Fatigue analysis methods must account for various effects in rubber that are observed under constant amplitude testing. Crack nucleation and crack growth approaches for analyzing the fatigue behavior of rubber components are complementary paradigms that can be applied depending on analysis objectives. Fatigue crack nucleation refers to the development, under the action of cyclic loading, of one or more cracks in a specimen, from an initial state nominally free of preexisting cracks. Fatigue crack growth refers to the development of cracks under cyclic loading, from an initial state in which the crack of interest is known. A linear crack growth rate prediction model analogous to Miner's linear damage rule has been proposed to predict the fatigue crack growth rate for variable amplitude signals using constant amplitude crack growth data as the basis for the predictions by Ryan J. Harbour et al.

Knowledge of the stress response of a material to the applied deformations is necessary for many engineering analysis and applications. This paper addresses the observed effects of load sequencing, Mullins effect, and multiaxial loading. on the constitutive behavior of rubber under variable amplitude conditions for a series of experiments using multiaxial ring test specimens. Two filled rubber materials were used and compared in this study; natural rubber which strain crystallizes, and styrene butadiene rubber (SBR), which does not. A pseudoelastic approach is used to model the cyclic stress-strain response for both materials. The implications of inelasticity when using hyperelastic material models are also discussed. Based on temperature results for the multiaxial ring specimen obtained via a thermal imaging system for SBR, a model capable of accurately predicting surface temperature for the multiaxial ring specimen as a function of hysteresis area and test frequency has been developed.

The orientations of cracks as they develop in a material indicate the planes that have experienced the maximum damage. For the purpose of fatigue life analysis and prediction, these planes are referred to as the failure or critical planes. In order to study the planes on which cracks develop for different types of loading, the development of cracks was observed during constant and variable amplitude experiments using the multiaxial ring specimen. Two filled rubber materials were compared in this study: NR, which strain crystallizes, and SBR, which does not. Multiaxial test signals composed of alternating blocks of axial and torsion cycles (each of which acts on different critical planes) produced crack orientations that fell between those occurring for signals composed only of axial or of torsion cycles. Plane-specific fatigue damage parameters of cracking energy density and normal strain were evaluated for their ability to predict the experimentally observed planes of crack development.

This paper investigates the effects of variable amplitude loading conditions on the fatigue lives of multiaxial rubber specimens. Two filled rubber materials were used and compared to investigate the effects of strain-crystallization on crack development NR, which strain crystallizes, and SBR, which does not. The applicability of Miner’s linear damage rule for predicting fatigue lives of variable amplitude tests in rubber and the use of both scalar and plane-specific equivalence parameters to characterize fatigue life results were also investigated. A fatigue life prediction approach that utilizes normal strain to find the critical plane and the cracking energy density on that plane to determine fatigue life is introduced and compared to other approaches. The effects of load sequence and temperature on fatigue life, as well as differences in fatigue lives using both stiffness and critical crack length failure criteria are discussed.

Loading conditions for rubber components are often more complex than the constant amplitude signals used in material characterization. During a series of uniaxial fatigue crack growth experiments on filled SBR under variable amplitude loading conditions, test signals that included a dwell period produced higher crack growth rates than corresponding constant amplitude test signals without a dwell period. These test signals alternated periods of cyclic loading with dwell periods ranging from 1 to 100 seconds at a near zero stress level. Dwell period tests produced average experimental crack growth rates up to 30 times greater than constant amplitude crack growth rates in filled SBR. The length of dwell time and the number of applied cycles between dwell periods were the most significant influences on the crack growth results. An empirical model was developed that captured the dwell effect based on these parameters. Dwell periods also produced increased crack growth rates in natural rubber, but the effect was less significant. It is proposed that the effect is caused by the time-dependent recovery in the rubber microstructure at the crack tip producing a localized and temporary elevated stress-state during loading events immediately following a dwell period. Current fatigue prediction methods do not account for the dwell effect.

Service conditions experienced by rubber components often involve cyclic loads which are more complex than a constant amplitude loading history. Consequently, a model is needed for relating the results of constant amplitude characterization of fatigue behaviour to the effects of variable amplitude loading signals. The issue is explored here via fatigue crack growth experiments on pure shear specimens conducted in order to evaluate the applicability of a linear crack growth model equivalent to Miner's linear damage rule. This model equates the crack growth rate for a variable amplitude signal to the sum of the constant amplitude crack growth rates associated with each individual cycle. The variable amplitude signals were selected to show the effects of R-ratio (ratio of minimum to maximum energy release rate), load level, load sequence, and dwell periods on crack growth rates. In order to distinguish the effects of strain crystallization on crack growth behaviour, two filled rubber compounds were included: one that strain crystallizes, natural rubber, and one that does not, styrene-butadiene rubber. The linear crack growth model was found to be applicable in most cases, but a dwell effect was observed that is not accounted for by the model.