Fatigue Damage Evaluation At Mesoscale Based On Metal Magnetic Memory Technology

The physical short crack stage accounts for a much higher percentage of the overall fatigue life at the fine scale than the long crack stage.The growth rate is faster than that of long crack,which already poses a potential hazard to the material.Traditional non-destructive testing techniques cannot effectively detect early hidden damage such as physical short cracks,and it is difficult to observe microscopic field in engineering practice,so the introduction of metal magnetic memory technology(MMMT)that can detect early hidden damage and abnormal stress concentration is explored for physical short crack expansion at the fine scale.In this paper,we analyze the fatigue short crack mechanism of ferromagnetic materials,combine in-situ observation of short cracks with MMMT,reveal the magnetic characteristics of the physical short crack expansion process,study the correlation between magnetic characteristics parameters and short crack expansion rate,establish the magnetic memory characterization of short crack expansion rate and remaining life prediction model,and establish the physical short crack expansion criterion based on magnetic characteristics parameters based on the criterion establishment method of fuzzy probabilistic fracture mechanics.In order to study the variation of magnetic property parameters during the evolution of physical short cracks,this paper uses Q235 steel three-point bending specimens as the test material and uses TSC-5M-32 MMMT detector with metallographic microscope to observe and record the crack extension evolution process and magnetic property parameters in situ,and obtain the characterization law of magnetic property parameters on physical short crack extension at the fine scale.Experimental results show that in the crack nucleation stage,non-extending cracks appear,short cracks compete with each other,Hp(x)jumps dramatically from-29.84 A/m to-55.09 A/m,and Hp(y)changes polarity from-29.84 A/m to 2A/m.In the grain boundary hindrance stage,the short crack extension rate is almost zero,and Hp(x)and Hp(y)remain constant at-59 A/m and 18 A/m,respectively.In the breakthrough grain boundary stage,most short cracks stop expanding and the extended short crack breaks through the grain boundary,the Hp(x)curve rotates clockwise and the Hp(y)curve moves upward.In the breakthrough grain boundary stage,most short cracks stop expanding and the extended short crack breaks through the grain boundary,the Hp(x)curve rotates anticlockwise and the Hp(y)curve moves upward.In the crack extension stage,the short cracks converge to form the main crack,which advances along the zigzag path,accompanied by the"Y"bifurcation,and the Hp(x)and Hp(y)are linearly varying.To effectively characterize the physical short crack expansion specificity,it is not rigorous to select the traditional magnetic memory signals such as Hp(x),K(x),K(y),etc.only by subjective selection.For this purpose,20 magnetic characteristic parameters in single and synthetic directions such asΔHp,M,K_a,etc.are extracted,and Pearson and Spearman correlation coefficients are introduced to synthetically select the magnetic characteristics with the highest correlation to the physical short crack expansion rate.The results show that the maximum relative errors of the expansion rate and remaining life models are 6.475%and11%,respectively.In order to establish the threshold of physical short crack and long crack based on magnetic property parameters,the mathematical relationship between magnetic property parameters and mechanical parameters is established by taking the energy change in the crystal as the entry point and using energy as a bridge by considering the effect of grain boundary obstruction.Based on the established mathematical relationship,the physical short crack extension specificity is analyzed,and the stress intensity factor amplitude thresholdΔKth of fine mechanics is introduced as the physical short crack threshold parameter,and the probability density distribution of the force-magnetic parameter is studied and K-S test is performed based on the fuzzy probabilistic fracture mechanics criterion establishment method.The fuzzy fracture mechanics criterion is based on the magnetic property parameters to establish the physical short crack and long crack critical point criterion,which provides new ideas for damage evaluation.