any reaction rate based on a temperature driven process. Because the
relationship of the rate of diffusion to activation energy and temperature is exponential, a
small change in temperature or activation energy causes a large change in the rate of
diffusion. Activation energy of the diffusion process is determined by calculating D at
different temperatures T, plotting the logarithm of D against 1/ T on a graph, and determining
the slope of the straight line that best fits the points. A linear fit across the entire regime
indicates dominance of a single moisture driven deteriorative mechanism, whereas a kink
indicates the point of transition between two diffusion regimes.
The diffusion coefficient also depends on the moisture concentration of the
environment, chemical structure of polymer matrix and imperfections like micro- cracks in the
polymer matrix, and the degree of cross- linking of the polymer. It has been proved that the
14
rate of degradation of polymers exposed to moisture is directly related to the rate of moisture
sorption of the polymer [ 18].
The process of moisture sorption is primarily influenced by internal factors – fiber
volume fraction, orientation of the fibers, and external factors- moisture concentration and
temperature [ 19, 20]. It has been observed that, in general, diffusion coefficients decrease
with increase in fiber volume fraction [ 21].
2.1.4 Factors Affecting Equilibrium Moisture Content
Experimental evidence indicates that the maximum moisture content is insensitive to
the temperature but depends on the moisture content of the environment. For a material
immersed in liquid, the maximum moisture content, Mm is a constant [ 2]. Equilibrium
moisture content is also affected by the previous thermal history, existing damages in the
composite and the chemical stability of the resin.
2.2 Hygrothermal Ageing of Composites
The degradation of the mechanical properties of polymeric composites, after
exposure to a combination of moisture and temperature is referred to as hygrothermal ageing.
Hygrothermal ageing is the summation of physical and chemical changes in the composite
material. Changes in the mechanical properties of the composites due to hygrothermal ageing
can be reversible or irreversible or a combination of the two depending on the exposure time
and temperature [ 22]. It should be noted that these changes can be affected through
application of sustained load. However, this will not be considered in the current
investigation.
15
The chemical effects of moisture on polymeric composites result from the interaction
between the water molecules and one or more of the matrix constituents and/ or the fibers.
Water molecules hydrolyze the polymer bonds, leading to dissolution and leaching of water-soluble
polymer molecules. In addition, the dissolution products react with the polymer
molecules, leading to further degradation [ 3]. Since polymeric composites are made with a
combination of various fibers and polymeric matrices, the degree of chemical interactions
with moisture depends on the physical and chemical composition of the composite material.
Damage caused to fibers, matrix cracking and debonding of fiber/ matrix interface due to
chemical changes in the composite permanently alters the mechanical properties of the
composite [ 22].
2.2.1 Hygrothermal Effects on Polymer Matrices
Moisture affects polymeric composites physically by plasticizing the matrix and thus
lowering its glass transition temperature. Changes caused due to plasticization and swelling
can usually be reversed on removal of the sorbed moisture from the material. The
plasticization phenomenon is related to the increase in the free volume of the polymer and to
the destruction of intra- molecular hydrogen bonds. Glass transition temperature Tg is an
important physical property of thermosetting polymers like vinylesters and polyesters. Glass
transition temperature is defined the critical temperature at which polymers undergo a change
from a glassy/ elastic to soft rubbery/ viscoelastic state.
At low temperatures, polymers are in a glassy state and are characterized by high
values of modulus of relaxation and elastic behavior. The only molecular motion possible is
vibration around fixed positions, because there is not enough thermal energy to facilitate
rotation and translation. When the temperature is increased, the increase in thermal energy
16
makes rotation and translation possible. The polymer then becomes like resilient leather
characterized by a sharp drop in the relaxation modulus. This region is called the transition
region. Following the glass transition, the modulus reaches a plateau [ 23] ( Fig 2.3). Thus
above the Tg, the strength and stiffness properties of the polymer decrease relative to its
properties below the Tg [ 24].
Moisture sorption by the polymeric matrix lowers its Tg, thereby causing the polymer
to soften at lower temperatures. Allred reported the effect of glass transition temperature on
the behavior of Kevlar/ Epoxy composites [ 25, 26]. It has been shown that the moisture
sorption decreases the Tg thus lowering the mechanical properties.
Fig 2.3 Modulus E as a function of temperature for a typical amorphous polymer
( After Tissaoui [ 23])
17
Chateauminois et al. [ 27] studied the static fatigue behavior of hygrothermally aged
unidirectional Glass/ Epoxy composites and the failure mechanisms associated with fatigue
damage were investigated under three- point flexural loading. Depending on the ageing
temperature, two failure modes were observed: fiber microbuckling on the compression side
or progressive cracking on the tensile side. Microbuckling was related to the reversible
plasticization of the epoxy matrix and the cracking on the tensile side was attributed to the
irreversible weakening of the fibers and the interface at higher ageing temperatures.
Glass transition temperature has been used characterize the physical effects of
moisture on polymers by Ghorbel and Valentin [ 1] and Birger et al [ 28]. Another physical
effect of moisture on polymers is the generation of internal stresses due to the accumulation
of water molecules in the micro- cracks and voids of the polymer matrix [ 3]. These internal
stresses cause localized failures in the matrix.
Apicella et al. [ 29] studied the influence of the chemistry of polyester resins on the
retention of their mechanical properties after exposing Glass/ Polyester composites to water at
different temperatures ( 25 and 90 oC). It was found that the relative hydrolytic damage
decreased as follows: isophthalic resins > bisphenol- B > bisphenol – A > vinylester. The
authors suggested that the susceptibility to hydrolytic attack increased with an increase in the
number of ester groups in the polymer repeat unit. Apicella et al. [ 30] also investigated the
influence of water sorption on the mechanical properties of glass fiber- reinforced polyester
composites immersed in water at temperatures of 20, 40, 60, 90 and 100 oC. The mechanical
properties of the polymeric matrix showed significant reduction in the glass transition region,
due to the progressive softening of the initially glassy system. The degradation mechanisms
were associated with both the low chemical resistance and the possible migration of some of
18
the components initially present in the thermoset, which was evident from the weight losses
observed for samples aged at higher temperatures.
2.2.2 Hygrothermal Effects on Fibers
Glass fibers, unlike graphite fibers, which are inert, are prone to attack by moisture
and aqueous solutions [ 24, 31]. It has been observed that the amount of strength reduction in
GFRP composites due to long- term load application is more pronounced when the composite
is wet than when it is dry [ 31].
The chemical effects on glass fibers can be demonstrated with the following
equations, which present a sequence of reactions leading to cleavage of silicon- oxygen bonds
and to their conversion to hydroxysilane [ 3].
2
2
Si ONa H O Si OH NaOH
Si O Si OH Si OH Si O
Si O H O Si OH OH
− −
− −
− + → − +
− − + → − + −
− + → − +
The overall reaction, which is autocatalytic due to the gradual increase in the pH
level, results in degradation and flaw formation at the glass fiber surface and in significant
strength reduction of the glass- fiber reinforced composite.
Tensile, Compressive and Interlaminar shear strengths are known to decrease in
GFRP composites exposed to hygrothermal ageing. Carol Williams [ 31] provides a
comprehensive review of the response of GFRP and CFRP composites to moisture sorption.
Wyatt and Ashbee provide a comparison of behavior of GFRP and CFRP composites on
exposure to water [ 32]. The differences in the behavior of glass fibers and graphite fibers
have been attributed to their different affinities to water ( the surface of a carbon fiber being
hydrophobic and that of a glass fiber hydrophilic) and the interface they form with the matrix.
19
Carbon fibers are almost immune to moisture attack at lower temperatures and any
degradation whatsoever is due to the degradation of the polymer matrix. This has been clearly
demonstrated by Wyatt and Ashbee [ 32]. However the CFRP composites showed debonding
at the fiber/ matrix interface at temperatures higher than 100 oC. On the other hand GFRP
composites showed significant damage due to fiber pitting and debonding at the fiber/ matrix
interface. Ehrenstein and Spaude subjected different types of individual glass fibers to
moisture and various corrosive media and reported axial or spiral cracking in the glass fibers
[ 33].
Water has been found to accelerate the rate of crack growth in glass fibers [ 34]. This
is due to two factors- first, water reduces the surface energy of the glass fiber, resulting in less
energy required for crack formation and the second, water reduces the energy required to
break the Si- O bond by a considerable amount, thus helping in the propagation of cracks.
Pultruded glass- fiber reinforced vinylester matrix composites were subjected to
environmental ageing in water and salt solutions at 25 oC and 75 oC by Liao et al [ 35]. Aging
in water and salt solutions results in degraded flexural and tensile properties of the pultruded
E- glass fiber reinforced vinylester composite. Also, comparison of the sizes of fracture
mirrors on the broken ends of the fibers in aged and un- aged samples suggested that
environmental ageing degraded the glass fibers. In addition, degradation of the fiber/ matrix
interface region during the aging process was also reported.
2.2.3 Hygrothermal Effects on the Interfacial Region
The interface is defined as the non- homogenous region that lies between the matrix
and the fibers. The adhesion between the fiber and matrix has to be good for the polymer
composite to have properties that are advantageous. In order for the composite to maintain its
20
properties on exposure to moisture, the interface must resist degradation due to moisture
sorption [ 36].
Ishida and Koenig [ 37] have published reviews addressing the mechanisms of
reinforcement of glass- fiber composites under wet conditions. To identify the mechanisms of
attack at the interface, it is necessary to understand the chemistry, structure and morphology
at the interface. The fibers are treated with coupling agents to enhance their adhesion with the
polymer matrix. In glass- fiber reinforced composites, the coupling agents react chemically
with glass fibers, through silicon hydroxyl groups and also with the resin through an organic
functional group that is compatible with the chemistry of the resin. Experimental studies of
the interface formed through the coupling agent revealed complicated multi- layered structure.
The deposition of coupling agents from water results in three layers on the glass- fiber
surface: a monolayer, a chemi- sorbed layer, a physic- sorbed layer [ 36].
Plueddemann [ 38] suggested that water is necessary to aid fiber- matrix bonding. He
proposed a theory in which coupling agents provide a bond at the interface that is capable of
using the hydrolytic intrusion of water, with self- healing, as a means of stress relaxation
without interrupting the bond between polymer matrix and fiber.
Several investigations of the interfacial region and its influences on the strength of
the composites have been done. Straub et al. [ 39] conducted microbond tests on P-Aramid/
DGEBA Epoxy composites exposed to temperatures ranging from 21 – 130 oC. The
interfacial shear strength was found to decrease with the increasing testing rate and the effect
was more pronounced below the glass transition temperature.
Liao [ 40] investigated the reaction between the coupling agent and epoxy matrix in
E- glass fiber reinforced epoxy composites, using Fourier Transform Infrared Spectroscopy
21
( FTIR). He found that a greater amount of coupling agent is needed for composites in
hydrothermal conditions than is required for dry conditions. His experiments demonstrated
that the interface will be more stable when the amount of coupling agent increases at the
interface, since the layers of the interface can be leached out when subjected to hydrothermal
conditions.
Gautier et al. [ 41] subjected two types of glass- fiber reinforced polyester composites
to immersion in water at different temperatures ( 30, 50, 70 and 100 oC). Osmotic cracking in
matrix, interface and interfacial bonding were identified. Decrease in inter- laminar shear
strength was reported which was attributed to interfacial debonding induced by differential
swelling.
The effect of fiber coatings on the mechanical properties of unidirectional glass-reinforced
composites was studied by Podgaiz and Williams [ 42]. It was reported that the
coating of fibers with an elastomer leads to a significant improvement in the impact strength
together with a slight decrease in the tranverse tensile strength.
2.2.4 Effect of Humidity on Composites
The equilibrium moisture content reaches a constant value when the material is fully
submerged in water. But its value varies with the relative humidity when the material is
exposed to humid air [ 43]. The equilibrium moisture content for materials exposed to humid
air can be expressed as
b
Mm = aφ ( Equation 2.2)
where a and b are constants and φ is the relative humidity.
22
Bonniau and Bunsell [ 16] studied the water sorption behavior of glass- fiber
reinforced epoxy composites subjected to humid air at relative humidities ranging from 0- 100
% and temperatures of 23 0C, 40 0C, 60 0C, 80 0C, 90 0C. Damage was reported in the
composites subjected to relative humidity levels of 90 – 100 % for exposure times exceeding
two weeks. Micro cracking of the resin surface was attributed for the damage.
Birger et al. [ 28] studied the response of graphite- epoxy composite specimens
subjected to flexural loading, after exposure to humid air at 95 % relative humidity and at a
temperature of 50 0C. It was reported that the mechanical properties and failure mechanisms
of the composites under flexural loading are affected by hygrothermal ageing. Also, ageing in
95 % relative humidity at 50 0C resulted in a drop in the glass transition temperature.
Collings [ 44] subjected carbon/ epoxy composites to humid environments at various
temperatures, which were representative of six different climates at different locations in the
world. The effect of these climates on total moisture level and distribution is reported for
various thicknesses of the carbon/ epoxy laminate. A constant relative humidity environment
that will produce a representative moisture level in all parts of the composite is proposed.
The effect of humidity of on glass fiber reinforced polyester and vinylester
composites was studied by Springer et al. by subjecting them to humid air [ 45]. Tests were
performed at temperatures of 23 0C and 93 0C with the composites exposed to humid air at 50
% and 100 % relative humidities. The weight gain of specimens for the specimens exposed to
humid air at 100 % relative humidity followed Non Fickian behavior. A decrease in ultimate
tensile strength, short- beam shear strength, tensile modulus and shear modulus was observed
with increase of exposure time.
23
2.2.5 Summary of Previous Research
Table 2.1- 2.5 present the summary of some previous research on hygrothermal
ageing of polymeric composites.
23
Table: 2.1 Resin systems subjected to environmental ageing
Author( s) Fiber Matrix Test Environment Test Temperature
( oC)
Test
Duration
Chin,
Nguyen and
Aouadi [ 56]
-
-
-
Vinylester,
Isopolyester,
Epoxy
Distilled water, Salt
solution, and
artificial concrete
pore solution
22, 60 400 hours
Ghorbel and
Valentin [ 1]
-
-
Polyester
Vinylester
Immersion in water 60 3900
hours
Roy et al. [ 7] - Epoxy Resin Salt water solution 23, 50, 60, 70 6 months
Table: 2.2 Kevlar fiber- reinforced polymer composites subjected to environmental ageing
Author( s) Fiber Matrix Test Environment Test Temperature
( oC)
Test
Duration
Aditya and Sinha
[ 21]
Kevlar
Kevlar/
Carbon
Epoxy
Epoxy
Relative Humidity 95 % 70 900 hours
Allred [ 25] Kevlar
49 Epoxy Immersion in
Distilled Water 21, 90, 150
-
24
Table: 2.3 Glass fiber- reinforced polymer composites subjected to environmental ageing
Author( s) Fiber Matrix Test Environment
Test
Temperature
( oC)
Test Duration
Aditya and Sinha
[ 21]
Glass
Glass
Epoxy
Polyester
Relative Humidity 95 % 70 900 hours
Bonniau and
Bunsell [ 16] E- glass
Bisphenol A
Epoxy
0 – 100 % RH 25 to 90 -
Chateauminois
et al. [ 27]
R- Glass DGEBA –
based Epoxy Distilled water 30, 50, 70 and 90 100 days
Gautier,
Mortaigne and
Bellenger [ 41]
Glass Polyester Immersion in Water 30 to 100 10000 hours
Ghorbel and
Valentin [ 1]
Glass
Glass
Polyester
Vinylester
Immersion in water 60 3900 hours
Karbhari [ 67] E- glass Vinylester
Immersion in water
Relative Humidity 56%
5, 23, 40, 60
23
225 weeks
Marsh, Lasky,
Seraphim and
Springer [ 62]
E- glass Epoxy Immersion in Water
under pressure
50, 75,85 and
100 145 hours
25
Table 2.3 contd.
Author( s) Fiber Matrix Test Environment
Test
Temperature
( oC)
Test Duration
Phani and
Bose [ 4] E- glass Isophthalic
Polyester Immersion in Water
50
80
100
480 hours
72 hours
25 hours
Pritchard and
Speake [ 52] Glass Isophthalic
Polyester Immersion in water 30, 45, 60, 70,
80 and 100 30 days
Rao et al. [ 19]
Jute
Glass
Epoxy
Epoxy
Immersion in distilled
water
Relative Humidity,
32 %, 76%, 92%, 98%
25, 40 and 60 2500 hours
Springer,
Sanders,
Tung [ 45]
E- glass Polyester
Saturated Salt water
No. 2 Diesel Fuel
Lubrication oil
Antifreeze mixture
Indolene
Humid air 50 % RH
Humid air 100 % RH
23, 93
23, 93
23, 93
23, 93
23, 93
3, 93
23, 93
6 months
6 months
6 months
6 months
6 months
6 months
6 months
Wyatt and
Ashbee [ 32] E- glass Polyester Immersion in water 20 and 100 1500 hours
26
Table: 2.4 Carbon/ Graphite fiber- reinforced polymer composites subjected to environmental ageing
Author( s) Fiber Matrix Test Environment
Test
Temperature
( oC)
Test Duration
Birger et al.
[ 28] Graphite Epoxy
Thermal Ageing
Immersion in water
Relative humidity 95
%
170
23
50
100
50
626 hours
960 hours
155 hours
115 hours
960 hours
Han and
Nairn [ 71] Carbon Polyimide
Immersion in Water
Relative Humidity- 62,
50, 76%
80
80
1000 hours
Loos and
Springer [ 61] Graphite Epoxy
No. 2 Diesel Fuel, Jet A
fuel, Aviation oil,
Saturated Salt water,
Distilled water
Humid air 100 % RH
Humid air 40 %, 60%
RH
Humid air 25 % RH
27 to 49
50, 70, 92
65
92
300 days
27
Table 2.4 contd.
Author( s) Fiber Matrix Test Environment Test Temperature
( oC)
Test
Duration
Mazor,
Broutman and
Eckstein [ 68]
Carbon
Graphite
Epoxy
Epoxy
Relative Humidity 0%
Distilled water
Sea water
Room
Temperature
Room
Temperature
Room
Temperature
Parvatareddy
et al. [ 66]
Carbon
Cyanate
Ester
Ambient air, At
reduced air pressure,
nitrogen
150 9 months
Shen and
Springer [ 43]
Graphite
Humid air- 0, 50, 75,
100 %
Saturated Steam
Immersion in water
27, 48, 70, 92
120, 150
70, 92, 150
-
Wyatt and
Ashbee [ 32]
E- glass Polyester Immersion in water 20 and 100 1500 hours
28
2.3 Performance Prediction Models
A number of empirical and theoretical models have been proposed for performance
prediction of Fiber Reinforced Polymer Composites. A brief description of some of the
models, which are frequently used, is given below.
2.3.1 Arrhenius Prediction Model
The Arrhenius Prediction Model is one of the commonly used life prediction
models in accelerated life testing [ 46]. It is a very convenient model to use in cases where
the acceleration variable is temperature. The model is derived from the Arrhenius reaction
rate equation proposed by the Swedish Chemist Svandte Arrhenius in 1887. The Arrhenius
reaction rate equation is given by,
R( T) Aexp Ea
KT
= ⎡⎢⎣ − ⎤⎥⎦
( Equation 2.3)
where R is the rate of the reaction,
A is a non- thermal constant,
Ea is the activation energy in Joules,
T is the absolute temperature ( Kelvin),
K is the Boltzmann’s constant, 1.38 x 10- 23 J/ K.
The Arrhenius life- stress relationship is formulated by assuming that the life is proportional
to the inverse reaction rate of the process.
L( T) Cexp B
T
= ⎡⎢⎣ ⎤⎥⎦
( Equation 2.4)
where L( T) represents the quantifiable life measure,
29
T is the temperature,
C is one of the model parameters to be determined,
B is another model parameter to be determined.
The Arrhenius life- stress relationship is linearized by taking natural logarithms on both
sides of the equation and the property retention data is fitted through the linearized model.
The result is a linear relationship between the percent retention of the property and the
natural logarithm of time. This relationship is then utilized for deriving an equation relating
the percent retention and the different temperatures to which the composite was subjected.
These relationships obtained can be used for prediction of life at temperatures other than
those used in the experiment.
2.3.2 Phani and Bose Prediction Model
Phani and Bose investigated the strength characteristics of a E- glass/ Polyester
chopped strand mat ( CSM laminate) immersed in water, using flexural strength tests. The
characterization of hydrothermal ageing of the laminates by means of acousto- ultrasonic
technique shows that the flexural strength σt after exposure time t is given by the relation
[ 47],
σ t ( σ0 σ ) exp[ t / τ ] σ ∞ ∞ = − − + ( Equation 2.5)
where σ0 and σ∞ are the flexural strength at times 0 and ∞, respectively and τ is a
characteristic time dependent on temperature.
It was found that the reduction of the strength of CSM laminates due to
hydrothermal effects is a rate process for which the temperature influences only the rate
constant. The rate constant follows the Arrhenius equation [ 48],
30
0
1 1exp a E
τ τ RT
= ⎡⎢⎣ − ⎤⎥⎦
( Equation 2.6)
where 1/ τ is the rate constant,
Ea is the activation energy in Joules,
1/ τ0 is a constant,
R is the universal gas constant ( 8.314 J/ mol),
T is the temperature ( Kelvin).
From equations 2.1 and 2.6 it is evident that the rate constant is nothing but the
diffusion coefficient. The percent retention data is fitted to the equation 2.5 using regression
analysis. This analyses yields relationships between the flexural strength and time at
different temperatures. By plotting 1/ τ against 1/ T, the values of Ea and τ0 are found. Using
the value of the constants calculated, equations 2.5 and 2.6 are combined to give the
strength degradation with time and temperature.
Time and Temperature Superposition principle ( TTSP) is applied to the degradation
process and a master curve for the process is obtained by shifting the data on the
logarithmic time scale. This master curve makes determination of strength at any
temperature possible, if the activation energy of the process is known. Thus, strength
retention experiments need to be conducted only at one temperature to estimate the
degradation at different temperatures.
2.3.3 Time and Temperature Superposition Model
Time and Temperature Superposition is a well known principle that works for
certain types of viscoelastic materials and relates the effect of time and the effect of
31
temperature, enabling us to substitute time with elevated temperature [ 49, 50, 51]. When
stiffness or strength properties are plotted against the logarithm of time for different
temperatures, they form a set of smooth curves. TTSP is based on the assumption that these
curves match each other when shifted horizontally along logarithmic time
scale.
Fig 2.4 Time Temperature Superposition Principle ( After Kuraishi [ 50])
Time dependent data at a particular temperature is selected as reference to
determine the shift factor on the time scale. The properties for each test temperature are
plotted on a logarithmic time scale, the data for the reference temperature are held fixed,
and the other curves are shifted horizontally along the time scale until the points form a
single curve. A small vertical shift can be applied to achieve the best superposition. The
32
resulting curve is called the master curve, which can be used to predict the strength and
stiffness properties at temperatures other than those used in the experiments.
It should be noted that superposition is mostly an approximation and therefore
extrapolation for long- term exposures, outside temperature ranges used in the experiments
is not reliable. The TTSP principle does not work, if there are multiple degradation
processes involved. If the data is determined with sufficient accuracy over a large enough
time range ( three or more decades), superposition will show that the curves actually do not
form a single curve. In many cases, data is available only for a limited amount of time, and
under these conditions superposition may appear to work, when it does not.
2.3.4 Pritchard and Speake Prediction Model
Pritchard and Speake describe a predictive model for the mechanical property
degradation in E- glass/ Polyester composites due to immersion in water at different
temperatures [ 52]. The degradation in the properties was found to be a function of the
absorbed moisture content, but was shown to be independent of the temperature of sorption
even when the sorption temperature exceeded the glass transition temperature of the resin.
According to Pritchard and Speake [ 52], two steps are necessary to predict material
properties: 1) the prediction of water sorption kinetics at temperatures outside the
experimental range, and 2) the establishment of empirical relationships between moisture
content and the properties.
The Fickian absorption model can be extended to find the absorption curve for
temperatures outside the experimental range. By plotting maximum moisture content from
the Fickian absorption model against temperature, it is possible to estimate the values of
moisture uptake at temperatures outside the experimental range. The diffusion coefficients
33
for the temperatures outside the experimental range can also be found by extrapolation of
the Arrhenius plots.
A curve- fitting program was used to obtain an empirical relationship between the
mechanical properties and the absorbed moisture contents. The best fits obtained from the
program were of the form,
p a( 1 ebexp[ cMt]) d = − − − + ( Equation 2.7)
where p is the residual property,
Mt is the moisture absorption at time t,
a, b, c and d are empirical constants.
This equation can be used to predict the residual mechanical properties at various
temperatures. The validity of these predictions depends on all degradation processes being
functions of absorbed water content, and on their being accelerated by temperature in the
same way and to the same extent as the water absorption process.
2.3.5 Phillips Prediction Model
Phillips [ 53] investigated the stress rupture in glass- fiber reinforced polyester
composites exposed to air and aqueous environments. He assumed that below the level of
stress, which causes immediate failure σ0, there exists a functional relationship between the
time to failure, t and the corresponding stress σt. The Phillips prediction model relates the
stress and the time to failure as follows:
0
t A Blogt
σ
σ
= − ( Equation 2.8)
34
where σt is the stress at time t,
σ0 is the initial stress,
A and B are empirical constants.
The property retention data can be fitted to equation 2.7 using regression analysis. The
empirical constants are found from the regression analysis and thus prediction of long- term
strength properties is made possible.
35
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36
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37
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41
Chapter 3
Materials and Test Procedures
3.1 Material Constituents
The composite system used in the study is a unidirectional E- glass/ Vinylester
composite with a volume fraction of 50- 55 %. The E- glass/ Vinylester composite specimens
were manufactured by the Resin Infusion Process using the Dow Derakane 411- 350
vinylester resin.
3.1.1 Glass Fiber Properties
The properties of E- glass fiber used to manufacture the unidirectional composites
are listed in Table 3.1.
3.1.2 Vinylester Matrix Properties
Vinylester resins are being widely considered for use in civil infrastructure, marine
vessels and offshore structures due to their ability to be easily fabricated through processes
like resin infusion. The vinylester resin used for the composite is Dow Chemicals Derakane
411- 350. Dow Derakane 411- 350 is based on bisphenol- A epoxy resin and has been widely
used in a wide range of end- use applications due to its ability to be used in a wide range of
fabrication techniques. Derakane 411- 350 provides resistance to acids, alkalis and organic
compounds and also provides good corrosion resistance. The resin is characterized by
superior elongation and toughness, which provides the composites with better impact
resistance and less cracking due to cyclic temperature and mechanical shocks [ 2]. The
42
liquid resin properties of Dow Derakane 411- 350 are listed in Table 3.2 [ 2]. Table 3.3 gives
a list of properties of the post- cured clear cast resin.
3.1.3 Fabrication Method
The E- glass/ Vinylester composites were fabricated using the Resin Infusion Process
[ 1] and are of 2.54 mm ( 0.1 in) thickness each. The specimens were post- cured at 120 oC for
24 hours. The fiber volume fraction of the specimens was then assessed by burn- off tests
and it was found to be 50- 55 %.
3.2 Environmental Conditions
The E- glass/ Vinylester test specimens were subjected to different environments
encompassing immersion in deionized water and humidity at different temperatures. The
list of the testing environments is given below:
1. Ambient conditions at 23 oC and 30 % Relative Humidity
2. Immersion in deionized water at 23 oC
3. Immersion in deionized water at 45 oC
4. Immersion in deionized water at 60 oC
5. Immersion in deionized water at 80 oC
6. Immersion in deionized water at 95 oC
7. Exposure to humid air with 0- 5 % relative humidity at 23 oC
8. Exposure to humid air with 45 % relative humidity at 23 oC
9. Exposure to humid air with 60 % relative humidity at 23 oC
10. Exposure to humid air with 75 % relative humidity at 23 oC
11. Exposure to humid air with 98 % relative humidity at 23 oC
12. Exposure to humid air with 0- 5 % relative humidity at 95 oC
43
13. Exposure to humid air with 45 % relative humidity at 95 oC
14. Exposure to humid air with 60 % relative humidity at 95 oC
15. Exposure to humid air with 75 % relative humidity at 95 oC
16. Exposure to humid air with 98 % relative humidity at 95 oC
The set of conditions were chosen to enable testing over a range og hygrothermal exposures
which would also enable useof acceleration procedures.
Table 3.1 Properties of E- glass Fibers ( Kaw [ 1])
Property Value ( SI) Value ( FPS)
Specific gravity 2.54 2.54
Young’s modulus 72.40 GPa 10.5 Msi
Ultimate Tensile Strength 1447 MPa 210 Ksi
Coefficient of thermal expansion 5.04 μm/ m/ oC 2.80 μ. in/ in/ oF
Poisson’s Ratio 0.2 0.2
Axial Shear Modulus 35.42 GPa 5.136 Msi
Shear strength 35 MPa 5.08 Ksi
Chemical Composition
54% Silicon oxide, 15% Aluminium oxide, 17%
Calcium oxide, 4.5% Magnesium oxide, 8% Boron
oxide
Table 3.2 Typical Liquid Resin
Properties of Dow Derakane 411- 350 Vinylester Resin *
Property Value
Density 25 oC/ 77 oF 1.046 g/ mL
Dynamic Viscosity 25 oC/ 77 oF 370 mPa. s
Kinematic Viscosity 350 centiStokes
Styrene content 45 % by weight
Shelf Life 25 oC/ 77 oF 7 months
* www. dow. com
44
Table 3.3 Typical Properties of Clear Resin Castings
( From [ 2])
Property Value ( SI) Value ( FPS)
Tensile Strength
73 MPa
10500 psi
Tensile Modulus 2.8 GPa
4 x 105 psi
Tensile Elongation, Yield 4.8 % 4.8 %
Flexural Strength 122 MPa 17600 psi
Flexural Modulus 3.1 GPa 4.5 x 105 psi
3.3 Test Procedures
A brief account of the testing procedures followed for the moisture absorption tests
and mechanical characterization tests is given in the subsequent sections.
3.3.1 Moisture Sorption
Moisture is known to react with one or more of the matrix constituents and can
hydrolyze the polymer bonds leading to the dissolution and leaching of water- soluble
components. Moisture affects polymeric composites physically by plasticizing the matrix
and thus lowering its glass transition temperature [ 3]. Moisture absorption in FRP
composites not only affects the dimensional stability but also affects the mechanical
properties of the composites. Thus determination of the moisture content and the rate of
moisture diffusion in composites after exposure to hygrothermal ageing is necessary
because moisture sorption has profound effects on short- term and long- term durability of
the composite system.
The measurement of moisture uptake was conducted by the gravimetric method.
Five specimens with dimensions of 25.4 mm x 25.4 mm ( 0.1in x 0.1 in) and 2.54 mm ( 0.1
45
in) thickness were placed in each of the environments listed in section 3.3 and the uptake
was measured at periodic intervals.
3.3.2 Tensile Characterization
Tensile tests are important because they are main characterizing element that
defines the in- plane tensile properties of the composite specimen [ 4]. Tensile data on
unidirectional composites are often used as one of the key factors in materials selection and
also provide basic ply properties, which are used in laminate design [ 5]. The ultimate tensile
strength and tensile modulus are the two important parameters that are obtained from this
test in addition to other tensile properties. However the tensile tests carried out under
controlled conditions and close observation can also yield additional information about
failure initiation and development [ 4]. Polymeric composites being non- homogenous
exhibit weakness in a particular loading direction, while having high strength in other
directions. Therefore the direction of loading is of utmost importance for polymeric
composites for the determination of tensile properties. The tensile tests on the E-glass/
Vinylester composite specimens were performed in accordance with ASTM D 3039M
[ 6]. The composite specimens measuring 254 mm x 25.4 mm x 2.54 mm ( 10 in x 1 in x 0.1
in) were tested in an Instron testing machine with the grips set to a gage length of 177.8 mm
( 7 in). The specimen was loaded at a rate of 1.27 mm/ min ( 0.05 in/ min).
3.3.3 Flexure Characterization
The use of flexural tests to determine the mechanical properties of polymeric
composites is widely prevalent because of the relative simplicity of the test method,
instrumentation and testing equipment required. Flexure mode tests can also be used to
determine the interlaminar shear strength ( using a short beam) and interlaminar fracture
46
toughness of the composite laminates. Although it is frequently found that the flexure tests
give results, which are very similar to those from other tests ( tension and compression), it is
generally recognized that test methods applying flexure as a means of loading do not
produce results of design data quality. But flexure tests continue to be used widely because
of their relative simplicity [ 7].
The flexure tests for the E- glass/ Vinylester specimens were done in accordance
with ASTM D 790 [ 8]. The composite specimens used measured 12.7 mm ( 0.5 in) in width
and 2.54 mm ( 0.1in) in thickness. The span of the specimen measures 152.4 mm ( 6 in)
making the span to depth ratio 60: 1. The specimen is loaded at a constant rate of 5.08
mm/ min ( 0.2 in/ min). The specimen is loaded until rupture occurs.
3.3.4 Short Beam Shear Characterization
Fiber- reinforced composite are known to exhibit poor resistance to shear
deformation, especially in material planes dominated by matrix properties. Relatively low
values of shear strength and shear modulus often leads to use of optimized arrangement of
laminate stacking sequences to maximize shear resistance. This in turn can lead to the
compromise of other mechanical properties [ 9]. Development of in- plane and out- of- plane
shear test methods for the determination of shear modulus and strength of fiber- reinforced
composites is difficult because a region of pure and uniform shear stress has to be provided
in the test section of the specimen. The difficulty of inducing pure shear increases with
increasing anisotropy and inhomogeneity of the material. Because of this there are a wide
variety of methods employed to determine the shear characteristics of a fiber- reinforced
composite specimen, which are listed below:
􀂉 ± 45o Tension test – ASTM D 3518
􀂉 Rail Shear Test – ASTM D 4255
47
􀂉 V- notched beam test – ASTM D 5379
􀂉 Plate- twist test – ASTM D 3044
􀂉 Short Beam Shear test – ASTM D 2344
In this study, the short beam shear test ASTM D 2344 [ 10] was employed to find
the shear characteristics of the E- glass/ Vinylester composite specimens. Short beam shear
tests are performed on composite specimens 12.7 mm ( 0.5 in) in width and 2.54 mm ( 0.1 in)
in thickness and 12.7 mm ( 0.5 in) length ( span). A span to depth ratio of 5: 1 was employed
for the test. The specimens were loaded at a rate of 32.25 mm/ min.
3.3.5 Dynamic Mechanical Thermal Analysis
Dynamic Mechanical Thermal Analysis ( DMTA) is used to determine the change in
the mechanical properties of materials either under isothermal conditions or as a function of
temperature. The technique is often used to measure the damping properties of materials
and the glass transition temperature of polymers [ 11]. The technique uses measured natural
frequencies of dynamically excited specimens to derive stiffness properties of the material.
DMTA tests were performed as per ASTM E1640 [ 12], using three- point bending.
The DMTA test was performed on the composite specimens and the glass transition
temperature was measured for the specimens immersed in water at 23 oC, 40 oC, 60 oC, 80
oC and 95 oC.
48
3.4 References
1. Kaw A. K, “ Mechanics of Composites Materials” CRC Press, 1997.
2. V. M. Karbhari, unpublished results.
3. Marom G., “ The Role of Water Transport in Composite Materials”, Polymer
Permeability, Comyn J., Ed., Elsevier, New York, 1975, pp. 341- 374.
4. Godwin E. W., “ Tension”, Mechanical Testing of Advanced Fiber Composites, J. M.
Hodgkinson Ed., Cambridge, England, CRC Press, 2000.
5. Whitney J. M. and Knight M., “ The relationship between Tensile Strength Flexure
Strength in Fiber- Reinforced Composites”, Experimental Mechanics, June 1980, pp.
211- 216.
6. ASTM D3039M, “ Standard Test Method for Tensile Properties of Polymer Matrix
Composite Materials”, American Society of Testing Materials, 1997, Vol. 15.03.
7. Hodgkinson J. M., “ Flexure”, Mechanical Testing of Advanced Fiber Composites, J. M.
Hodgkinson Ed., Cambridge, England, CRC Press, 2000.
8. ASTM D790M, “ Standard Test Methods for Flexural Properties of Unreinforced and
Reinforced Plastics and Electrical Insulating Materials”, American Society of Testing
Materials, 1993, Vol. 08.01.
9. Broughton W. R., “ Shear”, Mechanical Testing of Advanced Fiber Composites, J. M.
Hodgkinson Ed., Cambridge, England, CRC Press, 2000.
10. ASTM D2344M, “ Standard Test Method for Apparent Interlaminar Shear Strength of
Parallel Fiber Composites by Short- Beam Method”, American Society of Testing
Materials, 2000.
11. Ban