Study On Metal Magnetic Memory Testing Of The Crack Initiation And Propagation Of The Ferromagnetic Material

Metal magnetic memory (MMM) is a new non-destructive testing technique. Itutilizes the interaction of magnetic field and the inner stress of the material to detectthe material defects which are mainly characterized by stress concentration. However,there exit many interference factors that can affect the surface magnetic field of thematerials in the actual structure, making it difficult to quantify the connection betweenmagnetic memory signal and the inner damage of the materials. To investigate theproblems in the MMM testing technique at present, the paper implements a series ofexperiments to analyze the changing regularities of MMM signals of a kind of carbonstructural steel-Q235B in the static tensile and fatigue testing, and evaluates thedegree of fatigue damage on the structure quantitatively.Firstly, a static tensile test with the tensile load increasing linearly was carriedout on notched specimen, and the MMM of the fixed position on the surface ofspecimen was detected in the process of tensile test. Meanwhile, the residual stressesin the specimen were analyzed using the finite element method. The results indicatethat: The MMM signals fit the residual stresses calculated well. During the elasticdeformation stage, the MMM signals have a positive linear relationship with thetensile stresses，while，their gradients have a negative linear relationship with thetensile stresses. The growth tendency decreases in the following plastic deformationstage and the magnetic memory signals and their gradients reach the saturation pointswhen the tensile loads are up to110MPa (0.47ReL).Secondly, two groups of low cycle fatigue tests were carried out on notchedQ235B test specimen and welded butt joints with different load waveforms and stressratios, and the MMM signals of the fixed measuring channels on the surface ofspecimen was detected every certain times of cycle, in order to study the influences ofload waveforms and stress ratios on MMM signals. The results demonstrate: duringthe crack initiation stage, the MMM signals detected in the notched tip do not emergezero points and the strength gradients have no peaks. Those phenomena occur in thecrack extension stage. The amplitudes of MMM signals and peak values of theirgradients increase with the increase of the damage degree. Load waveform and stressratio can affect the stress concentration in the specimen and the detected MMM signals. MMM signal and its feature vectors can be used to predict the fractureposition and characterize the damage degree of specimen. The existence of weld jointswill shorten the fatigue life cycle of the specimen and enhance the amplitude ofMMM signals.