Multiaxial crack propagation tests on NR Pure Shear samples to study crack features from relaxing and non-relaxing loads

Abstract

Extensive studies of complex non-relaxing fatigue loads on AE2 samples have been performed to investigate the lifetime reinforcement as well as the crack propagation features (crack roughness, crack branching and propagation in or out of the critical plane) in (Warneboldt et al., 2019; Warneboldt et al., 2022). This test database includes tension-torsion loading, both for relaxing and non-relaxing conditions and for different temperatures between 23°C and 80°C. The principal findings are: Observations confirm connection between lifetime reinforcement and crack features; Temperature influences relaxing and non-relaxing tests (lifetime & crack features); Critical plane predictions fail for lifetime reinforcing multiaxial configurations; Analyses of crack features and lifetimes point to a local microstructure modification. To extend the study of crack propagation features, an Arcan-like test fixture has been designed and is used for multiaxial crack propagation tests on Pure Shear (PS) samples of natural rubber (NR). This original device allows to apply tension as well as shear loadings to the mounted planar specimen. Additionally, one of these axes can be fixed to impose a pre-load, while the other one imposes cyclic fatigue crack propagation loads. This setting enforces similar crack propagation conditions as the tension-torsion loadings of the mentioned test database on the AE2 specimens.Our exploratory investigations confirm indeed that similar local conditions on the AE2 and the PS specimens lead to comparable crack propagation features. This finding opens a new research axis for these types of loadings. The planar PS sample has many advantages: Propagation behavior of pre-initiated cracks can be studied, Digital Image Correlation (DIC) analyses around an initiated crack are easier on the planar specimen surface; the study of the crystallinity via wide-angle X-ray diffraction analyses (WAXD) is possible (see for instance (Toki et al., 2000)); simpler FEA modelling is given. In the following we present exploratory study results for these tests which extend further our understanding of the fatigue damage mechanisms for complex, non-relaxing fatigue loads.