The thermoelastic surface strip source for laser-generated ultrasound

Abstract

The vertical ultrasonic surface displacement generated by a laser-induced, thermoelelastic, rectangular surface strip source is formulated. The main features of this source, ultrasonic rise times comparable to those of the generating light pulse derivatives, and a large amplitude, double-pulsed waveform, are advantageous for ultrasonic spectral-and time-domain measurements for nondestructive material evaluation. The analysis shows that, for laterally symmetric sources in the thermoelastic regime, only one tangential thermoelastic stress component contributes to the vertical displacement. Therefore, the strip source is equivalent to two, tangential line forces acting outward at the strip's front and back edges. The leading-edge rise time of the signal is virtually independent of the lateral extent of the source, which mostly affects the trailing portions of the ultrasonic pulses. Consequently, a particularly simple expression, which compares favorably with experimental results, is obtained for short strips (subtending small angles at the observation point). In conjunction with this formulation, the thermoelastic strip source is an important tool for quantitative, laser-based, ultrasonic nondestructive material evaluation.

Compact Laser Driver for Ultrasonic Arbitrary Position and Width Pulse Sequences Generation

“…Power density used in high power ultrasound usually is close to the ablation threshold, 10 5 -10 7 W/cm 2 [9], [10], [31]- [45]. The peak power has to be from tens of kW (42 kW in [53]), to hundreds of kW ( [74], 833 kW in [43], 555 kW in [41], 300 kW in [19], 384 kW in [21]) or even MW (1.25 MW in [75], 1.7 MW in [36]). Usually researchers aim to use large (mm order) laser spot because this increases longitudinal waves energy directed normal to the surface, but power densities remain in MW/cm 2 range.…”

“…It should be noted that asymmetric beam profile might influence the produced acoustic output directivity: such a beam profile is equivalent to line source excitation [22], [74], [104]- [106]. If the surface is not constrained, the Rayleigh surface waves will dominate.…”

Subsurface, thermoelastic line source excitation of a transversely isotropic half space

“…One of the most frequently studied is the laser line source in which light is focused to a line-like geometry in/on a material and acts as a thermoelastic source in regions where optical absorption occurs [6][7][8][9][10][11][12].…”

Numerical simulation of the reflected acoustic wave components in the near field of surface defects

“…Laser ultrasonics generated great hope within the nondestructive evaluation (NDE) field due to its non-contact features in the measuring process and its ability of broadband signal generation [1][2][3][4].…”

“…Traditional techniques for the detection of surfacebreaking cracks rely on monitoring the reflections (pulseecho) or the changes in the amplitude of the transmission (pitch-catch) of the given incident signal caused by the presence of a fatigue crack. Since the reflected acoustic signal 3 Author to whom any correspondence should be addressed. may form the basis of a remote crack sizing technique in metals, the characteristic waveform and the forming process of the reflected surface wave components need to be fully understood.…”