Abnormal Twinning Behavior and Constitutive Modeling of a Fine-Grained Extruded Mg-8.0Al-0.1Mn-2.0Ca Alloy under High-Speed Impact along Various Directions

To understand the mechanical and twinning behaviors of a fine-grained extruded Mg-8.0Al-0.1Mn-2.0Ca alloy under high-speed impact, impact tests were carried out using a split Hopkinson pressure bar, and microstructures at strains of 0.05, 0.10 and 0.20 were obtained using a series of stop rings manufactured by high-strength steel. The stress response and twinning behavior are closely related to loading direction and applied strain rate. The true stress-true strain curves are s-shaped in extrusion direction (ED) and c-shaped in transverse direction (TD), showing apparent anisotropy, while the yield strength is insensitive to loading direction. Almost identical strain-rate sensitivity is demonstrated by the stress in ED and TD. Interestingly, de-twinning is apparent as the applied strain increases to 0.20, and it is enhanced with increasing the applied strain rate. In contrast, the twin density in ED samples is clearly higher than that in TD samples. By modifying the terms of strain hardening and strain rate hardening in the classical JC model, an optimized model is built, which can accurately predict the stress response behavior of the studied alloy under high-speed impact along ED and TD. The correlation coefficient (R) and average absolute relative error (AARE) are 98.63% and 0.0199 for ED, and 96.88% and 0.0202 for TD, respectively.     

Effects of cross-scale h-BN grains and orientation degree on the mechanical and thermal properties of BN-matrix textured ceramics

h-BN is a two-dimensional ceramic material with a lamellar structure, known for its typical orientation characteristics on mechanical and thermal properties. By optimizing the size and arrangement of h-BN grains in the matrix, the anisotropic characteristics of h-BN ceramics can be fully utilized to obtain ceramic materials with high thermal conductivity or high strength. In order to study the effect of grain orientation distribution on the mechanical and thermal properties of materials, the index of orientation distribution (IOP) was used to quantitatively characterize the orientation degree of h-BN grains and analyzed the effect of h-BN grain size on material properties. The results show when the initial h-BN size is 13.50 μm, the ceramic has the highest orientation degree/-507, and the mechanical and thermal properties show obvious anisotropy. While the related properties of BN-YAG ceramics varies significantly with the decrease of initial h-BN grain size.     

Numerical analysis of anisotropic stiffness and strength for geomaterials

In numerical modelling, selection of the constitutive model is a critical factor in predicting the actual response of a geomaterial. The use of oversimplified or inadequate models may not be sufficient to reproduce the actual geomaterial behaviour. That selection is especially relevant in the case of anisotropic rocks, and particularly for shales and slates, whose behaviour may be affected, e.g. well stability in geothermal or oil and gas production operations. In this paper, an alternative anisotropic constitutive model has been implemented in the finite element method software CODE_BRIGHT, which is able to account for the anisotropy of shales and slates in terms of both deformability and strength. For this purpose, a transversely isotropic version of the generalised Hooke's law is adopted to represent the stiffness anisotropy, while a nonuniform scaling of the stress tensor is introduced in the plastic model to represent the strength anisotropy. Furthermore, a detailed approach has been proposed to determine the model parameters based on the stress–strain results of laboratory tests. Moreover, numerical analyses are performed to model uniaxial and triaxial tests on Vaca Muerta shale, Bossier shale and slate from the northwest of Spain (NW Spain slate). The experimental data have been recovered from the literature in the case of the shale and, in the case of the slate, performed by the authors in terms of stress-strain curves and strengths. A good agreement can be generally observed between numerical and experimental results, hence showing the potential applicability of the approach to actual case studies. Therefore, the presented constitutive model may be a promising approach for analysing the anisotropic behaviour of rocks and its impact on well stability or other relevant geomechanical problems in anisotropic rocks.     

Effect of anisotropic transport properties of porous layers on the dynamic performance of proton exchange membrane fuel cell

To study the influence of anisotropic transport properties of porous layers (PLs) on the dynamic performance (DP) of fuel cells (FC), a numerical model was developed. The model uses self-compiled codes to couple the actual structure of PLs with its transport capability, while introducing a non-homogeneous model that reflects the effect of the actual agglomerate structure of the catalyst layer on the electrochemical reaction, based on the consideration of the internal heat transport and external heat exchange of FC, the three-phase transformation of water, the change of momentum, energy, species, and water transport with time, and finally realizes the transient changes of complete transport and reaction process inside the FC. Using the current density as the input load, the law of the anisotropic transport properties of PLs on the FC's DP was explored. The results show that among all properties, the proton conductivity has the most significant influence on DP indexes such as steady-state transition time, over/undershoot amplitude, the electrical conductivity and diffusivity have similar and significant effects, the thermal conductivity and permeability have minor and most negligible effects, respectively. When designing the material, structure and composition of the PLs, the focus should be on protons, electron conductivity and diffusivity.