Time of flight methods for thickness measurement of aluminium plane plate using the thermoelastic regime in laser UT

Md. Abdur Rahim, Yoshio Arai · 10.63414/jeas.v8.n2.2024.43

Abstract

Conventional LUT methods often struggle with significant noise in low-intensity waveforms, which compromises measurement accuracy. Instead of generation pulse, reflection pulses which are affected by low velocity waves after back reflection are normally used to calculate the time of flights. Accuracy obtained about 10%, 0.100, 0.050 mm etc. This paper presents a combined numerical/experimental effort to accurately measure thickness in case of plane aluminium plate experimentally using Laser Ultrasonic Technique (LUT) and find suitable time of flight methods and investigate the wave propagation characteristics using simulations. Lower generation laser power is used therefore no visible ablation spot on the material surface which is highly expected, and the phenomena is referred to as thermoelastic regime. Comparatively higher amplitudes of waveforms are produced which is easy for obtaining precise peaks with curve fitting methods which includes also the generation pulse. Time dependent multiple waveforms data are averaged to get waveform with less noise and stable pulse from which better method of time of flight is evaluated. Variation of time of flights occur which is revealed by simulation results. The inclusion of generation pulse for calculating the time of flight using -6dB method would be the better method for thermoelastic regime.

Conclusion

The proposed ultrasonic method using average waveforms gives reasonable values of time of flight in thermoelastic regime. The time of flights of -6dB at rising pulse between generation to 1st reflection and between the reflection pulses are the better methods for calculating the time of flight with more stability in thermoelastic regime. Strong edge generated shear wave generated with velocity higher than longitudinal wave in the thermoelastic regime. Therefore, wave propagation velocity is higher in case of -6dB method when calculated from the particle wave velocity waveforms from simulation results. The findings offer advancements in non-contact, non-destructive testing, with potential applications across various industries that rely on precise material characterization. The methods can be used to materials other than aluminium even for multilayer thickness measurement, modern composite materials etc.

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