Experimental mesoscopic investigation of the local cyclic plasticity of a non-oriented electrical steel

Complementary mesoscopic investigations have been carried out on a notched specimen, made of Fe–3%Si steel and submitted to fatigue loading. Load control fatigue testing was conducted to ensure localised cyclic deformation and the resulting displacement fields were measured by means of Digital Image Correlation (DIC). Electron Channelling Contrast Imaging (ECCI) and Electron Back-Scatter Diffraction (EBSD) complete the DIC data by giving a different view of the local plasticity, as they image the dislocation structure and map their related characteristics, such as misorientation. Hence, two misorientation based criteria, namely Grain Reference Orientation Deviation (GROD) and Grain Orientation Spread (GOS) were considered for EBSD analysis. Based on a previous study, a − Δε_t abacus has been coupled with a − DIC abacus in order to evaluate locally the encountered strain field and link this latter to an equivalent total strain variation. The obtained results have been discussed from the evolution of the dislocations structures, as observed by ECCI. This combination of mesoscopic techniques clearly emphasizes the local deformation by means of both qualitative and quantitative measurements. On one hand, DIC and EBSD measurements indicate the presence of strain localisation which can be correlated to a certain level of cyclic strain amplitude. On the other hand, the variation of the strain levels and its distribution when moving away from the notch are confirmed by the dislocation structures evolution seen by ECCI. Such structures vary from cells, vein and channels and entangled dislocations regions as the analysed areas go away from the notch. At a local scale, the DIC measurements behave as numerous virtual extensometers while its combination with EBSD and ECCI data confirms the correlation with the accumulated cyclic strain.