Quantitative nondestructive evaluation of plastic deformation in carbon steel based on electromagnetic methods

Plastic deformation may occur in a mechanical structure during its manufacturing and service process, and may cause serious problem in the structural integrity. Therefore, a reliable pre-service or in service quantitative non-destructive evaluation (NDE) of plastic deformation is very important especially for a structure after suffering giant load such as a large earthquake. However, there is still no satisfactory method being established for the quantitative NDE of plastic deformation in key structures such as those of a nuclear power plant. For this purpose, three electromagnetic NDE methods, i.e., Magnetic Barkhausen Noise (MBN), Magnetic Incremental Permeability (MIP) and Magnetic Flux Leakage (MFL) method are studied via experiments in this paper to investigate their feasibility for evaluation of plastic deformation in carbon steel SS400. A special testing system integrated these three electromagnetic NDE methods is established to measure the magnetic property of test-pieces with different plastic deformation, which was introduced by a tensile testing machine. It is found that the measurement signals of all these three methods have clear correlation with the plastic strains and show coincident tendency, which reveals the validity of these methods for the quantitative evaluation of plastic deformation. Among these methods, the MFL signals are of higher stability and repeatability but of relative low spatial resolution. The MBN method can give better resolution but of bigger standard deviation and is also not valid to evaluate a plastic strain of large scale. On the other hand, the MIP signals are more sensitive to the liftoff of sensor and to the remanent magnetization status, i.e. of more noise. Therefore,to measure the feature parameters of these three methods at the same time with an integrated testing system and to evaluate the plastic strain through signal fusion may give a better detectability and evaluation precision.