Cumulative Damage Under Constant Amplitude and Variable Amplitude Loading: Some New Physical Aspects

Synopsis

From systematic investigations of typical stages of metallic materials, subjected to different types of loading histories, a strong correlation between the cyclic deformation processes, characteristic changes of temperature and the microstructural mechanisms could be identified. For a further verification of this correlation a normalized unalloyed steel was investigated under constant amplitude loading with positive and negative mean stresses and also under simple variable amplitude loading histories as biharmonic loading and block program loading. The amount of cyclic deformation and the temperature development as a result of the plastic deformation were determined. The microstructural processes at the surface of the specimen were observed by using electron- microscopy. Both under constant amplitude loading conditions and under simple variable amplitude loading, where the main parts of the loading acted in the tension range, large surface deformations accompanied by a characteristic slip line formation and crack initiation predominantly at grain boundaries and at phase boundaries were observed. When the main parts of the loading acted in the compression range, only a few single slip bands were formed. Crack initiation took place at these slip bands. These microstructural peculiarities have far-reaching effects on the damage accumulation analysis.