Failure analysis by advanced magnetic techniques

Abstract

Now-a-days attempts are being made to use the inherent ferromagnetic properties to characterize steels. Out of all the available methods, mag¬netic hysteresis loop (MHL) and micromagnetic Barkhausen emissions (MBE) are relatively newer and very promising techniques in the area of component integrity and failure analysis. The MHL parameter coercivity (He) decreases with the tensile stress, grain size and increases with the hardness of the materials while the reverse effect is observed in case of remanence (B.). The micromagnetic Barkhausen emissions (MBE) which are originated due to irreversible domain wall motion in the presence of alternating magnetic field are also found to be sensitive to stress and mi¬crostructure of the materials. The present paper deals with the two case studies where MHL and MBE parameters were used to analyse failure of components. The cases were (i) failure of in-service flange used in fossil power plant and (ii) premature failure of shot peened suspension spring used in automobile. In the former case MBE and MHL parameters were used to find the extent of creep damage at various positions of the compo¬nent while in the latter case MBE parameters were used to evaluate the residual stresses at different positions of the spring component.