Numerical Simulation of Defect Detectability in Non-Isotropic Welded Joints

Mathematical models are helping to optimise the welding process. Nevertheless it is necessary to obtain a feed back by quality control, even if the optimised process of welding has lead to a clean and defect free weld itself. By nondestructive testing ofweldedjoints with ultrasound it is assumed that the material is nearly homogeneous and that the longitudinal and shear wave propagation will not be disturbed on the way from the probe to the weld and back to the probe again. In austenitic welds where the propagation direction of the waves is changed as a function of the elastic constants and the grain orientation, this homogeneity and isotropy cannot be assumed. Defects in the seam of the weld or cracks starting from the root of the weld can be easily missed or are buried in the noise. In order to predict the wave propagation direction and the distortion of the sound field as a function of propagation distance and the interaction of the sound field with typical weld defects, a computer code ‘3D-CAD-RAYTRACE’ has been developed which runs on Windows NT on a PC. It allows the real component and the real shape of the weld to be modelled and the elastic constants and the grain orientation of the weld to be entered in three dimensions. Defects of different shapes, sizes or orientations can be placed into the weld to simulate lack of side wall fusion, slag inclusions or inner surface connected cracks. The ultrasonic probe is positioned virtually on the surface of the pipe containing a circumferential weld and scanned along a given path. Longitudinal, SV and SH wave propagation show different spreading effects. The simulation has been applied to a focusing phased array probe assuming a lack of side wall fusion defect and to a 3D defect/probe scenario where the probe has been tilted towards the scan direction by 12°.