We investigated the relationship between the material properties and axial collapse behavior of hat-section hollow columns made of high-strength steels. Dynamic collapse tests of the columns and FEM simulations were conducted using high-strength steels with various tensile strengths and n-values. Materials that exhibited yield point elongation (YPEl) underwent enhanced accordion-type deformation in axial collapse owing to the occurrence of long wave buckling on the hat walls in the early stage of collapse. In addition, it was important to avoid fracture in the initial buckling region to induce progressive crumpling behavior. A high n-value of the material was favorable for avoiding fracture by increasing the bend radii and decreasing the strain concentration at the buckling regions on the hat walls, thereby resulting in stable progression of accordion-type folds. In conclusion, steels exhibiting YPEl and high n-values are favorable for inducing stable accordion-type deformation during axial collapse.
This research is on the development of a new extrusion method of a fuel cell separator with an orthogonal channel. Separators are successfully produced separators by this new extrusion method. It is important to determine the most suitable extrusion condition to enable stable continuous production. We decided to attempt the production of a fuel cell separator with a serpentine channel, which is a complicated shape. However, its formation has so far been impossible by the method of supplying material from the upper surface of the separator with the serpentine channel. Supplying the material from the side of the product channel was proposed as a new extrusion method for separators like the serpentine type. The flow of the material at the time of product formation was observed and the flow of the material required to achieve stable production was controlled by the position and geometry of the front guide at the position from which the product is extruded.
A method for identifying the yield function using simple material tests is proposed. The identification accuracy of this method is verified by finite element analysis. In the proposed method, a polygon circumscribing the equal plastic work contour is defined through uniaxial tensile tests, hydraulic bulge test, and plane strain tensile tests. The parameters of the anisotropic yield function are identified as a smooth curve inscribed in the polygon. The virtual material tests of three material models, namely, mild steel, aluminium alloy, and pure titanium, are conducted by the finite element method. On the basis of the results, stress measurement error and yield surface identification error are discussed. When the material exhibits strong in-plane anisotropy, the measurement error of equi-biaxial stress in the hydraulic bulge test becomes large. When the initial gradient of work hardening is large, the stress value of the circumscribed line of the small plastic strain stage in the plane strain tensile test is estimated to be lower than that on the true yield surface. These errors affect the identification accuracy of the material parameters of the yield function quantitatively, but the difference between the identified locus and true yield locus is small. The proposed method is feasible as a simple identification method of the yield function.
In this paper, we describe the robustness of thermosonic flip chip bonding when the bottom chip is tilted in the chip-on-chip assembly. Experiments were carried out using a bottom chip with 12 Au ball bumps and a top chip with Al pads. A bonding condition that causes gross sliding at the bonding interface was chosen to improve the robustness to chip tilt. The bottom chip was tilted around the axis perpendicular to the direction of the ultrasonic vibration. We found that die-shear force and the sum of the contact area between bumps and the top chip electrodes were constant in spite of tilting of the bottom chip. Although the contact area between each bump and top chip electrode depended on different bonding forces generated by bottom chip tilting, the fractional area of Au-Al alloy of each bonding area became nearly equal. These were caused by the deformation stress of the gold ball bump under the application ofultrasonic vibration being constant in the deformation range in this study.
In this paper, we present an inspection technology using Helmholtz resonance for small holes. As for the Helmholtz resonator, resonance frequency changes depending on the shape of the opening of the resonance container. The opening part is replaced with small hole parts and the hole diameter is inspected by measuring the resonance frequency of the inspection container. Small hole parts processed into a metal plate by press stamping or electric discharge method are the subject of examination. The prototype inspection system is constructed of a cylindrical container, a signal generator,and a sound signal analyzer. The container has a built-in standard microphone and speaker and resonance frequency is measured with respect to the container volume and sample hole diameter. The test sample of about 0.05 mm-2.0 mm in diameter had a hole formed by precision drill processing using planer-type jig borer and the electric discharge method. Test sample hole diameter inspection is enabled by measuring the resonance frequency and first fourier transform of measurement data.