Fatigue of polymer composites: Life prediction and environmental effects

Fiber reinforced polymer composites offer significant advantages over traditional structural materials, but also many challenges, particularly with regard to their long term behavior. The fatigue performance of composites has been explored for the last few decades but it is still difficult to predict the behavior of a particular material under different loading and environmental conditions. Many researchers have worked to develop fatigue life prediction models and others to characterize the effect of environmental changes on static composite material properties, but experimental work in both of these areas can be difficult to perform and thus many observations have not been fully understood.;In order to further explore the effect of environmental conditions on fatigue life, a regimen of test was performed on a glass/vinyl ester composite. The testing included bending fatigue tests, tension-tension fatigue tests, and immersion conditioned tension-tension fatigue tests, with each type being performed under varying environmental conditions in the forms of elevated temperatures and the presence of salt water. From the experimental data, it was observed that the presence of salt water caused as much as a 50% decrease in fatigue life but was dependent on the time of exposure and had little effect on short duration tests. Elevated temperatures had an even more detrimental effect and exhibited a linear relationship with the number of cycles to failure. Immersion conditioning at room temperature decreased the fatigue life of the material to around 50--65% while immersion conditioning at 100°F decreased the fatigue life to around 15--25%.;Additionally, a strain energy-based fatigue life prediction model proposed by researchers at the West Virginia University Constructed Facilities Center was evaluated by applying the model to data collected through the experimental work, as well as to a large amount of fatigue data from the DOE/MSU Composite Material Fatigue Database. Test results for 109 different composites (1254 individual tests) were analyzed. From the variety of coupon fatigue data used to evaluate the model, it was found that the model was able to fit GFRP materials with over 80% of the results falling within +/-5% of the log number of cycles to failure. The model was also shown to be able to predict the fatigue life of polyester GFRPs to within +/-5% of the log number of cycles to failure using only two experimental values with a success rate of over 75%; using three increased the success rate to over 80%, but using more had little effect on its accuracy. A single-sample estimation model based on resin volume fraction was also developed and had reasonable success with polyester GFRPs. Of the two component fatigue results tested, both were able to be predicted by the fatigue model to within +/-2.5% log error. The strain energy fatigue model appears to provide both a good fit and a good prediction for the tension-tension fatigue life of GFRP composite materials