Key Engineering Materials Vols. 535-536

Abstract: Lightweight materials have been in focus in recent times for their use in automobiles, planes and protective structures for numerous benefits ranging from reduction in fuel consumption and increased payload in vehicles to lighter and stronger protective structures. For efficient use of materials in applications where they are subjected to unusual higher sudden loads, it is necessary to understand their mechanical behaviour under such conditions.In present study, the effect of strain rate on deformation of magnesium alloy AZ31Bunder compression has been investigated. The alloy is subjected to various strain rates as 10-4s-1, 500s-1 and 2500s-1 and the microstructural analysis was performed to see the changes in the microstructure of the alloy and their effect on the mechanical response of the alloy is portrayed.

Abstract: The specimen size has always been crucial in defining the materials behaviour and becomes more important when materials are subjected to high rates of loadings. In the current study, the effect of specimen size on the mechanical behaviour of AZ31B alloy has been investigated under dynamic compression using the Split Hopkinson Pressure Bar (SHPB) and results are presented. Specimens were made in different sizes with fixed slenderness ratio (l/d) of 0.5 and with bar to specimen diameter ratio varying between 0.47 and 0.79. When deformed at the same strain rate 1500±50s-1, the smaller specimens give higher stresses and smaller strains. The smaller size specimens give more uniform strain rate as compared to the larger size specimens. However, some spurious oscillations are observed in the stress-strain curves for smaller size specimens. The alloy shows higher hardening behavior for larger size specimen; the hardening exponent n is larger for larger size specimens.

Abstract: An efficient numerical framework suitable for three-dimensional analyses of brittle material failure is presented. The proposed model is based on an (embedded) Strong Discontinuity Approach (SDA). Hence, the deformation mapping is elementwise additively decomposed into a conforming, continuous part and an enhanced part associated with the kinematics induced by material failure. To overcome locking effects and to provide a continuous crack path approximation, the approach is extended and combined with advantages known from classical interface elements. More precisely, several discontinuities each of them being parallel to a facet of the respective finite element are considered. By doing so, crack path continuity is automatically fulfilled and no tracking algorithm is necessary. However, though this idea is similar to that of interface elements, the novel SDA is strictly local (finite element level) and thus, it does not require any modification of the global data structure, e.g., no duplication of nodes. An additional positive feature of the advocated finite element formulation is that it leads to a symmetric tangent matrix. It is shown that several simultaneously active discontinuities in each finite element are required to capture highly localized material failure. The performance and predictive ability of the model are demonstrated by means of two benchmark examples.

Abstract: Although localized atomic rearrangements have been considered to be the underlying mechanism of plastic deformation in metallic glass, the nucleation and evolution of such plasticity events are still elusive. With the aid of molecular dynamics analysis, this study revealed that a series of localized atomic rearrangement events would occur in metallic glass as demonstrated by the formation of high-kinetic-energy clusters. It was found that atomic clusters of average sizes of 1 to 2 nm nucleate during elastic deformation, and become prevailing after yielding. The cores of these clusters contain several high-velocity atoms, which drive the local structural change and accommodate plastic strain. The nucleation and evolution of the local plasticity events are shown clearly by the strong dynamic signature, attributed to the spontaneous structural reshuffling after crossing an energy barrier.

Abstract: Spall Strength, uniaxial tensile strength and fracture toughness, are three typical parameters describing the fracture properties of materials subjected to different loadings. Actually, these three macroscopically parameters are connected to the tensile fracture (Model I) properties, and many papers have been trying to find the intrinsic connection between each other. In this work, ZL205A aluminum is conducted by varies experiments: the spallation test loaded by a light gas gun, the dynamic uniaxial tensile test using the Split Hopkinson Tensile Bars (SHTB), and the dynamic fracture toughness obtained with a three point bending specimen loaded by Split Hopkinson Pressure Bars (SHPB). The three parameters are compared with the view of energy. The results show that the cavity expansion model is successfully used to set up a connection between spallation strength and dynamic uniaxial tensile strength of this material, while the energy release rate or the surface energy can give a good prediction of dynamic tensile strength and fracture toughness.

Abstract: An AZ91D magnesium alloy was tested in either compression or tension, and extensive observation of the tested samples was carefully carried out to understand the fracture initiation and propagation processes. Cracking of β-Mg17Al12 intermetallic compounds was found to occur easily in plastic zones under either compressive or tensile loading. However, the cracking did not necessarily result in fracture propagation. It is argued that the fracture is controlled by the microcrack propagation.

Abstract: In spite of this progress in predicting ductile failure, the development of macroscopic yield criteria for describing damage evolution in HCP (hexagonal close-packed) materials remains a challenge. HCP materials display strength differential effects (i.e., different behavior in tension versus compression) in the plastic response due to twinning. Cazacu and Stewart [1] developed an analytic yield criterion for a porous material containing randomly distributed spherical voids in an isotropic, incompressible matrix that displays tension-compression asymmetry. The matrix material was taken to obey the isotropic form of the Cazacu et al. [2] yield criterion, which captures the tension-compression asymmetry of the matrix material. In this paper, finite element calculations of a round tensile bar are conducted with the material behavior described by the Cazacu and Stewart [1] yield criterion. The goal of these calculations is to investigate the effect of the tension-compression asymmetry on the necking induced by void evolution and propagation.

Abstract: This study involves the stability of plastic flow of thermoviscoplastic materials. The general instability criterion is proposed for determining the onset conditions of instability and the transition conditions among various plastic deformation behaviors. By using the phase diagram method, the transition of material instability modes under complex loading conditions is described and the analytical results are further validated with numerical examples.

Abstract: In the framework of crystal plasticity, a new cyclic polycrystalline viscoplastic model is constructed to describe the uniaxial ratchetting of a body centered cubic (BCC) metal. At the intra-granular scale, a combined kinematic hardening rule similar to the Ohno-Abdel-Karim model is employed to address the ratchetting within each single crystal grain; and two sets of slip systems with different resistances to dislocation slip, i.e., primary and secondary ones, are considered to capture the physical nature of dislocation slips in a body centered cubic (BCC) metal. At the inter-granular scale, an explicit transition rule is adopted to extend the single crystal approach into a polycrystalline version. It is shown that the proposed model describes the uniaxial ratchetting of annealed 42CrMo steel, a body centered cubic (BCC) metal, reasonably. Also, it is seen that the crystal orientation influences significantly the ratchetting of body-centered cubic (BCC) single crystals.

Abstract: Recently, the demand for energy is growing at a very high rate all over the world. The fossil fuels eventually lead to the foreseeable depletion of limited fossil energy resources. Hydrogen is considered a promising candidate to remedy the depletion of fossil fuels. The bipolar plate is the second most important component of a proton exchange membrance (PEM) fuel cell stack after the membrance electrode assembly (MEA). Its primary roles are to supply reactant gases to the fuel cell electrodes and provide electrical connection between adjacent cells in the stack while removing product water from the cell and transferring away the heat of reaction. Historically, machined graphite had been chosen as a good compromise between all of these requirements, but alternatives are emerging. New materials are light metals. In this study, rubber pad forming process was employed as the manufacturing method for metallic bipolar plates. The rubber pad and the sheet metal plate were pressed together by the punch, and the repulsive force of the deformed rubber is loaded at the plate, and can contribute to improving formability. And then, its surface was coated with TiN. After coating process, the performance characteristics of single stack in the condition of PEMFC using the metal bipolar plate have been investigated.