| Due to extensive utilization of epoxy polymers in industries, the durability and reliability of these materials become more and more critical issues for applications. The aim of this research project is to investigate the uniaxial and multiaxial fatigue behavior of an epoxy polymer (EPON 826/EPI-CURE Curing Agent 9551) under various loading conditions.;A fatigue testing platform, including a non-contact real-time strain measurement system based on the Digital Image Correlation Method (DICM), has been established. This system eliminates the problem brought by using the traditional extensometers, which would introduce considerable stress concentration on the surface of soft materials and thus accelerate the failure. By tracking the in-plane displacements of the pre-painted marks on the surface of the specimen, both axial strain and shear strain can be measured simultaneously in real-time mode. An optimized search algorithm enables high precision and fast data acquisition frequency in the strain measurement.;Series of uniaxial and multiaxial fatigue tests have been carried out under either stress-controlled or strain-range-controlled mode. Moreover, mean strains were superimposed in strain-range-controlled fatigue tests to investigate the mean stress/strain effect on fatigue life. Ratcheting strain effect was also studied by stress-controlled fatigue tests with mean stresses. Pure shear fatigue tests with and without mean strains, along with combined axial-shear fatigue tests with mean strains, were also performed to investigate the multiaxial fatigue behavior.;Three types of multiaxial fatigue life prediction theories, i.e. stress-based, strain-based and energy-based approaches, were examined in correlation with the comprehensive fatigue test results of the epoxy polymer studied. Mean stress/strain effect was included in these theories by incorporating mean stress/strain functions into the damage parameters. It was found that the Stassi criteria are more appropriate to evaluate the equivalent stress/strain in multiaxial fatigue for the studied polymer material which shows anisotropy in tensile and compressive behaviors. Good agreement between the prediction curves and the experimental data was obtained by using stress-based and energy-based criteria, among which the energy-based approach performed the best. Relatively large dispersion exists in the strain-based approach for the multiaxial fatigue data with mean strains, which could be attributed to its inability to account for the effect of mean stress relaxation. |
