Research On Corrosion-stress-magnetic Coupling Model And Stress Corrosion Of Metal Based On Magnetic Memory Method

Most modern industrial equipment is susceptible to corrosion due to the external environment in service.The defects caused by corrosion will seriously threaten the safety of engineering structures,among which the stress corrosion defects are the most harmful and easy to cause sudden and catastrophic accidents.The Metal Magnetic Memory(MMM)method has been developed as a new passive nondestructive testing(NDT)technique without an excitation source,and its testing process is relatively fast and convenient.It can be applied to simultaneously detect multiple damage forms such as stress concentration,cracks,etc.However,there is a lack of systematic theoretical and experimental research on corrosion defect detection based on this method.Based on analysis of the corrosion mechanism,this thesis establishes a Corrosion-Stress-Magnetic coupling model.Experiments and finite element analysis based on the proposed model are conducted to study the characterizations and variation rules of magnetic flux leakage(MFL)signals under whole and local stress-free corrosion(stress-free corrosion after loading-unloading,no stress exists during corrosion)and stress corrosion(stress and corrosion coexist).The contents and main conclusions are summarized as follows:(1)It is verified by the chemical post-treatment experiment that the influence of corrosion layer on the magnetization of a specimen can be ignored.A uniform plane current model is developed by analyzing the double-layer model at the electrode/electrolyte interface.The spatial expression of the magnetic field generated by corrosion current is obtained.Finally,a Corrosion-Stress-Magnetic coupling model is established.(2)The magnetic leakage field of whole uniform corrosion specimens under stressfree corrosion and stress corrosion is experimentally and numerically studied.It is verified that the proposed model is correct and effective in predicting the effect of stress-free corrosion and stress corrosion on the magnetization of a specimen.The influence of stress-free corrosion and stress corrosion on MFL signals is obtained.(3)The magnetic leakage field of local corrosion specimens under stress-free corrosion and stress corrosion is experimentally and numerically studied.It is verified that the proposed model is effective in predicting the MFL signals produced by stressfree corrosion defect(under stress-free corrosion)and stress corrosion defect(under stress corrosion).The influence of stress-free corrosion defect and stress corrosion defect on MFL signals is obtained.(4)The MFL signals of stress-free corrosion defect,stress corrosion defect,and stress defect(only the action of load)are compared,and different signal characteristics are obtained,which effectively distinguish different types of defects.The characteristic parameters of MFL signals of stress-free corrosion defect and stress corrosion defect are extracted and the unified optimal evaluation parameters are obtained.