How microscopic stress and strain analysis can improve the understanding of the interplay between material properties and variable amplitude fatigue

Abstract

Lightweight construction is one of the most demanded technologies in many engineering systems. In order to guarantee the safety of the whole system, it is mandatory to improve models that describe and predict its behavior under load. Fatigue, the damaging of materials under cyclic loading, is the main phenomenon leading to failure in e.g. automobile and aerospace components. Cyclic loading during service does usually not happen with constants amplitudes, rather there are complex patterns of different load levels. High load variations in these patterns lead to deviations from the linear Paris behavior. Strong decelerations occur as consequence of a single increased tensile load, which is known as the overload effect. Nevertheless, this effect does not affect all materials the same, there are materials that show a strong overload sensitivity and others on which overloads only have a minor influence. Reasons for this can be seen in the interplay of the underlying mechanisms of the overload effect: plasticity induced crack closure and compressive residual stresses. While both effects lead to crack tip shielding and a reduction of stress intensity, crack closure delays the opening of the crack tip and thereby reduces the effective ΔK range, whereas compressive residual stresses superimpose with crack tip stresses and thereby reduce Kmax. Possible reasons for differences in the sensitivity can be differences in the strain hardening, both in the static and in the dynamic case, as well as in changes of the sign of stresses (Bauschinger effect). Since crack propagation is driven by local stresses and strains, measurements to examine differences in them have to be performed on a microscopic scale.

We could show that by the combination of modern measurement techniques – magnetic Barkhausen noise and digital image correlation in scanning electron microscope – we were able to image, separate and evaluate the mechanisms of the overload effect quantitatively. The calibrated magnetic Barkhausen noise microscope allows us measurements of residual stresses with a spatial resolution of 10 µm. From the digital image correlation results we could evaluate the crack tip driving forces namely the crack opening behavior, changes in the stress intensity Kand in the strain energy release rate via the J-integral. Using a simple model based on these results, we were furthermore able to predict the crack growth behavior due to the overload effect. These results will be used to extend crack growth models, while taking the interaction of materials´ properties with the mentioned mechanisms into account. This should enable a physically based, improved lifetime prediction and material selection for certain load patterns.