Dynamic fractures in microscale laser peen forming

Microscale laser peen forming (µLPF) is a novel plastic forming process which utilizes laser induced shock waves to deform the 3D profiles. In this paper, three fracture modes were discovered during µLPF processes, which were fracture at the centre of formed dome (mode I), tensile-tearing near the die entrance (mode II), and shear fracture at the fringe of laser spot (mode III). The dynamic explicit finite element method together with the method of constrained-nodes-failure were employed to simulate the formation and propagation of the fractures in µLPF, in which four nodes of four shell elements were tied together instead of being merged in common method, and would split at the onset of failure. The experimental and numerical simulation results show that, (1) With the increase of the strength of forming materials, the fracture mode transfers from mode III to mode II, and then to mode I; (2) With the increased shock wave pressure, the location of fractures occurred is from the centre of the formed dome to the die entrance, and then to the fringe of the laser spot; (3) As the diameter of die increases, the fracture mode transfers from mode II to mode I.