Crack initiation and propagation under multiaxial fatigue in a natural rubber

The ever growing use of elastomers and polymers in structures leads to the need of pertinent multiaxial fatigue life criteria for such materials. Thus, the understanding of the fatigue crack initiation micro-mechanisms and their link to the local stress and/or strain history is essential. Scanning electron microscopy and Energy Dispersive Spectroscopy (EDS) have been used to investigate those micromechanisms on a natural rubber. Rigid inclusions were systematically found at the crack initiation. Depending on the type of inclusion (identified by EDS), cavitation at the poles or decohesion are the very first damage processes observed. Cracks orientations are compared to local principal stress orientation history, the later being obtained from finite element calculations (FE). It is shown that if large strain conditions are correctly taken into account, cracks are found to propagate systematically in the direction given by the maximal first principal stress reached during a cycle, even under non-proportional loading. A fatigue life criterion is proposed.