Multiaxial deformation and fatigue of rubber under variable amplitude loading

Conditions experienced by rubber components are often more complex than a constant amplitude loading history. Consequently, a method of relating the results of constant amplitude fatigue behavior characterization to variable amplitude loading conditions is needed. While the ultimate goal is to apply fatigue analysis to actual service histories, this dissertation uses simple variable amplitude signals to study the behavior of rubber under variable amplitude conditions. Two carbon black filled compounds were used to study the effects of strain crystallization: natural rubber and SBR.; Fatigue crack growth experiments were conducted on pure shear specimens to evaluate the applicability of a linear crack growth model that equates variable amplitude crack growth rate to the sum of the constant amplitude crack growth rates for each cycle. Crack growth rates were observed to be highly sensitive to R-ratios for natural rubber, but not for SBR. The selected variable amplitude test signals studied the effects of R-ratio, load level, load sequence, and dwell periods on crack growth rate. The linear crack growth model was found to be applicable in most cases, but a significant dwell effect was observed that is not accounted for by current models. A model was developed that successfully captures the dwell effect on crack growth rates.; Multiaxial experiments were also conducted to investigate the effects of variable amplitude loading. Multilevel tests varied the load level on one loading axis, while multiaxial tests applied both axial and torsional cycles. Varying the load level during the test was observed to have an effect on the stress-strain response of the material for the lower strain levels, while no significant effect on stress-strain level was observed during the multiaxial tests that alternated blocks of axial and torsion cycles. A model for the specimen temperature during a test as a function of hysteresis energy and test frequency was developed based on the temperature results measured via a thermal imaging system.; Natural rubber developed a high density of cracks that remained small, while SBR developed a few cracks that grew longer. The orientation of cracks was found to be dependent on the relative number of axial and torsional cycles during multiaxial tests that combined cycles that acted on different cracking planes. The observed crack orientations were compared to predicted crack orientations based on cracking energy density (CED) and normal strain parameters and found to agree for many of the test paths. The linear crack growth rate model based on Miner's linear damage rule concept was applied to the crack growth rate results from the multiaxial ring specimens and was reasonably accurate in predicting the variable amplitude crack growth rates in both materials.; The fatigue lives for the variable amplitude tests were predicted using Miner's linear damage rule. The predicted lives were generally within a factor of 2 for natural rubber and a factor of 3 for SBR, indicating relatively good agreement for the load signals used in this study. With regards to the use of equivalence parameters, CED produced the best correlation to the constant amplitude data in both materials, as well as producing good agreement between the predicted and experimental variable amplitude fatigue lives. While CED is the preferred approach, a more computationally efficient alternative fatigue life analysis approach that used maximum normal strain to identify the critical plane and CED to predict fatigue life was found to produce acceptable results.