Tae-Wan Ku

Experimental and Numerical Investigation on Impact Performance of Carbon Reinforced Aluminum Laminates

It is known that fiber metal laminates (FML) as one of hybrid materials with thin metal sheets and fiber/epoxy layers have the characteristics of the excellent damage tolerance, fatigue and impact properties with a relatively low density. Therefore, the mechanical components using FML can contribute the enhanced safety level of the sound construction toward the whole body. In this study, the impact performance of carbon reinforced aluminum laminates (CARAL) is investigated by experiments and numerical simulations. Drop weight tests are carried out with the weight of 4.7 kg at the speed of 1 and 2 m/s, respectively. Dynamic non-linear transient analyses are also accomplished using a finite element analysis software, ABAQUS. The experiment results and numerical results are compared with impact load-time histories. Also, energy-time histories are applied to investigate the impact performance of CARAL.

Finite Element Analysis of Multi-Stage Deep Drawing Process for High Precision Rectangular Case with Extreme Aspect Ratio

Abstract Deep drawing process for rectangular drawn section is different with that for axisymmetric circular one. Therefore deep drawing process for rectangular drawn section requires several intermediate steps to generate the final configuration without any significant defect. In this study, finite element analysis for multi-stage deep drawing process for high-precision rectangular cases is carried out especially for an extreme aspect ratio. The results of analysis show that the irregular contact condition between blank and die affects the occurrence of failure, and the difference of aspect ratio in the drawn section leads to non-uniform metal flow, which may cause failure. A series of experiments for multi-stage deep drawing process for the rectangular cases are conducted, and the deformation configuration and the thickness distribution of the drawn rectangular cases are investigated by comparing with the results of the numerical analysis. The numerical analysis with an explicit time integration scheme shows good agreement with the experimental observation.

Finite element analysis of micro-rolling using grain and grain boundary elements

Abstract The recent trend towards miniaturization causes an increased demand for parts with very small dimensions. The conceptual miniature rolling process enables the production of such parts with high productivity and accuracy while the dimensions decrease, the grain structure of the working material remains constant. This leads to so-called size effects. As a result, specific characteristics have to be considered for the design of micro-forming processes. This study proposes a new numerical approach to simulate the micro-strip rolling. The new numerical scheme is proposed to simulate intergranular micro-strip rolling process by the finite element method. Grain element and grain boundary element are introduced to simulate the micro-strip rolling. The grain element is used to analyze the deformation of individual grain while the grain boundary element is for the investigation on the movement of the grain boundary such as compression and shearing. The new approach is possible to design the process parameters which satisfy the desired micro-forming.

Numerical and Experimental Study on Plate Forming Process using Flexible Die

A flexible forming apparatus is composed a number of punches which have spherical pin tip shape instead of conventional solid die. The flexible forming tool consisted of punch array in a matrix form was proposed as an alternative forming method to substitute the conventional line heating method which use heat source to induce residual stress along specified heating lines. In this study, application of the flexible forming process to the small scale curved plate forming was conducted. Numerical simulations for both solid and flexible die forming process were carried out to compare the shape of the products between flexible and conventional die forming process. In addition, spring-back analysis was conducted to figure out the feasibility of the flexible forming process comparing with the die forming process in view of final configuration of the specimens. Moreover, experiment was also carried out to confirm the formability of the process. Consequently, it was confirmed that the flexible die forming method has capability and feasibility to manufacture the curved plates for shipbuilding.

Study on Application of Flexible Die to Sheet Metal Forming Process

Flexible forming process for sheet material using reconfigurable die is introduced based on numerical simulation. In general, this flexible forming process using the reconfigurable die has been utilized for manufacturing of curved thick plates used for hull structures, architectural structures and so on. In this study, numerical simulation of sheet metal forming process is carried out by using flexible dies model instead of conventional matched die set. The numerical simulation and experimental verification for sheet metal forming process using a flexible forming machine that is more suitable for thick plate forming process are carried out to confirm the appropriateness of the simulation process. As an elastic cushion, urethane pads are utilized using hyperelastic material model in the simulation for smoothing the forming surface which is discrete due to characteristics of the flexile die. In the flexible forming process for sheet metal, effect of a blank holder is also investigated according to blank holding methods. Formability in view of occurrence of dimples is compared with regard to the various punch sizes. Consequently, it is confirmed that the flexible forming for sheet material using urethane pad has enough capability and feasibility for manufacturing of smoothly curved surface instead of conventional die forming method.

Development of Stretch Forming Apparatus using Flexible Die

A stretch forming method has been widely used in sheet metal forming process. Especially, this process has been adopted in aircraft and high-speed train industries for skin structure forming having a variety of curvature. Until now, solid dies, which are designed with respect to the specific shapes and manufactured as a single piece, have been usually applied to stretch forming process. Therefore, a great number of solid dies has to be developed according to the shapes of the curved skin structure. Accordingly, a flexible die is proposed in this study. It replaces the conventional solid dies with a set of height adjustable punch array. A usefulness of the flexible die is verified through a formability comparison with the solid die using finite element method considering an elastic recovery and the stretch forming apparatus with the flexible die is developed.

Process Simplification of Multi-Stage Forging for the Outer Race of a CV Joint

Constant velocity (CV) joint with six inner ball grooves has been conventionally produced by a multi-stage warm forging process that includes forward extrusion, upsetting, backward extrusions, necking, ironing, and sizing. In this study, multi-stage cold forging is presented with four simplified stages of forward extrusion, modified upsetting, backward extrusion, and combined necking–ironing–sizing. A three-dimensional numerical simulation is performed to ensure the appropriateness of the proposed process, and experimental investigations are carried out using spheroidized and phosphophyllite-coated SCr420H billet material. It is shown that the proposed multi-stage cold forging process could be successfully applied to the production of the outer race.

Evaluation of Strain, Strain Rate and Temperature Dependent Flow Stress Model for Magnesium Alloy Sheets

The formability of magnesium alloy sheets at room temperature is generally low because of the inherently limited number of slip systems, but higher at temperatures over . Therefore, prior to the practical application of these materials, the forming limits should be evaluated as a function of the temperature and strain rate. This can be achieved experimentally by performing a series of tests or analytically by deriving the corresponding modeling approaches. However, before the formability analysis can be conducted, a model of flow stress, which includes the effects of strain, strain rate and temperature, should be carefully identified. In this paper, such procedure is carried out for Mg alloy AZ31 and the concept of flow stress surface is proposed. Experimental flow stresses at four temperature levels (, , , ) each with the pre-assigned strain rate levels of , and are collected in order to establish the relationships between these variables. The temperature-compensated strain rate parameter which combines, in a single variable, the effects of temperature and strain rate, is introduced to capture these relationships in a compact manner. This study shows that the proposed concept of flow stress surface is practically relevant for the evaluation of temperature and strain dependent formability.

Experimental Study on Non-Axisymmetric Rectangular Cup using Multi-Stage Deep Drawing Process

For multi-stage deep drawing process including ironing operation and biaxial forming in this study, tool developments are achieved, and the developed tool sets are applied to experimental investigations. In process and tool designs, a contact condition between intermediate blank and lower die is considered as the sequential one. In this study, the material used is cold-rolled thin sheet (SPCE) with the initial thickness of 0.4mm. From the experimental approaches, several failures such as tearing, localized thickening and thinning, are observed. To solve these failures, the contact surface on the lower die is modified. As the experimental results by applying the modified lower die, it is investigated that the failures are not occurred, and the excessive deformation behavior due to the thinning and thickening effects are decreased. Furthermore, the thickness distributions on the major axis and the minor axis of each intermediate blank are investigated to be already satisfied the target (ironing) thickness, respectively. By this systematic approach, it is confirmed that the experimental results show good agreements with the designed and required configuration of each deformed and final products.

Numerical and experimental approach on energy dissipation in nano colloidal damper

Mechanical damping systems have been widely used to various mechanical structures and systems, and are mainly hydraulic and pneumatic devices nowadays. New damping system such as nano colloidal damper (NCD) is complementary to the hydraulic one, having a cylinder-piston-orifice structure. This study includes numerical and experimental investigation about energy dissipation of NCD by using porous silica particles. In numerical approach, the dissipated energy was obtained between compression and relaxation processes for porous silica particle in NCD according to the capillary tube theory. Furthermore, for colloidal damper, the hydraulic oil was replaced by a colloidal suspension that was consisted of a nano-porous matrix with controlled architecture and a lyophobic fluid. NCD test rig and the measuring technique of the hysteresis were described in this study Performance of the energy dissipation between numerical and experimental results was investigated and compared. As a result, the proposed NCD was proved to efficiently dissipate the mechanical energy.