Effect of stress triaxiality on micro-mechanisms of void coalesence and micro-fracture ductility of materials

The micro-mechanisms of void coalescence at different stress triaxiality levels were investigated employing axisymmetrical smooth and notched tension specimens of spheroidized 37Mn5 steel. It is observed that voids coalesce mainly by internal necking of the ligaments between voids at lower triaxial stress levels and by lateral growth of voids at higher triaxial stress levels. The critical void growth at failure and the 3-D dimension of dimples on the fracture surfaces were measured for different specimens. It is revealed that the ratio of depth to planar radius of dimple, related to the critical aspect ratio of voids, decreases monotonously with increased stress triaxiality. A new expression for micro-fracture ductility of materials was given and proven to be in better accordance with macro-fracture ductility measured experimentally at different stress triaxiality.