Non-destructive ultrasonic measurement of residual stress in metallic materials

The relationship between the applied stress σ, and the measured shear wave birefringence B was measured in the laboratory for several low alloy steel. The birefringence response was measured with a dual polarization electromagnetic transducer comparing the time of flight for the two polarizations, in the rolling direction and the transverse direction. This stress-birefringence relationship showed a linear dependence and can be described with the relation σ = K * B + σ0. The slope of this relationship when applying a tensile stress in the transverse direction of the sample, K, was measured to − 1280 MPa/B(%) and was independent of material chemistry, whereas the position/offset of the curves, i.e., σ0 , varied in between steel grades, assumed to be due to microstructural texture. A non-contact EMAT probe has been installed in a levelling mill at SSAB Special Steel in Oxelösund, Sweden, to measure the residual stress dependence on the levelling process, in daily production of steel plates. The EMAT probe has been provided by Nordinkraft AG, Remchingen, Germany. This single EMAT probe has been applied along the centerline of plates. Measurements have been made on more than 2000 plates. The responsive or reactive residual stress was successfully measured and calculated for all plates using the above stress-birefringence relationship, showing typical residual stress difference values on the production plates after levelling in the range of less than ± 100 MPa.