A Novel Framework for Understanding Multiaxial Fatigue of SBR Through Monitored Tension‐Torsion Tests

ABSTRACT In the present work, mappings of two invariants of the true strain tensor, that measures the magnitude of the distortion and that quantifies the mode of distortion, are used to investigate the multiaxial fatigue of AE2 and AE42 diabolo samples under tension/torsion conditions. These mappings are used to set up two types of experimental tests. The first is called unimodal because it ensures a constant value of ; therefore, the mode of distortion remains the same along the cycles; three modes have been chosen: uniaxial tension (), simple shear () and a mix mode (). The second is called multimodal because the value of changes along a cycle, oscillating between simple shear and uniaxial tension with a constant magnitude of the distortion . Experimental results show large difference between the fatigue lifetime of the two samples: For both unimodal and multimodal tests, failure of AE42 samples occurs earlier than failure of AE2 samples for the same conditions. Numerical analysis show that, under the same conditions, the principal stresses and stretch ratios are identical for the two samples questioning the relevance of these quantities for multiaxial fatigue lifetime predictions. Finally, it appears that the stress gradient differs from one geometry to another, and it could be an important mechanical quantity to take into account. This experimental work highlights one of the weaknesses of the fatigue criteria proposed in the past in the literature: The geometry of the test specimens must be taken into account. Here, we emphasize that this can be accounted for using the spatial stress gradient.