The effect of surface roughness on laser-induced stress wave propagation

We investigate laser-induced acoustic wave propagation through smooth and roughened titanium-coated glass substrates. Acoustic waves are generated in a controlled manner via the laser spallation technique. Surface displacements are measured during stress wave loading by the alignment of a Michelson-type interferometer. A reflective coverslip panel facilitates capture of surface displacements during loading of as-received smooth and roughened specimens. Through interferometric experiments, we extract the substrate stress profile at each laser fluence (energy per area). The shape and amplitude of the substrate stress profile are analyzed at each laser fluence. Peak substrate stress is averaged and compared between smooth specimens with the reflective panel and rough specimens with the reflective panel. The reflective panel is necessary because the surface roughness of the rough specimens precludes in situ interferometry. Through these experiments, we determine that the surface roughness employed has no significant effect on substrate stress propagation and smooth substrates are an appropriate surrogate to determine stress wave loading amplitude of roughened surfaces less than 1.2 μm average roughness (Ra). No significant difference was observed when comparing the average peak amplitude and loading slope in the stress wave profile for the smooth and rough configurations at each fluence.