Research On Deformation And Fracture Of Ultrathin Plate In Microscale Laser Peen Forming

Microscale laser peen forming (μLPF) is a novel plastic forming process which utilizes a high pressure plasma to deform the 3D profiles and it has great potentials for the manufacturing of MEMS parts. The high pressure plasma is created when transient, intensity laser pulse impinges on a black ablative layer painted on the surface of the target material.In this paper, three fracture modes were discovered duringμLPF processes, which were spallation 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 based on grain and grain boundary were employed to simulate the formation and propagation of the fractures inμLPF, 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. Through the independent developed mesh generation program MicroMesh, mesh of ultrathin plate with the hexagonal grain including two trapeziform elements was generated. The experimental and numerical simulation results show that, (1) 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; (2) With the increase of the duration of laser shock wave, the fracture mode transfers from the spallation at the center of the formed dome to tensile-tearing near the die entrance, and then to the shear fracture at the fringe of laser spot; (3) With the increase of the strength of forming materials, the fracture mode transfers from mode III to modeⅡ, and then to modeⅠ; (4) As the diameter of die increases, the fracture mode transfers from mode II to mode I.Dynamic plastic behavior of a circular plate inμLPF is very difficult to predict for its complicated boundary and loading conditions. In this paper, an analysis of small deflection dynamic plastic response of a circular plate inμLPF was preliminarily studied. Some simplifications were proposed that the target material was treated as a simply supported rigid-perfectly plastic plate, the spatial loading condition was simplified to be rectangular and rectangular-triangular pulse pressure distribution, and the temporal loading condition was regarded as triangular distribution. In addition, Tresca yield criterion was used as well as the associated flow rule to derive the analytical solution ofμLPF with small deflection. The final profile and deformation height of circular plate were obtained.