Torsional Fatigue Life Prediction of 30CrMnSiNi2A Based on Meso-Inhomogeneous Deformation

In this paper, torsional fatigue failure of 30CrMnSiNi2A steel which exhibited non-Masing behavior was studied under different constant shear strain amplitudes, using thin-walled tubular specimens. The relationship between shear fatigue and the evolution of meso-deformation inhomogeneity and the prediction method of the torsional fatigue life curve were investigated. Shear fatigue of the material under constant amplitude was researched by numerical simulation with reference to tests, by using crystal plasticity of polycrystalline representative volume element (RVE) as the material model. Considering the non-Masing behavior of material, when determining the parameter values of the crystal plasticity model the correlation between these parameters and strain amplitude was taken into account. The meso-deformation inhomogeneity with increments in the number of cycles was characterized by using the statistical shear strain standard deviation of RVE as the basic parameter. Considering the effect of strain amplitude on fatigue damage, ratio cycle peak stress/yield stress was taken as the weight to measure the torsional fatigue damage and an improved fatigue indicator parameter (FIP) to measure the inhomogeneous deformation of the material was proposed. The torsional fatigue life curve of 30CrMnSiNi2A steel was predicted by the critical value of the FIP and then the result was confirmed.

Predicting Plastic Flow Behaviour, Failure Mechanisms and Severe Deformation Localisation of Dual-Phase Steel using RVE Simulation

In this work, the von Mises stress and plastic strain distribution of Ferrite-Martensite–Dual-Phase (FMDP) steels are predicted at various stages of deformation. The failure modes and volume fraction effect are identified based on Representative Volume Element (RVE). FMDP steel consists of a typical ferrite-matrix phase, in which martensite-islands are dispersed. Recently FMDP steels are increasingly used to the various car parts in demand. 2D-RVEs are also utilised to predict the orientations effect of the martensite phase in the FMDP steels. Based on the position of the element, the boundary conditions (BC) are given in the RVE of FMDP steel microstructures. The failure modes are examined in the form of severe plastic strain localisation. While the distribution of islands in the microstructure varies, as a result, the deformation inhomogeneity increases with a rise of martensite fraction. The results of numerical computation and the trend of experimental failure shown in the literature are compared. This is signifying that the overall macro-behaviour of FMDP steel, as a consequence of stress-strain partitioning and influence of martensite-island volume fractions (MVFs), can be predicted by the finite element (FE) based 2D-RVE modelling.

Investigation of Deformation Inhomogeneity and Low-Cycle Fatigue of a Polycrystalline Material

Considering the relationship between inhomogeneous plastic deformation and fatigue damage, deformation inhomogeneity evolution and fatigue failure of superalloy GH4169 under temperature 500 °C and macro tension compression cyclic loading are studied, by using crystal plasticity calculation associated with polycrystalline representative Voronoi volume element (RVE). Different statistical standard deviation and differential entropy of meso strain are used to measure the inhomogeneity of deformation, and the relationship between the inhomogeneity and strain cycle is explored by cyclic numerical simulation. It is found from the research that the standard deviations of each component of the strain tensor at the cyclic peak increase monotonically with the cyclic loading, and they are similar to each other. The differential entropy of each component of the strain tensor also increases with the number of cycles, and the law is similar. On this basis, the critical values determined by statistical standard deviations of the strain components and the equivalent strain, and that by differential entropy of strain components, are, respectively, used as fatigue criteria, then predict the fatigue–life curves of the material. The predictions are verified with reference to the measured results, and their deviations are proved to be in a reasonable range.

Prediction of Strain Fatigue Life of HRB400 Steel Based on Meso-Deformation Inhomogeneity

The relationship between strain fatigue life and evolution of meso-deformation inhomogeneity was studied, through the cyclic process of numerical simulation of crystal plasticity compared with the fatigue test of steel hot-rolled ribbed-steel bar 400 (HRB400). The statistical characterization parameters at grain level, including the standard deviation of the dot product of longitudinal stress and strain, the product of the macro stress and the standard deviation of the longitudinal strain, and the product of the macro stress ratio and the standard deviation of the longitudinal strain, were proposed and respectively applied to measure the meso-deformation inhomogeneity of materials. These parameters take the effect of peak stress into account, distinct from the pure strain statistical parameters. The numerical results demonstrate that the low-cycle fatigue life curves of materials are predictable using the new parameters as FIPs (fatigue indicator parameters), and the predictions are more rational than by utilizing the FIPs without considering the peak stress effect.

Investigation on the Strain Distribution in Tube High-Pressure Shearing

The Finite-element method (FEM) and experiments were used to investigate the geometric factors and material parameter on the strain distribution during tube high-pressure shearing (t-HPS). The results show that t-HPS could be realized successfully either by pressurizing on both ends of the tube, or by pressurizing using the wedge effect; and in both cases, the “dead metal zone” could be found at both ends of the tube. The grain size distribution from the experiment confirmed this strain distribution feature. In the case of t-HPS pressurized using the wedge effect, the half cone angle has little effect on the strain distribution. Decreasing the strain-hardening exponent leads to increased deformation inhomogeneity in both the ideal t-HPS described by theoretical equations and the close to practical t-HPS described by FEM. This feature of t-HPS stands out from other SPD processes like HPT, and makes practical t-HPS behavior more predictable using the analytical formation than any other SPD processes, and places it an advantageous position in understanding the basics of deformation physics through the coupling between practical experiments and theoretical approaches.

Dynamic Uniform Deformation for Electromagnetic Uniaxial Tension

To compare with quasi-static uniaxial tensioning, researchers designed an electromagnetic uniaxial tension method using a runway coil. However, the requirements to obtain a uniformly deformed sample and the ways the stress changes on the sample using a runway coil have not been studied in the past. In this study, a three-dimensional (3D) sequential coupling method was developed to analyze the factors affecting on-sheet deformation inhomogeneity under electromagnetic uniaxial tension. Two main process parameters, comprising the die type and the relative position of the coil and sheet, were evaluated. Under the optimal parameters, the experiment and simulation both obtained uniformly deformed samples with different discharge conditions, and the simulation method had a high accuracy in modeling the deformation process. The stress state of the sample is approximately unidirectional tensile stress before 240 μs. After 240 μs, the three main stresses showed significant oscillations.