Fatigue Damade Evaluating And Fatigue Life Predicting Based On Magnetic Memory Technology

Fatigue damage of metal material, the key factor leading to catastrophic failure, is the major concern in engineering practice. Today, how to evaluate fatigue damage and fatigue life accurately has been an important and urgent task in order to avoid catastrophic accidents.The evolution of fatigue damage, from crack initiation to crack growth, is an accumulative process. With the evolution of fatigue damage, magnetic property of ferromagnetic material has been varying that magnetic resistance increases and magnetic conductivity decreases near the damage zones. Based on this magnetic property variation, a new nondestructive testing (NDT) technology called magnetic metal memory testing (MMMT), can not only detect plastic deformation and macroscopic crack, but also inspect stress concentration zones at early damage stage.MMM technology overcomes the shortage of traditional early damage testing NDT methods, such as ultrasonic inspection, eddy current testing and X-ray testing. So, MMM technology has potentials of fatigue damage evaluation and remaining life prediction.However, fatigue damage evaluation not only demands advanced NDT methods, but also depends on microscopic observing means. Fatigue damage and fatigue life couldn't be evaluated accurately until macroscopic features of fatigue damage were studied by combining damage evolution micro-mechanism.This paper aims at applying MMMT to fatigue damage evaluation and life prediction. By combining macroscopic and microscopic view, a new fatigue damage measurement technique which integrates CCD microscopic video observing into macroscopic MMM testing, called fatigue damage MMM——Micro_ Observing in situ method, is first successfully introduced to investigate MMM attributing regularity of fatigue damage evolution micro-mechanism. A new crack initiation life model is obtained. The correlation between crack growth and MMM effect is investigated including short crack and long crack. Based on experimental data, total fatigue life prediction is made. The contents of this paper are as follows.Based on magneto-mechanical effect deriving from thermodynamical equilibrium equation, magnetic memory effect of single fatigue cycle is studied. Further investigation of multi-cycle magnetic memory effect is made through spontaneous nagnetization theory, magnetic domain theory and ferromagnetic energy balance theory. At the same time, the principle of magnetic memory testing is the key of its application, and magnetic dipole theory is cited to prove magnetic memory testing principle.Using fatigue damage MMM——Micro-Observing in situ method, three-point bending fatigue crack initiation experiments of Steel 45 and Q235, are made under different load levels and stress ratios. By means of FEM, the relation between stress distribution and MMM signal is investigated from three aspects: longitudinal direction, transverse direction and three-dimension of the crack tip zone. Combining dislocation theory and magnetoelasticity coupling theory, MMM attributing regularity of fatigue crack initiation micro-mechanism is given, and characteristic signal corresponding to critical fatigue crack initiation is analyzed.On the basis of phase change theory during crack nucleation and magnetic domain spontaneous nagnetization, energy fluctuation of ferromagnetic material complies with the law of Gibbs free energy minimum. From the view of energy variation before and after crack nucleation, a new crack initiation life model is presented and verified based on MMM parameter, microstructure parameter and loading condition.Stress intensity factor range and crack growth rate are two important parameters to evaluate crack growth. Due to the specificity of physically-short crack growth, the LEFM long crack method can't be used to study physically-short crack growth. Therefore, by analyzing crack clogging effect and by adopting MMM signal Hp (y) and effective stress intensity factor rangeΔK eff, this paper unifies physically-short crack and long crack successfully. Through three-point bending fatigue crack propagation experiments of Steel 45 and Q235, MMM attributing regularity of fatigue crack growth is also given, and the correlation between effective stress intensity factor rangeΔK eff, crack growth rate da / dNand MMM signal Hp (y) is investigated in detail.On the basis of least square fit method, the model of the correlation between crack growth rate da / dNand MMM signal Hp (y) is built which overlays physically-short crack and long crack. The results of proving experiment show that the agreement of predicting values and testing values is found to be good, and especially the agreement of long crack steady growth stage is better than that of the physically-short crack growth stage and the long crack unsteady growth stage. Furthermore, the influence of average load, stress ratio, thermal refine on the correlation model of da / dN~ Hp (y) is studied.Considering the dynamic nonlinearity of fatigue damage evolution, it is very difficult to build precise mathematic model for fatigue life. In this paper, the newest neural network, called Dynamic Multi-resolution Analysis Wavelet Neural Network (DMRA-WNN), is applied to built total fatigue life prediction models for Steel 45and Steel Q235 which are based on the theory of physically-short crack growth specificity and the analysis of crack clogging effect. The parameters of the DMRA-WNN MMM models are normalized Hp (y) values corresponding to different load levels and stress ratios. With the advantages of Neural Network, Wavelet Transform and Multi-resolution Analysis, the DMRA-WNN avoids blind construct design of Neural Network and the dimensional catastrophe of Static Wavelet Neural Network. The predicting results show that the DMRA-WNN MMM model has a prospect of engineering practice.