Laser-ultrasound evaluation of longitudinal stress in rails

New laser-ultrasonic method for evaluation of residual stress in rails was developed. It is based on laser generation of sub-surface ultrasonic wave and its registration with piezoelectric transducer. The values of residual stress in rails are determined by measuring the velocity of the ultrasonic pulse. The laser-ultrasonic transducer which induces and registers sub-surface ultrasonic waves was constructed. We theoretically analyzed relation between ultrasound wave propagation velocity and the value of residual stress, and used these results to suggest and implement experimentally the method of absolute calibration based on formation of stress in a sample with the hydrotension device UNG-75. Calibration allowed us to justify the linear dependency between relative velocity change (in the range of percent deciles) and applied tension/compression stress in the range of reversible deformations.