Structural health monitoring technologies have provided extensive methods to sense the stress of steel structures. However, monitored stress is a relative value rather than an absolute value in the structure’s current state. Among all the stress measurement methods, ultrasonic methods have shown great promise. The shear-wave amplitude spectrum and phase spectrum contain stress information along the propagation path. In this study, the influence of uniaxial stress on the amplitude and phase spectra of a shear wave propagating in steel members was investigated. Furthermore, the shear-wave amplitude spectrum and phase spectrum were compared in terms of characteristic frequency (CF) collection, parametric calibration, and absolute stress measurement principles. Specifically, the theoretical expressions of the shear-wave amplitude and phase spectra were derived. Three steel members were used to investigate the effect of the uniaxial stress on the shear-wave amplitude and phase spectra. CFs were extracted and used to calibrate the parameters in the stress measurement formula. A linear relationship was established between the inverse of the CF and its corresponding stress value. The test results show that both the shear-wave amplitude and phase spectra can be used to evaluate uniaxial stress in structural steel members.

The stress fields of steel members in service are unevenly distributed under complicated loads. The existing single path ultrasonic (SPU) sensors can only measure stress in a single measurement point and direction. Multiple operations of the SPU sensors are required to measure stress at different measuring points and directions. The approach has low measurement efficiency and increases the measurement deviations caused by multiple operations. In this study, a modified method using an ultrasonic array sensor to measure the non-uniform absolute stress field of steel members was proposed. The relationship between the acoustic time differences of the longitudinal critically refracted (LCR) waves and the plane stress field was revealed in this method. The acoustoelastic equations of the three directional acoustic time differences and the plane principal stresses were derived. An innovative ultrasonic array non-uniform absolute stress field measurement scheme was designed, which integrated an ultrasonic array sensor capable of transmitting and receiving five LCR waves. Nineteen sets of acoustic time differences were measured by arranging the array sensor in the 0°, 45°, and 90° directions. Finally, the acoustic time differences were decoupled simultaneously to solve the acoustoelastic equation, thus achieving a non-uniform absolute stress field measurement of the steel member in service. The verification experiment indicates that the relative deviations of measurement stresses between the array sensor and strain gauges are less than 10%. Furthermore, the measurement accuracy and efficiency of the array sensor are higher than those of the SPU-LCR wave method and SPU-shear wave method, validating that the array sensor can rapidly and accurately measure the non-uniform absolute stress field of steel members.