Abstract: The miniaturization of products has driven the demand for better realization in the micro forming process. Micro sheetmetal forming becomes more susceptible to friction attributed to the decrease in the weighting of body force and the increase in surface force, as the size of workpiece decreases. In this work, micro cupping tests were utilized to test the influence of workpiece thickness and surface texturing both on stainless steel and copper sheets under an oil-lubricated condition. Three thickness gauges of workpiece, 0.05, 0.1 and 0.2 mm, were selected in order to represent the micro, meso and macro forming conditions, respectively. Longitudinal and isotropic lay conditions were obtained through imprinting the textured rolls produced by grinding and sand blasting methods. The result of the micro cupping tests show that the cup heights are comparable for both longitudinal and isotropic lays in forming the sheets of 0.1 and 0.2 mm thickness. However, isotropic lay has a greater cup height than that of longitudinal lay in forming the 0.05 mm sheets. This indicates that surface texturing becomes influential in the formability of micro sheetmetal forming.
Abstract: A tensile/compressive-torsional biaxial testing system was employed and tensile/ compressive-torsional tests were performed for the hollow specimen, which was loaded and the austenized specimen was cooled so that pealrite transformation accompanied by transformation plasticity occurred and axial and torsional strain were measured. Furthermore, the elastic-plastic constitutive equation due to phase transformation based on the hydrostatic pressure dependent model was proposed, and the validity of this equation was discussed experimentally. The test results showed the transformation plasticity coefficient due to pearlitic transformation of S45C depends on the loading direction, and these behaviour can be appropriately expressed by the hydrostatic pressure dependent model than the isotropic model.
Abstract: If TRIP steel with excellent mechanical properties dominated by strain induced martensitic transformation (SIMT) can be applied to a shock absorber in automobiles, it becomes possible to reduce a weight of the automobiles by decreasing the thickness of their components. In order to improve its reliability by clarification of the mechanical properties, it is necessary to evaluate continuous evolution of martensite during deformation in TRIP steel. In the previous studies, volume resistivity and impedance have been measured during deformation. However, these studies have not been succeeded for the evaluation of martensite with higher precision. Here, because of focusing on a characteristic which martensite indicates ferromagnetism, the evolution of martensite is evaluated by measuring relative magnetic permeability in TRIP steel during deformation at various strain rate.
Abstract: The paper presents the dynamic response of an integral bridge to an earthquake registered in Central Europe. The acceleration history of the shock was scaled up to peak ground accelerations predicted for this seismic zone (0.4 g). The seismic action was implemented in the form of two models of three dimensional kinematic excitation: uniform and non-uniform (spatially varying). In the uniform model the assumption was made that the motion of all supports of the bridge was identical. In the case of the spatially varying excitation the wave passage effect was taken into consideration, assuming that the seismic wave propagated along the bridge forcing subsequent supports of the bridge to repeat the same motion with a time delay dependent on the wave velocity. The structural system of the integral bridge consisted of steel girders and crossbars whereas the superstructure was made of a concrete material. To represent the inelastic behavior of the integral bridge during the earthquake, plastic models of both the steel and the concrete material were implemented. For the steel material the classical metal plasticity model with the dynamic failure model of progressive damage, provided by the ABAQUS software, was applied. For the concrete material of the superstructure the concrete damaged plasticity constitutive model was taken into consideration. It turned out that when the non-uniform excitation model was imposed, the tensile damage (cracking) and the degradation of the support zones of the concrete deck were more significant than in case of uniform excitation. The non-uniform excitation model also caused considerably higher inelastic strains of the steel girders and crossbars than the uniform model. This resulted from quasi-static effects caused by ground deformations imposed on the bridge supports during the seismic shock.
Abstract: In the Taylor impact test, obtained strain rate becomes in a range of 103~105/s corresponding to penetration of space debris to a space structure. According to this test, a stress value can be calculated by theoretical formulae. However, the formulae include some assumptions and the external force acting on a specimen is not directly measured by using the formulae. In the past study, the split Hopkinson pressure bar (SHPB) is employed instead of a use of a rigid wall which the specimen collides. However, there are two difficulties on this method. The first one is to be a similar range of measurable strain rate to the SHPB technique and the second is to require a sufficiently-large space for a testing apparatus. In contrast, by introducing a force sensing block, the apparatus becomes compact and longer measurable time is realized compared with the SHPB technique. Therefore, the stress value can be measured with higher precision since an extensive range of strain rate can be measurable. In this study, to enhance the precision of the test, it is suggested that the force sensing block is placed just behind the rigid wall for a direct measurement of a time history of external force.
Abstract: In this study, a framework to predict the onset of plastic flow localization is introduced. The Marciniak-Kuczyński type approach, which is a classical method to predict the strain localization, and a crystal plasticity model with a homogenization-based finite element method are combined, and forming limit strains that are defined as the onset of plastic flow localization for FCC polycrystals are computed. The forming limit strains with several kinds of textures are evaluated with the present approach, and the results are compared with those obtained by the Taylor model, which is a widely used conventional polycrystalline model. Within the present application, the present method and the classical Taylor model give similar forming limit strains for FCC polycrystal sheets. According to the present results, the use of the Taylor model in the sheet necking analysis might be justified, at least for FCC polycrystal sheets with various textures.
Abstract: This study considered four types of punch, which were flat, conical, spherical and standardized punched, to investigate the effect of the punch shapes on the micro backward extrusion of brass. Brass (JIS C2600) materials were annealed at 500 °C, 600 °C, and 700 °C and resulted in different microstructures with grain sizes of 50 μm, 70 μm and 111 μm, respectively. The annealed brass materials were carefully machined as cylindrical billets with the same dimension of 1.1 mm in diameter and length. The billets were later used in the experiments of the micro backward extrusion to form cups with 1.1 mm diameter and 0.2 mm thickness. The experimental results show that the type of punch affecting load in an increasing order are spherical, standardized, conical and flat punches. The results also show that load increases with the punch speed, and reduces as grain size increases.
Abstract: This paper is concerned with auto-body frames and roof structures against rollover motion which could take place frequently with TWVs compared to FWVs because of the major difference of the loading between the two motions. Topology optimization is utilized for conceptual design and preliminary design of the roof structure for various loading conditions. The roof structure for TWV consists of existing part of FWV and a new part called Cross-bar reinforcement. Although a test method for regulation of roof strength is with static loading, simulations are carried out for both static and dynamic cases. In the dynamic loading case, rate-dependency of a material needs to be considered for accurate results of FE analysis.
Abstract: Experimental observations and data are employed to elucidate the effect of indenter size on the deformation and energy absorption of composite sandwich beams. Unlike metal face sheets that yield and plastically deform to create an intact indentation zone; composite face sheets tend to fail in a brittle manner resulting in fibre breakage that leads to widespread fracture. This mode of failure can dictate how the load is transferred throughout the structure and directly affect the energy absorption character of the composite sandwich beam. Quasi-static and low velocity impact (LVI) three-point bending experiments with various indenter diameters were conducted to observe the interaction between indenter and face sheet and the energy absorption properties. The results are compared with existing analytical expressions.
Abstract: In this paper, a numerical procedure for plastic limit analysis of 3-D elastic-perfectly plastic bodies under complex loads is presented. The method is based on the lower-bound limit theorem and von Mises yield criterion so that the lower-bound limit analysis can be conducted by solving a nonlinear mathematical programming problem. A SQP algorithm and a dimension reduction-based technique are used to solve the discretized finite element optimization formulation. A conception of active constraint set is introduced, so that the number of constraints can be reduced greatly. The basis vectors of reduced residual stress spaces are constructed by performing an equilibrium iteration procedure of elasto-plastic finite element analysis. The numerical procedure is applied to carry out the plastic limit analysis of pipelines with part-through slots under internal pressure, bending moment and axial force. The effects of different sizes of part-through slots on the limit loads of pipelines are studied.