Effect of surface characteristics on strain distribution in air- bending

Abstract. This study is a continuation of previous studies; therefore, the idea is to determine the influence of surface roughness and hardness on the bendability of a UHSS grade, and it is investigated with 3-point bending tests, utilizing Digital Image Correlation (DIC) for measuring the strain distributions on the outer curvature. Investigated bending samples of a 4 mm thick commercial bainitic-martensitic steel sheet were tested in different surface conditions: (1) as- rolled, (2) dry electropolished, (3) ultrasonic burnishing and (4) a combination of dry electropolishing and ultrasonic burnishing. Ultrasonic burnishing increased the surface hardness and decreased the surface roughness. Bending results showed that smoother surface did not lead to improved bending angles and higher hardness in the subsurface decreased the bending capacity, i.e., reduced the maximum bending angle and critical strain.