A prototype method to evaluate the inner wall defects of ferromagnetic materials based on a two-stage magnetic combined detection

•A two-stage magnetic combined detection method to evaluate the inner wall defect.•Rapidly determine if there is an inner wall defect by metal magnetic memory (MMM).•Accurately quantify the size of inner wall defect by magnetic needle probe (MNP).•Hysteresis characteristic parameters of MNP are sensitive to local defect. As the ferromagnetic materials have been more and more widely used in engineering, the detection requirements of the inner wall defects on structural components such as pressure vessels and pipes become urgent. Under the effect of high temperature, high pressure and corrosive media, the inner wall defects of ferromagnetic materials including crack, pore and corrosion tend to appear. In order to improve the detection efficiency, it is wise to evaluate the inner wall defects from the outside of ferromagnetic materials in a nondestructive way. Considering the rapid detection capability of metal magnetic memory (MMM) and the high sensitivity of magnetic needle probe (MNP), we combine these two magnetic nondestructive testing (NDT) techniques together and use them at two different detection stages, respectively, to help to better evaluate the inner wall defects of ferromagnetic materials. During the first stage of MMM detection, the weak magnetic signals measured from the outside of samples show that both the amplitude of MMM signal tangential component HP(x) and the gradient of MMM signal normal component HP(y) change obviously at the location of inner wall defect. However, they cannot quantify the size of inner wall defect at the backside of samples due to the serious fluctuation of signal characteristics under the interference of environment. Next, during the second stage of MNP detection, the hysteresis loop obtained by MNP can be used to quantitative evaluation because the remanence Br and the coercivity Hc decrease sharply near the inner wall defect. Furthermore, the overall average values of remanence Br¯ and coercivity Hc¯ are proposed as the characteristic thresholds to accurately determine the location and the size of inner wall defect and the evaluation accuracy can reach up to 90%. In this paper, the two-stage MMM and MNP combined detection can provide a convenient and accurate prototype method to evaluate the inner wall defects of ferromagnetic materials from the outside.