Papers by Keyword: Hydrostatic Stress

Abstract: This paper aims to investigate the performance of a new three-parameter damage mechanics model which describes three basic damage mechanisms of quasi-brittle solids: tension, shear, and hydrostatic compression. The stress is first decomposed into its positive part and negative part, and then the latter is further decomposed into its deviatoric part and hydrostatic part, whereby a three-parameter damage description is formulated. Through matrix representation of the tensor formulation, specific forms of the three-parameter damage theory are illustrated in various stress states. It is found that the proposed framework of three-parameter damage theory can inherit the existing two-parameter models and extend them to a broader scope of application.

Abstract: Engineers often use nature as an inspiration for generating designs. Plants have optimised their structures over millions of years of evolution. A lotus root, rhizome of Nelumbo Nucifera Gaertn having gas canals was studied in the paper. The lotus root cross section subjected to an external water pressure was analysed and compared with other cross sections including a circular hollow cross section. It was found that the lotus root cross section has more areas containing higher triaxiality factors due to the holes. The hydrostatic stress in the lotus root cross section varies from zero to several times that of the external water pressure. However, the hydrostatic stress in a circular hollow cross section has a constant ratio to the external pressure. The research results show that engineering components with designed multi-holes can affect its failure behaviour. The structure of lotus root may be adopted in the design of engineering materials and structures.

Abstract: Petroleum-based fuel is facing significant depletion issue due to its limited reserves and increasing demand from various industries. Thus, various considerations from economical, environmental and political concerns have motivated researchers to develop alternative energy sources such as biofuel to decrease dependence on petroleum-based fuel. However, the changes in the fuel composition of biofuel affect the material compatibility. In engineering applications where elastomeric components are exposed to hostile environment such as palm biodiesel medium, at least two aspects contribute to the degradation of the materials during the service: diffusion of the liquids leading to swelling and fluctuating multiaxial mechanical loading leading to fatigue failure. Therefore, it is of utmost importance to study the mechanical responses of elastomers under this coupled diffusion-mechanical loading in order to predict accurately their fatigue failure. The present work investigates the swelling of elastomers under simultaneous diffusion of palm biodiesel and multiaxial large deformations.

Abstract: According to micromechanics, consider the porous shape memory alloy (SMA) as a composite-sphere model. Isolate a constant thickness spherical shell which is composed of SMA, and is traction free on its inner surface and subjected to the uniform hydrostatic pressure and the deviatoric stress on its external surface. Then, a constitutive model for porous SMA considering hydrostatic stress is proposed by elasticity solution. The stress distribution of the spherical shell was calculated. Corresponding to different applied stresses, the spherical shell is divided into different regions of pure austenite, pure martenite, and austenite/martensite mixture under isothermal circumstances. The martensite volume fraction is then obtained. The predicted results have been compared with the obtained experimental data by Zhao and Sia Nemat-Nasser. It shows that the modeling results are in good agreement with the experiments and the initial phase transition point for porous SMA is lower than the dense SMA.

Abstract: It analyzed the influence on parts’ shear surface quality by the stress and strain in the deformation area of materials. The analysis shows that the continuum linear rigid-plastic model and the fracture failure criterion in the light of damage factor, which is based on the microcosmic structure, and can accurately simulate the closed extruding fine-blanking processing; and that the higher the filling rate of outer ring is, the larger pressure stress are the deformation materials under; also that the greater the compressive stress of deformation area materials is, the higher does the deformation limit, the time of materials’ crack is delayed.

Abstract: Internal voids have to be eliminated for defect-free in some open die forging. The FEM analysis is performed to investigate the overlap defect of cast ingots during cogging stage. The measured flow stress data were used to simulate the cogging process of cast ingot using the practical material properties. Also the numerical analysis of void closure is performed by using the DEFORMTM-3D. The calculated results of void closure behavior are compared with the measured results before and after upsetting, which are scanned by the X-ray scanner. From this result, the criteria for deformation amounts effect on the void closure were estimated into effective strain of 0.6 by the comparison of practical experiment and numerical analysis.

Abstract: In this study, bottom-up type powder processing and top-down type SPD (severe plastic deformation) approaches were combined in order to achieve both full density and grain refinement of metallic powders with least grain growth, which is considered as a bottle neck of the bottom-up method that uses the conventional powder metallurgy of compaction and sintering. ECAP (Equal channel angular pressing), one of the most promising method in SPD, was used for the powder consolidation. In the ECAP process of not only solid but also powder metals, it is important to get a good understanding of the density as well as internal stress, strain and strain rate distribution. We investigated the consolidation, plastic deformation and microstructure evolution behavior of the metallic powders during ECAP using an experimental method. It was found that high mechanical strength could be achieved effectively due to the well bonded powder contact surface during ECAP process of gas atomized Al-Si powders. The experimental results show that SPD processing of powders is a viable method to achieve both fully density and nanostructured materials.

Abstract: Experiments show that the failure of ductile materials can be characterized by a rate-independent parameter, relative spacing d defined as the ratio of the distance between two voids and the radius of voids. In this study, this experimental phenomenon is analyzed via numerical simulations using 3-D finite element model. Considering that hydrostatic stress is a dominant factor in the evolution of microvoid nucleation, growth and coalescence in ductile materials, numerical simulations are performed to obtain the relationship between relative spacing d and hydrostatic stress in the ligament between voids. Numerical results show that hydrostatic stress along matrix ligament is sensitive to the change of the relative spacing. Further analysis shows that the failure of ductile materials can modeled by using a criterion of the threshold of local hydrostatic stress in the ligament. Based on such a criterion, a curve displaying the relationship between the strength of ductile material and strain rate is obtained numerically. It is concluded that the failure criterion of ductile materials can be described by using local hydrostatic stress and relative spacing between two voids, which is not sensitive to strain rates.

Abstract: Hardness is most likely to mean the resistance to indentation, and to the design engineer it often means an easily measured and specified quantity which indicates something about the strength and heat treatment of the metal. Especially, Vickers hardness is one of the most widely useful methods to obtain mechanical properties of a product. Firstly, in this study, a method to estimate hardness will be presented using FE simulations of Vickers hardness tests from the viewpoint that hardness indicates resistance to plastic deformation. To verify our method, the results of the simulations for several materials such as commercial aluminum alloy and steel will be compared with those of Vickers hardness tests for the materials. Secondly in this study, hardness numbers of the several materials will be obtained as a function of hydrostatic stress. Through the results of this study, the estimation of hardness number of a specific material will be very easy to obtain and access even though the material is under a kind of hydrostatic stress state.

Abstract: In this study, bottom-up type powder processing and top-down type SPD (severe plastic deformation) approaches were combined in order to achieve both full density and grain refinement of metallic powders with least grain growth, which was considered as a bottle neck of the bottom-up method using the conventional powder metallurgy of compaction and sintering. ECAP (Equal channel angular pressing), one of the most promising method in SPD, was used for the powder consolidation. In the ECAP process of not only solid but also powder metals, knowledge of the density as well as internal stress, strain and strain rate distribution is important for understanding the process. We investigated the consolidation, plastic deformation and microstructure evolution behavior of the metallic powders during ECAP using experimental and theoretical methods. Almost independent behavior of powder densification in the entry channel and shear deformation in the main deformation zone was found by the finite element method in conjunction with a pressure dependent material yield model. It was found that high mechanical strength could be achieved effectively as a result of the well bonded powder contact surface during ECAP process of gas atomized Al-Si powders. The SPD processing of powders is a viable method to achieve both fully density and nanostructured materials.