Calibration-Free Uniaxial Stress Measurement in Metallic Materials Using Orthogonal SV-EMATs

Electromagnetic acoustic transducers (EMATs) have significant potential for measuring stress in metallic materials due to their noncontact capability. Traditional stress measurement methods based on acoustoelastic theory require calibration, and their accuracy depends on precisely determining the relationship between acoustic time of flight (ToF) and stress. However, this relationship is difficult to establish directly because it is influenced by the specimen's density, second-order elastic constants (SOECs), and third-order elastic constants (TOECs). In practical applications, extensive calibration is necessary to ensure measurement accuracy, and recalibration is required whenever the specimen's material changes. To address this limitation, this study proposed a calibration-free uniaxial stress measurement method for metallic materials using orthogonal shear-vertical (SV) EMATs. In this method, two EMATs generate SV waves propagating in perpendicular directions, while two additional EMATs receive these waves. Theoretical analysis demonstrated that uniaxial stress measurements depend solely on the ToF of the two orthogonal SV waves and are independent of factors such as SOECs and TOECs. The feasibility and effectiveness of the proposed method were validated through finite element analysis and experimental testing. Experimental results showed that the maximum relative error for calibration-free uniaxial stress measurements of aluminum alloy specimens was 9.29%.