Fatigue, which is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading, is a major cause of component failure in many industrial domains. The fatigue lifetime of defect-free materials is mainly controlled by crack initiation which arises from microstructural changes induced by dislocations nucleation and glide. Thus, the development of a fatigue damage indicator related to the dislocations density and their configuration is useful in order to predict crack initiation sites and strain localisation.
In the present work, the ability of SEM-EBSD to provide a relevant signature of the cyclic strain accommodation is evaluated among other techniques such as TEM. Indeed, whereas SEM-EBSD has shown a common interest when evaluating the local strains within monotonic loadings, there are still few applications regarding to cyclic loading and more precisely low cycle fatigue. TEM remains actually the tool of interest since it links the materials macroscopic behaviour to microstructure and more precisely to dislocations arrangements. However, this technique is destructive, time consuming and the obtained signature is more-or-less qualitative.
Thus, a SEM-EBSD analysis was conducted on two steels submitted to low cycle fatigue. Depending on the applied strain variations, different macroscopic regimes are observed. In addition, the local strain accommodation can be related to planar slip, cross-slip or even multiple slip, which lead to various dislocations arrangements for which SEM-EBSD provides qualitative or quantitative signatures.
The first investigated steel exhibits mainly multiple slip. Dislocations arrange themselves to misoriented cells which are observed as well-defined grain boundaries. In addition, local strains are evaluated through misorientation criteria such as kernel based as well as grain based approaches. The second material, an electrical steel, exhibits planar slip at low strain variation and a mixture of vein-channel and wall-channel structure at high strain variation. Due to the weak lattice distortion, misorientation based criteria are only valid to distinguish the dislocations structures transitions. Nevertheless, EBSD shows a clear interest when performing electron channelling contrast imaging under controlled diffraction (cECCI) to highlight the encountered dislocations structures.
From those two applications, SEM-EBSD appears of great value to highlight the dislocations structure (though cECCI) and clearly gives a quantitative signature, when for example, dislocations leads to local misorientation changes.