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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.
Highlights
The laser-based generation and detection of ultrasound waves have been widely used to characterize materials remotely and non-destructively
The transit time for the longitudinal acoustic (LA) wave propagation across sample plate is ∼ 300 ns, as defined by the time interval spanned between the trigger pulse and precursor (i.e. LA peak), while the shear arrival time, estimated to be ∼ 540 ns, is defined by the time interval spanned between the trigger pulse and the downward step (i.e. TA peak)
Experimental results were found to be in good agreement with our finite element model (FEM) based on fully coupled elastodynamic and thermal conduction governing equations implementing “effective heat capacity” method, optimized spatial discretization and time-stepping, and shear modulus vanishing in molten region
Summary
The laser-based generation and detection of ultrasound waves have been widely used to characterize materials remotely and non-destructively. We have examined epicentral waveform for laser ultrasound in the surface melting regime of tungsten without automated control of incident laser pulse energy[16] and performed numerical simulations based on finite element model (FEM) of epicentral displacement.[17,18,19] Time resolved spectral pyrometry was used to measure melting points of stainless steel and platinum using nanosecond laser heating.[20] There has been intense research devoted to measure melting points of refractory materials using laser pulses on the scale of milliseconds[21,22,23] and microseconds.[24]. Developed model shows that laser-induced surface melting and corresponding complete loss of rigidity (i.e. vanishing of shear modulus) within molten pool along with rapid radially spreading molten mass results in the shear wave arrival delay, which was found to be due to generation of surface and volumetric thermoelastic sources from the heated zone surrounding the molten pool
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