Ultrasonic Stress Measurement: Application to Welded Joints

When a material is under mechanical load, the stresses modify the velocity of acoustic waves. This consists in acoustoelastic effect. This property can be exploited for stress measurement in the material itself when the stress concerns the surface of the material, or in the bulk material, as in bolts. Residual stress measurement. In the welding case, ultrasonic velocity varies not only by the acoustoelastic effect, but also in a smaller part with the microstructure variations produced in melted zone and heat affected zone. This may lead to overestimation of stress level. The work performed gives an answer to this measurement restriction. To achieve this aim, the velocity was measured on unstressed samples extracted from the different zones of the weld. The experimental study performed on P460 HLE and P265 X welded sheets lead to the validation of the method. This is encouraging for the use of the method in some cases for stress measurement of pressure equipment. This study was lead in collaboration between Cetim and Ecole des mines de Douai. 1. Theoretical Basis of Ultrasonic Stress Measurement The ultrasonic method of stress measurement is based on the application of the theoretical results developed in the articles (1) and (2). The elastic waves which are propagated in isotropic solids are characterized by their propagation velocity in material (Fig. 1). The velocity variation versus stress is given for the waves of compression (longitudinal) and transverse (shear) modes by linear equations. The following expression gives the velocity dependence for the longitudinal mode: