Research on ultrasonic longitudinal critical refraction wave stress measurement method based on the influence of separation temperature and coupling layer thickness

The accurate measurement of stress in steel structures is critical for the stable operation of high-end equipment. Currently, the coupling mechanism of temperature and the coupling layer in ultrasonic stress measurement remains unclear. This paper proposes a novel method for ultrasonic longitudinal critically refracted wave (LCR) stress precision measurement based on acoustic time difference and waveform index. By precisely controlling temperature and coupling layer thickness, the individual and combined effects of these two factors on the acoustic time difference and waveform index of LCR waves were investigated. Mathematical relationships between the acoustic time difference and waveform index, and the temperature and coupling layer thickness were established, with coefficients of determination reaching 0.999. This method enables the accurate separation and calculation of temperature and coupling layer thickness values. This lays a theoretical foundation for the accurate separation of the effects of temperature and coupling layer thickness on stress measurement. Subsequently, by precisely applying stress ranging from 50 MPa to 400 MPa in stages to both sides of the specimen, a mathematical model relating acoustic time difference to temperature, coupling layer thickness, and stress was established. This allowed for the separation of measurement errors induced by temperature and coupling layer thickness, thereby achieving precise stress measurement. The relative error between the stress values obtained by this method and the actual stress values is less than 3%. This study provides a theoretical foundation and a high-precision solution for the stress measurement of key equipment.