Effect of surface roughness on strain distribution during bending

The limited formability of ultra-high strength steels (UHSS) poses some challenges for the bending process in the form of strain localisation, surface defects and pseudo-polygonal “nut-like” shape of the bend. Bendability is well known to be affected by surface quality, and especially shot blasting. Therefore, in this study, the effect of surface roughness on bendability of UHSS grade is investigated with 3-point bending tests, utilising Digital Image Correlation (DIC) for measuring the strain distributions on the outer curvature. Investigated bending samples of 4 mm thick commercial martensitic were tested in different surface conditions: As-rolled (with scale), “lightly” shot blasted (SB I), “roughly” shot blasted (SB II), dry electropolished (As-rolled P) and a combination of “rough” shot blasting and dry electropolishing (SB II P). Shot blasting increased the surface roughness and subsurface hardness. Utilizing a commercial dry electropolishing process reduced the surface roughness, although this also had major effect on the hardness. Bending results showed that coarser surface roughness decreased the bending capacity, i.e. reduced maximum bending angle and critical strain. A strong correlation between surface roughness (with Rv: the maximum valley depth below the mean surface) and critical bending angle was found, likewise with the subsurface hardness and critical bending angle.