Non-contact measurements of residual stress distribution and grain size in titanium alloys with laser ultrasonic system

In this study, a laser ultrasonic method is proposed for nondestructive non-contact measurement of both the residual stress depth profile and grain size in Ti-6Al-4 V alloy processed by warm shot peening (WSP). The method utilizes a pulsed laser to generate a surface acoustic wave (SAW) along the cross-section of the sample, and a laser interferometer to detect SAW signals. Laser ultrasonic measurements were performed on the samples treated by WSP at different temperatures (32, 100, 200 and 300 °C). A theoretical model for calculating the residual stress profile and a model to correlate the center frequency shift and the mean grain size of the sample were presented. The results obtained by the laser ultrasonic method were compared with those obtained by X-ray stress measurement, center-hole drilling (CHD), and electron backscattering diffraction (EBSD). The proposed method can measure and locate the maximum compressive residual stress (CRS) in a peened surface layer with a high spatial resolution of approximately 30 µm, allowing for precise extraction of the residual stress profile. This method can also provide an estimation of the grain size through the same experiment. The developed method is promising for the quick and nondestructive measurement of the residual stress profiles and grain size of surface treated metallic materials.