Relations of Microstructural Attributes and Strength-Ductility of Zirconium Alloys with Hydrides

As the first safety barrier of nuclear reactors,zirconium alloy cladding tubes have attracted extensive attention because of its good mechanical properties.The strength and ductility of zirconium alloy are of great significance to the service process of cladding tubes,while brittle hydrides precipitate and thus deteriorate the overall performance.Based on the cohesive finite element method,the effects of cohesive strength,interfacial characteristics,and hydrides geometric characteristics on the strength and ductility of two-phase material(zirconium alloy with hydrides)are numerically simulated.The results show that the fracture behavior is significantly affected by the cohesive strength and that the overall strength and ductility are sensitive to the cohesive strength of the zirconium alloy.Furthermore,the interface is revealed to have prominent effects on the overall fracture behavior.When the cohesive strength and fracture energy of the interface are higher than those of the hydride phase,fracture initiates in the hydrides,which is consistent with the experimental phenomena.In addition,it is found that the number density and arrangement of hydrides play important roles in the overall strength and ductility.Our simulation provides theoretical support for the performance analysis of hydrogenated zirconium alloys during nuclear reactor operation.

Influence of inter-grain cementation stiffness on the effective elastic properties of porous Bentheim sandstone

Effective elastic properties of porous media are known to be significantly influenced by porosity.In this paper,we investigated the influence of another critical factor,the inter-grain cementation stiffness,on the effective elastic properties of a granular porous rock(Bentheim sandstone)using an advanced numerical workflow with realistic rock microstructure and a theoretical model.First,the disparity between the experimentally tested elastic properties of Bentheim sandstone and the effective elastic properties predicted by empirical equations was analysed.Then,a micro-computed tomography(CT)-scan based approach was implemented with digital imaging software AVIZO to construct the 3D(three-dimensional)realistic microstructure of Bentheim sandstone.The microstructural model was imported to a mechanics solver based on the 3D finite element model with inter-grain boundaries modelled by cohesive elements.Loading simulations were run to test the effective elastic properties for different shear and normal intergrain cementation stiffness.Finally,a relation between the macroscale Young’s modulus and inter-grain cementation stiffness was derived with a theoretical model which can also account for porosity explicitly.Both the numerical and theoretical results indicate the influence of the inter-grain cementation stiffness,on the effective elastic properties is significant for porous sandstone.The calibrated normal and shear stiffnesses at the inter-grain boundaries are 1.2×10^(5) and 4×10^(4) GPa/m,respectively.

Mechanical Properties of Boron Carbide/Reducedgraphene-oxide Composites Ceramics

Reduced graphene oxide(rGO)enhanced B_(4)C ceramics was prepared by SPS sintering,the enhancement effect of rGO on the microstructure and mechanical properties of composites was studied through experiments and numerical simulation.The results show that the composite with 2wt%rGO has the best comprehensive mechanical properties.Compared with pure boron carbide,vickers hardness and bending strength are increased by 4.8%and 21.96%,respectively.The fracture toughness is improved by 25.71%.The microstructure observation shows that the improvement of mechanical properties is mainly attributed to the pullout and bridge mechanism of rGO and the crack deflection.Based on the cohesive force finite element method,the dynamic crack growth process of composites was simulated.The energy dissipation of B_(4)C/rGO multiphase ceramics during crack propagation was calculated and compared with that of pure boron carbide ceramics.The results show that the fracture energy dissipation can be effectively increased by adding graphene.

Bioinspired design of hybrid composite materials

Mimicking the natural design motifs of structural biological materials is a promising approach to achieve a unique combination of strength and toughness for engineering materials.In this study,we proposed a 2D computational model,which is a two-hierarchy hybrid composite inspired by the ultrastructural features of bone.The model is composed of alternating parallel array of two subunits(A&B)mimicking‘mineralized collagen fibril’and‘extrafibrillar matrix’of bone at ultrastructural level.The subunit-A is formed by short stiff platelets embedded within a soft matrix.The subunit-B consists of randomly distributed stiff grains bonded by a thin layer of tough adhesive phase.To assess the performance of the bioinspired design,a conventional unidirectional long-fiber composite made with the same amount of hard and soft phases was studied.The finite element simulation results indicated that the toughness,strength and elastic modulus of the bioinspired composite was 312%,83%,and 55%of that of the conventional composite,respectively.The toughness improvement was attributed to the prevalent energy-dissipating damage of adhesive phase in subunit-B and crack-bridging by subunit-A,the two major toughening mechanisms in the model.This study exemplifies some insights into natural design of materials to gain better material performance.

The microbuckling failure of Dyneema~? composites under compression

Two grades of Dyneema composite laminates with the commercial designations of HB26 and HB50 were cut into blocks with or without an edge crack and compressed in the lon- gitudinal fiber direction. The cracked and uncracked specimens show similar compressive responses including failure pattern and failure load. The two grades of Dyneema composites exhibits different failure modes： a diffuse, sinusoidal buckling pattern for Dyneema HB50 due to its weak matrix constituent and a kink band for Dyneema~ HB26 due to its relatively stronger matrix constituent. An effective finite element model is used to simulate the collapse of Dyneema composites, and the sensitivity of laminate compressive responses to the overall effective shear modulus, interlaminar shear strength, thickness and imperfection angle are investigated. The change of failure mode from kink band to sinusoidal buckling pattern by decreasing the interlaminar shear strength is validated by the finite element analyses.