Measuring Spallation Strength of Epoxy by Laser Spallation Technique

Abstract

The laser spallation technique in combination with Michelson interferometry is employed to estimate the spallation strength of epoxy film. The epoxy layer of thickness 160 μm is deposited onto the microscopic glass (MG) substrate. The specimens are subjected to high amplitude short duration stress waves, developed by the pulsed laser ablation of sacrificial absorbing layer, deposited at the back surface of the substrate. The complex interaction of rarefaction waves with the mode converted tensile pulse from the free surface of the film develops high magnitude tensile region. For sufficiently high stresses spallation is observed in the top part of the epoxy layer. Profilometric analysis confirms spallation in the epoxy layer while the MG/epoxy interface remains intact. Interferometric measurements exhibit distinctly spaced fringe patterns. The temporal span of the wave arrivals confirms that the fringes correspond to the longitudinal and the rarefaction waves. Based on the space-time wave travel analysis the spallation depth is predicted which is in excellent agreement with the profilometric observations. By employing dynamic wave propagation analysis in combination with the interferometric data, the spallation strength of epoxy is estimated to be 260 ± 15 MPa.