NONLINEAR MODEL OF FLAW DETECTION IN STEEL PIPES BY MAGNETIC-FLUX LEAKAGE

A numerical model of crack detection by magnetic flux leakage that tak es into account the saturation of the induction flux density with the applied field is developed. The model contains no free parameters and is validated with detailed measurements of material properties and mag netic fields reported by Forster. it is then used to discuss the depen dence of signals on the tube wall thickness, crack depth and width, an d the strength of the applied field. An estimate of the noise level is used to analyse the signal-to-noise ratio and calculate the optimal a pplied field for crack detection. Difficulties arising in internal cra ck detection of shallow cracks in thick-walled pipes are also discusse d.