Microscopic stress- and strain analysis to investigate the influence of hardening on crack retardation mechanisms under cyclic loading with variable amplitudes (overloads)

• Applicants: Privatdozent Dr.-Ing. Michael Marx; Professor Dr. Christian Motz
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins ; Mechanics
• Term: from 2016 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 299357558

Crack growth behaviour under the influence of overloads and variable amplitude loading has been examined for over 40 years and its predictive modelling is only partly successful. The models are usually based on the corresponding crack growth curves and macroscopic (therefore not local) measurements of the crack opening behaviour. Due to the inaccessibility of measurement techniques for microscopic stress and strain fields, there have been discussions on the mechanisms that influences to the crack growth behaviour: the plasticity induced crack closure on the crack flanks and the residual stresses in front of the crack tip. In our work we could show that due to the combination of modern measurement techniques - magnetic Barkhausen noise and digital image correlation in scanning electron microscope - we are able to image, separate and evaluate quantitatively the mechanisms of the overload effect. Using a simple model based on these results, we were furthermore able to predict the crack growth behaviour due to the overload effect. There are materials that show a strong overload sensitivity and others on which overloads have only a minor effect. Our past results were achieved using an elastic-ideal plastic material and can therefore not explain this material behaviour. Possible reasons for this sensitivity can be found in differences in the strain hardening, both in static and in dynamic case as well as in change of loading direction (Bauschinger Effect). By using our analysis methods, we caninvestigate the influence of the hardening effects on the overload mechanisms. These new insights will improve models that predict crack growth, which would improve safety or - especially relevant for lightweight construction - a less conservative, but still safe construction. Our data can also be used as physically based input data for simulations or even allow new simulation approaches that will not only be based on crack growth curves but also on micromechanic effects. Furthermore, findings can be used for purposeful material selection regarding variable amplitude loadings considering not only strength but also the hardening behaviour.

• DFG Programme: Research Grants