Search

High entropy alloy has attracted much attention in the field of national defense due to their excellent low-temperature dynamic mechanical properties. Taking TiZrHfCu0.5 high entropy alloy as the research object, the compressive properties of the specimens under quasi-static and dynamic (strain rate range 600s−1-2600s−1and temperature range −60 °C–20 °C) conditions are systematically tested. Based on the static/dynamic stress-strain experimental results, the parameters of the original Johnson-Cook constitutive model are determined by fitting. On this basis, a modified Johnson-Cook constitutive model considering the coupling effects of strain, strain rate and temperature is proposed and its parameters are determined. The dynamic compression process of the specimens under different strain rates and temperatures is numerically simulated by ABAQUS finite element software, and the accuracy of the modified Johnson-Cook constitutive model to predict the dynamic compression behavior of TiZrHfCu0.5 high entropy alloy is verified. The experimental and numerical simulation results show that the TiZrHfCu0.5 high entropy alloy exhibits significant strain rate hardening effect and excellent low-temperature mechanical properties during dynamic compression. The ultimate stress can reach 1.79 GPa at −20 °C and strain rate of 2600 s−1. The predicted curves of the modified Johnson-cook constitutive model are in good agreement with the experimental results at low temperature and high strain rate. The modified Johnson-Cook constitutive model is embedded in the finite element software, which effectively improves the reliability of the numerical simulation of the compression performance of TiZrHfCu0.5 high entropy alloy at high strain rate and low temperature. The relative error between the predicted results of the modified Johnson-Cook constitutive model and the experimental results is greatly reduced.

A new constructive method, called optimized advance front method (OAFM), for ellipse packing is proposed. The OAFM allows particle rotation at several angles and movement along a local advance front. Combined with the ellipse approximated by four connected arcs and a series of sequential coordinate transformations, the OAFM generates dense ellipse packing with any imposed size, aspect ratio, and orientation distribution at a fastpacking speed, and the generated ellipse packing can satisfy an imposed spatial arrangement. Based on the approximation of ellipses by multicircles, Laguerre Voronoi Tessellation method constructs initial cells. The initial cells are then merged to create the convex polygon with the same size, aspect ratio, orientation, and location as that of the obtained ellipse. Three examples of ellipse packing and convex polygon demonstrate that the convex polygon generated can be statistically equivalent with the real material and satisfy an imposed spatial arrangement.

AbstractDue to depositional processes and other factors, such as shear banding and the intermediate principal stress, soils and rocks are typically cross-anisotropic. Strength anisotropy should be described by the proper anisotropic strength criterion. This paper presents an anisotropic unified nonlinear strength criterion to accurately characterize the strength anisotropy of geomaterials based on a unified nonlinear strength criterion (UNS). The anisotropic state variable in terms of a joint invariant and a unit stress tensor is brought into the UNS criterion to characterize the effect of the anisotropy. The new criterion had been illustrated in both the meridian and octahedral planes, leading it the oversimplification to describe strength anisotropy behaviors of soils and rocks under different loading conditions. Moreover, only conventional triaxial test is employed to calibrate the parameters of the new criterion. The results demonstrated the ability of the anisotropic strength criterion to be applied to a wide range of soils and rocks for true triaxial tests along with the complex stress condition in torsion shear tests. By comparing the predictions to test results reported in the literature, the new criterion proposed in this paper is able to provide excellent traits of the strength anisotropy for various geomaterials.

High‐entropy alloys (HEAs) are gradually replacing armor‐piercing core materials such as depleted uranium and tungsten alloys in high strain rate, high temperature, and high pressure service environments due to their excellent mechanical properties (high strength, high strain rate strengthening effect, and strong shear self‐sharpening characteristics). The dynamic compression performance of TiZrHf‐based HEAs with different elements, different strain rates, and multiple loadings is tested by the self‐constructed separate Hopkinson compression bar experimental system. The results show that the TiZrHfCu0.3alloy has the largest elastic modulus and the strongest deformation resistance. TiZrHfAl0.3and TiZrHfCu0.3show a certain strain rate effect. The elastic modulus increases with the increase of loading times. The ultimate strength of TiZrHfAl0.3remains unchanged after multiple loading, while the ultimate strength of TiZrHfCu0.3and TiZrHfNi0.3decreases gradually. The effects of trace element types, strain rates, and multiple loading times on the dynamic compressive mechanical properties of TiZrHfAl0.3, TiZrHfCu0.3, and TiZrHfNi0.3HEAs are studied. The alloys show a certain strain rate effect. The elastic modulus increases with the increase of loading times; the ultimate strength of TiZrHfCu0.3and TiZrHfNi0.3decreases gradually.