Papers by Keyword: Strain Gradient Plasticity

Abstract: Particle-reinforced trimodal composites, which typically consist of three constituent phases where at least one phase forms itself a composite with ceramic particles, feature a length-scale-dependent strengthening. Joshi and Ramesh [1] presented a secant Mori–Tanaka method to describe the strength of trimodal composites. They, however, seem to have used excessively high strength by mismatch in coefficients of thermal expansion between ultrafine-grained 5083 aluminum and B₄C particles. Since the ultrafine-grained 5083 aluminum features high strength, an additional enhancement of strength by this kind of thermoelastic mismatch between matrix and particle may hardly occur. Using finite-element methods with strain-gradient plasticity, this problem is examined computationally and an enhanced procedure of strength prediction for trimodal composites is suggested.

Abstract: This work focuses on the application of a higher-order gradient-dependent plasticity-damage model for microstructural modeling of dual-phase (DP) steels. Damage evolution is governed by the evolution of a nonlocal plasticity measure which is a function of the local equivalent plastic strain rate and its corresponding first-order gradient. Different RVEs of DP microstructures are virtually generated and simulated in order to predict the macroscopic mechanical response. Size effects and additional hardening due to evolution of geometrically necessary dislocations are predicted.

Abstract: Several methodologies have been proposed to model the effect of length scale parameters in constitutive equations. Most of them are developed based on strain gradient theory. The main restriction is contributed to the large scale of imposed plastic deformation in comparison with implementation of length scale parameters. Also comparing to the scale of dislocation movement and hardening mechanisms, the plastic deformation in microstructures and nanostructure materials is sufficiently large that finite plasticity theory could be well justified. Therefore, the main intention of this paper is to develop strain gradient deformation with the corporation of finite plastic and dislocation theory as physically based attribution in constitutive equations. This procedure is accomplished with intrinsic length scale relation, which is dedicated to develop phenomenological of plasticity laws for microstructures in finite plasticity. Finally, the result of new theory is indicated for microstructures and its predictable results are discussed for nanostructure materials.

Abstract: A finite element model is developed in this paper for simulating cyclic deformation in ultra-fined polycrystalline metals. The grain material is characterized by conventional theory of mechanism-based strain gradient plasticity (CMSG). A cohesive interface model was used to simulate the initiation and propagation of intergranular cracks. The simulation results show that inhomogeneous plastic deformation induces high strain gradient effects and severely plastic hardening in the grain interior, and the intergranular crack has a significant influence on the overall mechanical properties of ultra-fined polycrystalline metals subjected to cyclic loading.

Abstract: The conventional finite element method (FEM) cannot investigate the size effect on thermal residual stresses induced by the sintering process in micro multilayer ceramic capacitors (MLCCs). In this paper, a FE two-dimensional single layer model is developed for investigation of the effect of the micro scale on prediction of the residual thermal stresses in MLCCs. In this FE single layer model, the strain gradient effect is considered. It is found that with decreasing single layer thickness, the shear stress increases significantly in the ceramic layer near the electrode tip, which might cause cracking of the ceramic layer near the electrode tip. The numerical results also show that the predictions of the thermal residual stresses in MLCCs are strongly dependent on the micro scale. The residual thermal stresses induced by the sintering process exhibit strong size effects and, therefore, the strain gradient effect should be taken into account in the design and evaluation of MLCC devices.

Abstract: A comprehensive numerical study of tensile deformation on electro-deposited Cu with nano-scale growth twins is presented using a conventional theory of mechanism-based strain gradient plasticity (CMSG). Systematic research on the size effects of growth twins in Cu polycrystalline is carried out. The roles of many material parameters which affect the size effects, such as grain sizes, Young’s modulus, twin/matrix lamellar structures aspect ratio and volume fraction, as well as the plastic strain hardening exponent, are presented.

Abstract: Due to their dissimilar properties and different deformation mechanisms between grain interior (GI) and grain boundary affected zone (GBAZ) in the nanocrystalline (NC) materials, a two-phase composite model consisting of GI and GBAZ was developed and adopted to build strain gradient plasticity theory. Comparison between experimental data and model predictions at different grain sizes for NC copper shows that the developed method appears to be capable of describing the strain hardening of NC materials.

Abstract: An analytical model describing the deformation behaviour of copper during the high-pressure torsion (HPT) processing is presented. The model was developed on the microstructural basis where the material is partitioned in two ‘phases’, the dislocation densities in cell walls and the dislocation densities cell interior, entering the model as scalar internal variables. The resulting ’phase mixture’ model is combined with strain gradient theory to account for strain non-uniformity inherent in SPD. It was demonstrated that gradient plasticity model is capable of describing the experimentally observed trends and accounting for a homogenisation of the accumulated shear strain across the HPT sample. The predictions of the model with respect to the ultrafine grain size produced by HPT and evolution of dislocation densities are in good agreement with experimental results reported by other research groups.

Abstract: By incorporating the Taylor-based nonlocal theory of plasticity, the finite element method (FEM) is applied to investigate the effect of particle size on the deformation behavior of the metal matrix composites. In the simulation, the two-dimensional plane strain and random distribution multi-particles model are used. It is shown that, at a fixed particle volume fraction, there is a close relationship between the particle size and the deformation behavior of the composites. The yield strength and plastic work hardening rate of the composites increase with decreasing particle size. The predicted stress-strain behaviors of the composites are qualitative agreement with the experimental results.

Abstract: Behaviors of fracture and deformation in a Zr-Al-Ni-Cu bulk metallic glass(BMG) was investigated by using three-point bending tests. Apparent fracture toughness obtained by bending test was 40MPam1/2 which is comparable to the value of ductile crystalline metals. This high toughness of the BMG should be understood by the crack-tip plasticity as well as crystalline metals. It is well known that plastic deformation occurs very inhomogeneously when BMGs are deformed at room temperature. Such inhomogeneity is manifested by the appearance of surface steps caused by localized shear deformation. In the present study, the surface steps due to the localized shear bands near a fracture surface have been examined in detail by using SEM and AFM, where much attention has been paid on the variation of the surface step height measured along the localized shear band. The variation of the step height indicates the gradient of plastic shear deformation, and it can be understood, in principle, as the introduction of elastic singularities corresponding dislocations in the case of crystalline materials.