| In order to effectively prevent the occurrence of safety accidents, safety hidden dangershould be found and eliminated before the accident. An important symbol of thedangerous state is yield or partial yield of the equipment and components. Traditionalnon-destructive testing (NDT) methods can only detect the macro defects, but theycan’t effectively detect the premature defects and the hidden discontinuous change.Therefore, the immediate demand in engineering is to find a new evaluation theory ofsafety hidden danger.Metal magnetic memory (MMM) technique is a new kind of NDT methods and hasbecome the unique practical method to diagnose premature defects. Many researchershave found the phenomenon that uneven plastic strain can cause distortion of MMMsignals, but there is a lack of direct evidence to prove it. Moreover,expansion law of the magnetic distortion behavior and its essence are still unknownby now during the whole period of yield, which is very importantto assess the yield state and to find potential safety hazard in time.Materials of Q235and20g steels were taken as the examined objects which havebeen widely used in engineering. Methods of strain measurement and macro-slideobservation were used for the first time in the study of MMM signals so as tocomplete the research on mechanical behaviors of both overall and local yield and toobtain the change law of MMM signals during the whole yield period and themechanism of MMM signals’ distortion. A defect-free500mm-long flat platespecimen was designed by considering unfavorable factors such as the limitations ofthe short sample ever used, the range of the testing machine, etc. In order to completelarge strain measurement during yield, special processes of pasting and curing wereused after strain gauges were pasted on the surface of every specimen. Moreover, aspecial curing loading device was designed. Quantitative research was done to therelation between MMM signals’distortion and the extent of yield so as to obtain thecritical magnetic intensity (Hp) derivative (dHp/dx) which can quantitative describewhether yield occurs. The results show that:The mechanical behavior of local yield has the following characteristics: the initialyield location is random and plastic flow is non-uniform in the process of yield. Theprocess of local yield includes three main stages: strain increases linearly, the suddenincrease stage and strain stable region. Every local yield point has the strain stableregion before yield end, but the length of this stage is different for different points.The length has relation to the yield order of local yield points. The values of steadystate strain of different points are different. The change laws of both the yield strain and macro-slip line are almost the same.The change laws of MMM signals of the two kinds of materials are almost the same:the curve of the initial MMM signal is basically a horizontal line and it changed intocurve after loading. The intersections of every curves and the horizontal line arealmost the same, but their angles are different. The distortion of MMM signals occursduring yield stage and the location of MMM signals’ distortion are different underdifferent yield period. MMM signals’ distortion disappears when yield end. The mainreason for the fluctuations of MMM signals are strain, the elastic strain causedby the normal fluctuations of MMM signals and uneven yield strain caused by MMMsignals’ distortion.The behavior and expansion laws of both MMM signals’ distortion and yield arebasically the same. Magnetic distortion is directly related to the extent of the yield, thehigher extent of the yield is, the more obvious magnetic distortion is. A concept ofcritical magnetic intensity (Hp) derivative (dHp/dx) of yield was proposed, the critical dHp/dxvalues of both Q235and20g base metal are4(A/m)/mm, yield occurs when it larger than4andyield doesn’t occur when it less4. |
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