Abstract: Fatigue tests were carried out at frequent of 20 kHz for 5083 aluminum alloy. The loading way is uniaxial and bending loading. The S-N curve of uniaxial loading presents a duplex curve corresponding to surface fracture and interior fracture. However the S-N curve of the bending fatigue shows the continuous curve. This demonstrates that different loading ways lead to different S-N curve characteristics. For uniaxial loading, almost all crack initiated interior of specimen in the very high cycle regime. The crack source zone appears wear away because of the constant pressure and grinding of this area in the process of cyclic loading. For the symmetric bending loading, the crack of corner in the specimen expands at different rates and direction.
Abstract: In this study, macro/meso/micro elastic-viscoplastic analysis of plain-woven laminates is conducted based on a homogenization theory for nonlinear time-dependent composites. For this, a plain-woven laminate is modeled with respect to three scales by considering the laminate as a macrostructure, fiber bundles (yarns) and a matrix in the laminate as a mesostructure, and fibers and a matrix in the yarns as a microstructure. Then, an elastic-viscoplastic constitutive equation of the laminate is derived by dually applying the homogenization theory for nonlinear time-dependent composites to not only the meso/micro but also the macro/meso scales. Using the present method, the elastic-viscoplastic analysis of a plain-woven glass fiber/epoxy laminate subjected to on-and off-axis loading is performed. It is shown that the present method successfully takes into account the effects of viscoplasticity of the epoxy in yarns on the elastic-viscoplastic behavior of the plain-woven GFRP laminate. It is also shown that the results of analysis are in good agreement with experimental data.
Abstract: We develop new drawing technologies of rotary laser dieless drawing for the fabrication of micro-tubes without the need for tools such as dies, plugs and mandrels. A rotary laser dieless drawing apparatus with local heating using a laser irradiated from one direction as the heating source has been focused on for the fabrication of micro-tubes. In this study, a dieless drawing with a semiconductor laser setup with power of 30W is designed and developed. A workpieces was rotated about a tensile axis by rotary stage with chucks for circumferential uniform temperature distribution. A tube of stainless steel SUS304, with an outer diameter of 0.5mm and thickness of 0.13mm is used in the experiments. In this experiment, we verify the effectiveness of the developed rotary laser dieless drawing apparatus.
Abstract: Ni-Ti shape memory alloys have shape memory effect, that if they are deformed from martensitic phase state at a lower temperature, they will recover their original shape by heating them to austenitic phase state. To have them for an application using this shape memory effect, usually they undergo a constraint aging after plastic deformation. That is, they are fixed with tool set and together heat treated in a furnace after they are formed at room temperature. However a large load is needed to form them at room temperature. Thus, this study is aimed to lower the forming load by combining the forming and aging process together in a furnace at high temperature. In this study, a Ni-Ti shape memory alloy wire having a diameter of 0.63 mm is bent in a heated chamber at 450°C, 500°C, 550°C, and 600°C, respectively, by a V-shaped punch of 2 mm in radius to an angle of 60°, then held along with the die set at its dead center in the chamber for maximum one hour long, and then quenched in the water. All of the bent wires have the shape memory effect. That is, the wires recover their bent geometry once they are unbent at about 4°C and heated again at about 100°C. The experiment results showed that the bent wires can have the geometry accuracy as desired because of stress relaxation found in the process, which depends on the process temperature and duration. As a result, the higher the process temperature is and the longer the duration is, the better the accuracy of the formed wires is.
Abstract: Cellular material such as aluminium foam has been considered as a potential material for energy absorption upon impact and blast loadings. One of the most important properties that contribute to this feature is the densification strain. At high impact velocity, prediction of the densification strain from quasi-static engineering stress-strain curve has been found inadequate. Furthermore, theoretical prediction using the equation proposed by Reid et al. always over-predicts the dynamic crushing stress. Formation of the shock wave at high impact velocity is believed to further increase the densification level of the foam. However, this effect is disregarded when determining the densification strain quasi-statically. The present study aims to address this issue by determining the densification strain experimentally from impact tests. Forty cylindrical aluminium foams with three different lengths were used as projectiles and were fired towards a rigid load cell by using a gas gun. The peak forces generated from the impact were recorded and analysed. The experimental densification strains were determined physically by measuring the deformation of the foam projectiles after the tests. It is concluded that, at high impact velocity, the densification strain varies with the initial impact velocity. Therefore an appropriate value of densification strain needs to be used in the equation of dynamic crushing stress for a better approximation.
Abstract: This paper is concerned with the hardening behavior of 4340 steel at high strain rates from 104 s-1 to 106 s-1. Tension tests were conducted using Instron 5583, HSMTM and SHPB testing machines at a wide range of strain rate from 10-3 s-1 to 103 s-1. Three different impact velocities were performed for the Taylor impact tests to evaluate the reliability of Johnson–Cook model, modified Johnson–Cook model, modified Khan–Huang model, and Lim–Huh model at high strain rates for 4340 steel.
Abstract: A micro hydromechanical deep drawing is carried out using the pure titanium and the effect of fluid pressure on formability of pure titanium is investigated. The experiments are performed using the two kinds of pure titanium foils (TR270C-H and TR270C-O) and stainless steel foil (SUS304-H) with 50 thickness and the cylindrical and conical punches. As a result, it is found that the peeling off the oxide film of pure titanium can be reduced by applying the fluid pressure because the friction force and contact pressure between the blank and die decreases. However, the formability is lower for pure titanium than that for stainless steel because the tensile strength is low and the friction force is easy to increase as the friction force increases. In contrast, due to the low young modulus of pure titanium, the restriction of wrinkling, decrease of friction force and friction holding effect can be obtained at low fluid pressure.
Abstract: This study is focused on the influences of micro stretching process, miniaturized of micro square hole-flange to stainless steel (SUS304) material, and different thicknesses (0.2, 0.1, 0.05mm) of plate. By undergoing finite element program analysis of material parameter corrected by scale factor, we can discover the differences of different thicknesses of plate during micro stretching forming process. The finite element method in this paper is combined with the plastic flow rule of Dynaform and LS-DYNA solver, finite element deformed theory, and updated Lagrangian formulation to simulate the process of micro square hole-flange. The point of this research is by simulating and analyzing all datum of micro stretching forming process, relation between punch load and stroke, distribution of thickness, distribution of stress and strain, the maximum diameter of flange’s hole and the maximum height of flange. Design three pairs of micro square hole-flange tool undergoing micro stretching experience through SUS304 plate. Compare the experience to the results of the simulation to test the reliability of this analyzing program. Through finite element analysis and the results of the experience, we can discover that the minimum of the thickness, the biggest stress and major strain centralize areas where blank and punch corner meet.
Abstract: Three-point bending system with one end simple support and the other end fix support has been proposed to analyze the transformation plasticity (TP) behavior and obtain transformation plasticity coefficient. In this investigation two types of materials SCM440 steel and S45C steel have been studied. The specimens were heated to austenite temperature and the temperature kept constant for several minutes, then cooling and loading processes were performed. Austenite to martensite phase transformation with forced cooling for SCM440 steel and austenite to pearlite phase transformation with natural cooling for S45C steel due to bending stresses have been occurred. The deflections of specimen were measured during loading process. By obtaining the maximum deflection due to transformation plasticity, the transformation plasticity coefficient was determined.
Abstract: Molecular dynamics simulations were carried out to clarify the atomistic mechanism of transformation plasticity. As the first step for the purpose, a simple thin-film model consisting of 8640 atoms was prepared. Phase transformation was assumed to be expressed by switching the material parameters of Lennard-Jones potential function. As a preliminary calculation, phase transformation was forced to occur homogeneously in the whole region of the model, resulting in no extra strain except volumetric transformation dilatation. In that case, perfect single crystal structure was maintained in the new phase. Simulations were carried out under external load, but specific strain was not generated. On the contrary, when the transformation region was set partially in the model and the region was expanded with time, a large deformation was observed. In the middle process of the phase transformation, slip-like deformation behavior and the change in crystal orientation occurred, indicating that extra plastic strain was induced during phase transformation. The strain was observed even when external load is not applied, and hence it was concluded that not only external load but also local stress distribution may cause the transformation plasticity.