A study on influence of plastic deformation on the global conductivity and permeability of carbon steel

Non-destructive evaluation (NDE) techniques are indispensable for inspection of damage in structures of carbon steel, which is widely used in many industries. In this paper, experimental and numerical studies were conducted to investigate the electromagnetic property variations of carbon steel Q195 due to plastic strain by using the 4-probe potential drop method and ECT aiming at NDE applications. A series of specimens were fabricated and uniform global plastic strains of different levels were introduced to the specimens by tensile testing. 4-probe potential drop measurements were carried out to obtain the conductivity information at first. It is found that the macro conductivity is not affected significantly by the plastic strain once taking into account the influence of the cross-sectional area reduction of the specimens. Combining the results of numerical analysis using a code of Ar formulation with the measured ECT signals, the permeability of specimens with various plastic damages was evaluated through inverse analysis. It is proved that the ECT signal change due to plastic damages is mainly caused by the variation of permeability, which gives a good possibility to evaluate the plastic damage with a magnetic NDE method.