Detecting stress concentration, especially critical stress state leading to structure damage or failure, is one of the most important tasks of equipment diagnosis. Metal magnetic memory technique needs further research to evaluate stress concentration quantitatively due to ambiguous physical mechanism, though it has potential to detect early defects in ferromagnetic materials. Mild Q235 steel defective specimens in demagnetization state were loaded in tension up to visible necking, with magnetic memory signals measurement made at increasing stress levels. Magnetic signals varied greatly under first several loadings and subsequently tended to stability in the elastic region, which showed that the magnetization always approaches the anhysteretic magnetization curve and was explained by the theory of magnetomechanical effect. In the plastic stage, an abnormal wave occurred in the stress concentration zone and its height value was sensitive to plastic deformation levels and dependent on the distance between the probe and defect, in accordance with the simulation results based on the magnetic dipole model. Different magnetic signal characteristics in the elastic-plastic region indicate that the magnetic memory technique can identify macroyielding and early damage, which is of profound significance for ensuring safe operation of equipment in service.
In order to eliminate noise interference of metal magnetic memory signal in early diagnosis of stress concentration zones and metal defects, the empirical mode decomposition method combined with the magnetic field gradient characteristic was proposed. A compressive force periodically acting upon a casing pipe led to appreciable deformation, and magnetic signals were measured by a magnetic indicator TSC-1M-4. The raw magnetic memory signal was first decomposed into different intrinsic mode functions and a residue, and the magnetic field gradient distribution of the subsequent reconstructed signal was obtained. The experimental results show that the gradient around 350 mm represents the maximum value ignoring the marginal effect, and there is a good correlation between the real maximum field gradient and the stress concentration zone. The wavelet transform associated with envelop analysis also exhibits this gradient characteristic, indicating that the proposed method is effective for early identifying critical zones.
In order to propel the development of metal magnetic memory(MMM) technique in fatigue damage detection,the Jiles-Atherton model(J-A model) was modified to describe MMM mechanism in elastic stress stage.A series of rotating bending fatigue experiments were conducted to study the stress-magnetization relationship and verify the correctness of modified J-A model.In MMM detection,the magnetization of material irreversibly approaches to the local equilibrium state M0 instead of global equilibrium state Man under cyclic stress,and the M0-σ curves are loops around the Man-σ curve.The modified J-A model is constructed by replacing Man in J-A model with M0,and it can describe the magnetomechanical effect well at low external magnetic field.In the rotating bending fatigue experiments,the MMM field distribution in normal direction around cylinder specimen is similar to the stress distribution,and the calculation result of model coincides with experiment result after some necessary modifications.The MMM field variation with time at a certain point in fatigue process is divided into three stages with the variation of stable stress-stain hysteresis loop,and the calculation results of model can explain not only the three stages of MMM field changes,but also the different change laws when the applied magnetic field and initial magnetic field are different.The MMM field distribution in normal direction along specimen axis reflects stress concentration effect at artificial defect,and the magnetic signal fluctuates around the defect at late fatigue stage.The calculation results coincide with the initial MMM principle and can explain signal fluctuates around the defect.The modified J-A model can explain experiment results well,and it is fit for MMM field characterization.
To avoid the serious accidents caused by the failure fastening bolts on reciprocating compressor cylinder cover,a new nondestructive testing(NDT) technology,metal magnetic memory(MMM) testing,was applied to safety evaluating and failure analyzing for the fastening bolts.Based on the dynamic stress calculation of the failure bolts,MMM testing was carried out at workshop.Given are the MMM stress distribution characteristics of the failure bolts and fracture faces.It has been found that the MMM signal variation amplitude of the crack transition zone in the fracture surface is minimal,that of the crack initiation zone is in the middle,and that of the tear fracture zone is maximal.The failure reasons were analyzed with MMM effect.The results of the metallographic examination showed that the validity and feasibility of MMM testing and failure analysis.This means MMM technology is a new,fast and validity method of failure analysis.
