Air-Coupled Ultrasonic Lamb Wave in Situ Stress Measurement Method for Composite Plates

The rapid and accurate measurement of the stress state of composite plates is crucial for enabling health monitoring and evaluation in their application fields. To address the need for high-efficiency and in situ stress measurement of large component composite plates and the contradiction between the spatial resolution and accuracy, this article proposes an air-coupled ultrasonic Lamb wave stress measurement method based on the step-difference acoustic time difference. With the guarantee of stress measurement accuracy, the spatial resolution is enhanced from propagation distance to step-difference distance, which can be adjusted automatically and accurately through mechanical devices. To verify the effectiveness of the proposed method, seven uniaxial tensile specimens are obtained along the different fiber directions of 0°, 15°, 30°, 45°, 60°, 75°, and 90°, respectively. Utilize stress coefficient calibration results to realize the uniaxial stress measurement with a spatial resolution of 10 mm. The experimental results show that within the range of 0-100 MPa, the measurement errors are less than ±6.96 MPa, and the standard deviations of the repeatability measurement experiment do not exceed 10.84 MPa. The spatial resolution of stress measurement is enhanced from 150 to 10 mm, with an increase of 93.33%. The air-coupled ultrasonic Lamb wave in situ stress measurement method based on the step-difference acoustic time difference can provide a feasible technique for high-efficiency and in situ stress measurement of large component composite plates.