Examination of the epicentral waveform for laser ultrasound in the melting regime

S J Reese; Z N Utegulov; F Farzbod; R S Schley; D H Hurley

doi:10.1016/j.ultras.2012.11.007

Examination of the epicentral waveform for laser ultrasound in the melting regime

Ultrasonics. 2013 Mar;53(3):799-802. doi: 10.1016/j.ultras.2012.11.007. Epub 2012 Nov 28.

Authors

S J Reese¹, Z N Utegulov, F Farzbod, R S Schley, D H Hurley

Affiliation

¹ Idaho National Laboratory, Department of Materials Science and Engineering, Idaho Falls, ID 83415-2209, USA.

PMID: 23259982
DOI: 10.1016/j.ultras.2012.11.007

Abstract

A laser ultrasonic source just below the ablation regime is examined by recording an epicentral waveform in a high purity tungsten sample. Using pulse energy as a parameter, a slight delay in the shear wave arrival time is observed upon transition to the melting regime. This phenomenon is attributed to a change in character of the ultrasonic source. In the thermoelastic regime, shear waves are generated by mode conversion at the sample surface of longitudinal waves emanating from subsurface sources. Just above the melting threshold, a molten pool forms in the center of the generation volume. Shear waves are not supported by the molten pool. As a result, shear waves generated from off-axis thermoelastic sources are weighted more heavily. This results in a delay of the shear wave arrival time.