Multiaxial fatigue under complex non-relaxing loads

Automotive anti-vibration parts undergo complex and multi-directional fatigue loadings, which must be considered and finally validated in their design phase. Some parts have to endure Road Load Data (RLD) test-loads (i.e. stochastic signals, representative for the actual service conditions of a part), meanwhile being already pre-charged by a constant load e.g. by the engine mass or by swaging. These conditions might result in positive minimal load values. For natural rubber, it is well known that this leads to material reinforcement which is usually related to strain induced crystallization. The reinforcement effect is well studied and illustrated in the so-called Haigh diagram by various previous studies of uni-axial tension tests Cadwell et al. 1940, Champy et al. 2015). However, to be able to determine a mean-load correction factor for a part or a specimen under complex stochastic loads, the concept of the Haigh diagram must be extended to various complex and multi-axial test conditions. It is crucial for its fundamental understanding to perform a dedicated experimental test campaign on specimens under different complex periodic loading cycles. Such fatigue tests are conducted on hourglass-shaped natural rubber specimens, loaded by coupled and aligned tension and torsion actuators, which permits to induce numerous complex load states. Finite Element Analysis (FEA) is used to determine the respective local mechanical state in the rubber specimens. Different strain, biaxiality ratio and critical plane orientation histories over a loading cycle are achieved in different test series. Finally, the number of cycles to crack-initiation and the final fracture surfaces are carefully analyzed using an optical microscope and a Scanning Electron Microscope (SEM). The results are then compared with the outcomes of non-relaxing fatigue tests in simple tension and torsion with the same specimens.