Abstract:
Nanosecond laser pulse-induced melting thresholds in refractory (Nb, Mo, Ta and W)
metals are measured using detected laser-generated acoustic shear waves. Obtained
melting threshold values were found to be scaled with corresponding melting
point temperatures of investigated materials displaying dissimilar shearing behavior. The experiments were conducted with motorized control of the incident laser
pulse energies with small and uniform energy increments to reach high measurement accuracy and real-time monitoring of the epicentral acoustic waveforms from
the opposite side of irradiated sample plates. Measured results were found to be
in good agreement with numerical finite element model solving coupled elastodynamic and thermal conduction governing equations on structured quadrilateral
mesh. Solid-melt phase transition was handled by means of apparent heat capacity method. The onset of melting was attributed to vanished shear modulus and
rapid radial molten pool propagation within laser-heated metal leading to preferential generation of transverse acoustic waves from sources surrounding the molten
mass resulting in the delay of shear wave transit times. Developed laser-based
technique aims for applications involving remote examination of rapid melting processes of materials present in harsh environment (e.g. spent nuclear fuels) with high
spatio-temporal resolution.
