Research On Prediction Method Of Low-cycle Fatigue Life Of Metal Materials Based On Crystal Plasticity Analysis

Fatigue of metal materials has always been an important issue in the engineering field,because it is a key factor affecting the safe service of structures and components.The existing methods for evaluating the fatigue life of materials are based on experimental experience and lack of in-depth understanding of their mechanisms.This study attempts to combine the crystal plasticity theory that can reflect the meso-deformation mechanism of the metal with the material model that can reflect the polycrystalline structure,with reference to experiments,to simulate the strain fatigue process of the material.This research can not only reflect the macro-mechanical response of the experimental material,but it can also give an estimate of the inhomogeneity of the internal meso-deformation of the material.Only the stable hysteresis curves with different amplitudes and the fatigue life of a certain amplitude need to be obtained from the test can achieve the prediction of fatigue life of materials before experiments.The main research studies of this paper are:1.Complete the strain fatigue experiment and meso-structure observation of pure copper,and analyze the fatigue life law by traditional methods.A series of strain fatigue experiments were performed on T2 pure copper with FCC lattice structure,and its hysteretic behavior was analyzed.The fatigue life of pure copper was evaluated by Manson-Coffin formula and Morrow formula,there the relevant parameters of the formula could be obtained.Samples with observable microstructures were prepared,and they were observed through metallographic experiments under different strain cycles.On this basis,the inhomogeneous evolution of the density of the slip zone on the surface of the specimen was studied,and it was found that the slip of the material became more and more inhomogeneous with the increase of the cycle.It is the discovery that the plastic deformation at the meso-scale of the material tends to be inhomogeneous with the cycle,lays the experimental foundation for further theoretical analysis.2.Carry out numerical simulation of strain fatigue combining RVE model and crystal plasticity analysis,study the quantitative parameters of material deformation inhomogeneity,and use it to predict fatigue life.The cyclic hysteresis curve of pure copper was obtained through experiments,then the crystal plastic constitutive model parameters could be calibrated.After that,the entire fatigue process of pure copper was numerically simulated.The rationality and validity that the standard deviation of the longitudinal strain in the tensile direction and the mean value of the first principal strain in the representative volume element(RVE)was utilized as the characterization parameters for measuring the meso-deformation inhomogeneity of the material were discussed.The law of these two parameters increasing with the cycle number was analyzed,and they all fluctuated with loading and unloading,but each time they experienced a small increase in value.It is further confirmed that these two fatigue indicator parameters(FIPs)can be used to predict the low-cycle fatigue life of polycrystalline copper,and the results are within a reasonable error range.The obvious feature of this method of predicting the fatigue life of materials is that it only requires the stable hysteresis curve of the material and the measured fatigue life at a single amplitude.The stable hysteresis curve is used to calibrate the crystal plastic constitutive model parameters,and the measured fatigue life of a single amplitude is used to determine the critical value of the FIPs.3.Investigate the impact of different statistical scale on the measurement results of deformation inhomogeneity in the RVE.The study found that if different statistical gauges were selected in RVE,and the results were different.It is proved that the value of the meso-deformation inhomogeneity of the material is related to the measurement gauge selected.The smaller the scale,the more obvious the meso-deformation inhomogeneity.Studies have pointed out that due to the lack of a sufficiently small scale distribution measurement method;it is very difficult to quantitatively measure the inhomogeneity of meso-deformation experimentally under current conditions.4.Complete the strain fatigue experiment of BCC material HRB400 steel,use the characterization parameters of the meso-deformation inhomogeneity to predict the life,and propose the improved FIPs.The basic mechanical properties test and strain fatigue test of HRB400 steel were performed to analyze the hysteretic behavior of the material,and the life law was analyzed using the Manson-Coffin formula and Morrow formula.Based on the standard deviation of longitunal strain and the mean value of the first principal strain,three improved FIPs are proposed: the standard deviation of the dot product of mesoscopic longitudinal stress and strain,the product of the macro stress peak and the standard deviation of the mesoscopic longitudinal strain in RVE,the product of the macro stress ratio and the standard deviation of the mesoscopic longitudinal strain in RVE.The results of the new FIPs were validated with the test of HRB400 steel and pure copper,which confirmed their applicability and effectiveness.