Theoretical study on the influence of the overlay on the pressure of laser shock wave in photomechanics

The interaction between laser and material can induce strong shock wave, so a new subject of science and manufacture technology—photomechanics is being founded on the basis of the mechanical effect of laser induced shock wave which is applied in areas such as laser shock shaping and laser spallation measurement. One of the key problems is to enhance the pressure peak value of shock wave effectively, for which the overlay is generally introduced in laser shock processing to improve the shock effect. So it has important theoretical significance and application value to study the influence of overlays exerting on the shock wave pressure. With respect to rigid overlays, flexible overlays and fluid overlays respectively, we analyze the influence from the aspects of the state of surface of discontinuity of laser induced shock wave, the diffusion of gasified material and plasma and the reflection of shock wave. It is found that overlays do not raise the pressure peak value directly when the pulse width is shorter than the time for the shock wave to cross the gasified material. On the contrary, shot of laser pulse with width longer than the time of shock wave crossing the gasified material heightens the shock effects by heightening the pressure by way of restricting the diffusion of gasified material and plasma and prolonging the acting time of useful pressure through multiple reflection of the shock wave betiseen the surfaces of the workpiece and the overlay. The rigid overlay can raise the pressure peak value of shock wave greatly. But the advantage of the flexible and fluid overlays is the adaptation to the form of the surface of non-planar workpieces.