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

► The change in the laser ultrasonic source character below the ablation threshold was investigated. ► A delay in the shear wave arrival time is observed upon transition to the melting regime. ► A simple model was presented that accounts for the delay in the shear arrival time due to the formation of a molten pool. 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.