Three-dimensional stress measurement for structural steel plates using ultrasonic T-waves and P-waves

•A three-dimensional stress measurement method for steel plates is proposed.•The analytical expressions of stress-acoustic relations are deduced.•The acoustoelastic coefficients are calibrated with unidirectional compression test.•The AECs depend on the component, ultrasonic frequency and echo number n.•The center frequency of the transducer is selected as the optimal fitting frequency.