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In the past ** D.Y. Yang** has collaborated on articles with

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AbstractA theory of dual pump—probe spectroscopy is developed. At time t≈0 the medium is subjected to two pulses with different carrier frequencies ωL and ωS. Then the same sample is probed by two pulses, ωL and ωS, after a delay time td. The transmission lineshape is explored as a function of td. Time resolution during one period of the medium vibration can be achieved. The limitation on the pulse duration tp is determined by the dephasing time T2:tp⪡T2.

AbstractHydrofilm extrusion is a kind of hydrostatic extrusion which uses a minimum amount of oil as lubricant and pressure-transmitting medium. In hydrofilm extrusion energy dissipation in the fluid lubricant between the die and working material corresponds to sliding friction in ordinary lubricated extrusion using solid lubricants. Utilizing the upper-bound theorm, an “equivalent” friction factor is defined so that the overall frictional effect between the die and working material can be conveniently investigated in terms of geometrical parameters and press velocity. On the basis of this definition, the effects of various process parameters on the frictional characteristics in hydrofilm extrusion are discussed. It is consequently found that the dominant contribution to frictional energy dissipation is made by reduction of area and press velocity. Die length is found to have very little influence on the equivalent friction factor in so far as it is longer than billet diameter.

AbstractA new variational formulation is proposed for the rigid-plastic finite element analysis of large deformation processes. In the new formulation the solution for each step is determined in a step-by-step procedure for non-steady metal forming problems. The corresponding finite element equations are derived using the proposed formulation. The work-hardening effect is considered explicitly in each step. The present formulation gives more accurate and stabler results with less computational time than the conventional formulation when applied to large deformation problems in which the strain rate field has a high gradient and the work-hardening effect is dominant. The effectiveness of the present method is demonstrated through the numerical tests for some chosen examples.

AbstractA variational formulation is derived for the incremental analysis of the nonsteady large deformation of normal anisotropic rigid-plastic sheet metal obeying Hill's new yield criterion. The associated finite element equations are formulated for the analysis of the hydrostatic bulging of elliptical diaphragms. Using the natural convected coordinate system, the numerical procedure is improved. In the finite element analysis, the contact between sheet metal and die shoulder is considered by using a skew boundary condition. The relation between bulging pressure and deformation is studied and discussed in comparison with existing experimental results.

AbstractA new kinematically admissible velocity field is suggested for the upper-bound solution of hydroforming of longitudinally curved boxes with regular polygonal cross-section. In order to maintain the uniform wall thickness the back-up fluid pressure is controlled to vary with respect to the punch position during the hydroforming process. The pressure vs punch stroke curves for various process conditions are determined from theoretical analysis. The effects of punch shape, work-hardening exponent, drawing ratio and friction are then analysed and discussed. Experiments are carried out in a hydroforming press according to the computed pressure vs punch stroke curves. The hydroformed specimens have shown very little thickness variation within a 5% range. Thus, the validity of the assumption of uniform wall thickness has been confirmed. The flange deformation from the computation is found to be in good agreement with the experimental observation. When the computed reference curve of pressure vs punch stroke relation has been used, no defect, i.e. no necking or no wrinkling has been observed in the experimental specimens. It has been shown that the present method of analysis can be effectively used for the hydroforming process design of longitudinally curved boxes with regular polygonal cross-section.

AbstractThe method of weighted residuals with the use of the finite difference method based on a coordinate transformation to nonorthogonal curvilinear coordinate system to fit the boundary of an arbitrary shape is proposed to determine the stresses and strains in axisymmetric extrusion of rods through continuous dies. By using the body-fitted coordinate transformation, the finite difference expressions at and adjacent to the boundary do not need any interpolation between boundaries and interior grid points. The errors defined for governing equations and boundary conditions are minimized by introducing the method of weighted residuals. The work-hardening effect is incorporated by integrating the effective strain rate along the flow line. As computational examples, four types of dies with different reductions of area are chosen. Experiments are carried out for steel and aluminum alloys at room temperature. The computed results are compared with the experimental results as well as with those by the finite element method. The extrusion load and the flow pattern computed by the present method are in excellent agreement with the experimental results and those by the finite element method.

AbstractAn incremental formulation incorporating the effect of shape change is derived for the non-steady large deformation problem of a rate-independent isotropic elastic-plastic shell. The deformation during a step is studied using the natural convected coordinate system. To account for the spread of the plastic zone in the thickness direction, a layered model is used. Change of thickness is considered by using the incompressibility condition. The corresponding finite element equations are found in order to analyse the sheet metal deformation in which the effects of shape change and bending are significant. Based on the derived equations, the FEM program is developed to analyse the hydrostatic bulging process for rectangular diaphragms. The computational results are then compared with the available experimental results.

AbstractBy employing a method of numerical analysis based on the method of weighted residuals, the stresses and strains in three-dimensional cold extrusion of arbitrarily shaped sections from round billets through continuous dies are calculated with sufficient accuracy. The previous method using global flow functions lacks generality for its application. In the present work, instead of using global flow functions the velocity components and stress functions are taken in the form of global functions expressed in polynomial terms for the generalized three-dimensional plastic flow. The velocity boundary condition that the velocity component normal to the die surface and the velocity component normal to the plane of symmetry should be zero is considered by additional error equations. The stress and boundary conditions are also considered simultaneously. The errors defined for the governing equations and the boundary conditions are minimized by introducing the method of weighted residuals. The work-hardening effect is considered by integrating the effective strain rate following a streamline. As computational examples for arbitrarily shaped products, ellipse, clover, rounded square and trocoidal gear section are chosen for the extruded sections. Experiments were carried out for an aluminum alloy at room temperature. The computed results are compared with the experimental results as well as with those by the finite element method. The extrusion load and the flow pattern computed by the present method are in good agreement with the experimental results and those of the finite element method.

AbstractThe three-dimensional rigid-plastic finite element method is used to simulate the open pass profile ring rolling of a T-shaped section from an initially rectangular cross-section. Because the ring rolling process is incremental and the deforming region is restricted to the vicinity of the roll gap, only a ring segment spanning the roll gap is analysed in order to save computation time. As the thickness of the ring is reduced, the mesh system is progressively modified to treat profile development more effectively. Roll separating force, strain distribution, strain rate distribution and cross-sectional configuration of the deformed ring are calculated. Comparisons between computation and experiment show good agreement in roll separating force and cross-sectional configuration of the deformed ring.

AbstractA systematic approach of the energy method is proposed for analysis of three-dimensional sheet metal forming of noncircular cups with complicated shape. In the proposed method, the whole deforming region is divided into several zones by considering the geometric characteristics and contact boundary condition. The geometric shape is expressed by sweeping the section curves defined on the boundary of each zone. Velocity fields are expressed in a similar manner by sweeping the boundary velocity functions. The solution is found through optimization of the total energy dissipation with respect to some parameters assumed in the kinematically admissible velocity field defined over each zone. In order to check the effectiveness of the present method, three-dimensional deep drawing by the elliptic punch and the clover-type punch are analysed and compared with the corresponding experiments. In analysing the deep drawing process by the elliptic and clover-type punches, the total deforming region has been divided into five zones. The computed results are shown to be in good agreement with the experiment in punch load, deformed edge contour and distribution of thickness strain.

AbstractRecently the second-order sumdashfrequency generation (SFG) has become a powerful surface — spectroscopic technique. In this paper we present a molecular theory for SFG. To systematically treat this process, we have classified SFG into the resonance-resonance case, resonance-off-resonance case, off-resonance-resonance case and off-resonance-off-resonance case. Among these cases we may have IR-UV SFG and UV-UV SFG. Some of these SFG's are reported in this paper. In each SFG case, we have shown how to calculate the strength (or intensity) and band-shape function.

AbstractAn approach using the energy method is proposed for the analysis of three-dimensional sheet-metal forming. There are some limitations in analysis by the energy method, e.g., the analytical expression of the geometric configuration and the construction of a proper kinematically-admissible velocity field are very difficult and the estimation of strain rate at a point on the surface is very complicated. In the method suggested, the principal components of strain increment are calculated directly from the change of geometric profile for an arbitrary triangular element. The corresponding solution is found through optimization of the total energy consumption with respect to some parameters assumed in the kinematically-admissible velocity field and the geometric profile. In order to check the validity of the method proposed, the hydrostatic bulging of an elliptic diaphragm has been analyzed. In comparison of the computed results with existing experimental results, good agreement has been obtained for the pressure curve, the polar membrane strains and the strain distributions and it has thus been shown that the approach is applicable to the analysis of three-dimensional sheet-metal forming.

AbstractThe objective of the paper is to make comparison of the effectiveness of implicit, explicit and iterative implicit/explicit finite-element analysis methods for the analysis of static and dynamic sheet-forming problems. In the finite-element simulation of sheet-metal forming processes, the robustness and stability of computation are important requirements, since the computation time and convergency became major points of consideration due to the complexity of the geometry and the boundary conditions. The implicit scheme employs a more reliable and rigorous scheme in considering the equilibrium at each step of deformation, while in the explicit scheme the problem of convergency is eliminated at the cost of solution accuracy.In the present work the rigid-plastic finite-element method using both the usual membrane elements and bending-energy-augmented membrane elements (BEAM) is employed for computation. Computations are carried out for some typical sheet-forming examples by implicit, explicit, iterative implicit/explicit schemes, including deep-drawing involving two-dimensional hemispherical stretching, a square box and an oil pan. From comparison of the methods, their advantages and disadvantages are discussed.

AbstractIn order to manufacture a doubly curved sheet metal effectively, a flexible incremental roll forming process has been developed by adopting the advantages of the incremental forming process and the roll forming process by combining inherent flexibility of the incremental forming process and continuous deformation of the roll forming process. In the proposed roll forming system, a newly designed gripper system is attached in order to achieve automation and more precision manufacturing of the required sheet intended double curvature. The forming method has been further enhanced to form general quadrilateral blanks (including a square, a rectangle, a symmetrical trapezoid and an asymmetrical trapezoid, etc.) into doubly curved shapes by controlling the forming schedule developed by various experiments.

AbstractA flexible incremental roll forming process has been developed by adopting the advantages of the incremental forming process and the roll forming process: i.e., inherent flexibility of the incremental forming process and continuous bending deformation of the roll forming process. It has an adjustable roll set as a forming tool composed of one upper center roll and two pairs of lower support rolls, which plays a key role. Through the experiments, it is shown that the improved forming method of the proposed process is effective in the manufacture of various doubly curved sheet metals, and the proposed relationship of the experimental parameters and the radius of curvature of the formed sheet boundary is useful to control the final shape

In metal forming, there are problems with recurrent geometric characteristics and without explicitly prescribed boundary conditions. In such problems, so-called recurrent boundary conditions must be introduced. The present study deals with nonsteady-state three-dimensional finite element analysis for extrusion of a trocoidal helical gear through a curved die. The boundary-directed remeshing scheme based on the modular remeshing technique is developed to reduce the errors arising in fitting old and new mesh systems. The computed extrusion pressure in reaching the near steady-state loading stage is compared with the results of the experiment and the steady-state analysis. The three-dimensional deformed pattern involving warping at the extruded end due to torsional deformation mode is demonstrated.

AbstractHigh-quality cups of deep drawing ratio of more than four cannot be simply drawn by conventional drawing and redrawing process. In the present study, after the first deep drawing process, subsequent hydromechanical reverse redrawing with controlled radial pressure and final ironing to control the thickness and outer surface appearance are employed. In order to increase the deep drawing ratio much more than four, the radial pressure is controlled independently of the chamber pressure and thus an optimum forming condition can be found by varying the radial pressure. The final ironing process ensures the fine surface quality as well as improved deep drawing ratio without inducing any forming defect such as slight puckering and surface waviness. The process has been subject to finite element analysis by using the rigid-plastic material modeling considering all the frictional conditions induced by the hydrostatic pressure. The comparison of the computation with the experiment has shown that the finite element modeling can be conveniently employed for the design of the process with reliability from the viewpoint of formability.

SummaryIn cold lubricated extrusion of round tubes the material properties and surface quality of the extruded products are influenced by the die profile. Streamlined dies induce less redundant work and render desirable distribution of strength. In the present general analysis the shapes of the die and the mandrel are expressed by continuous functions. A new general kinematically admissible velocity field for the metal flow is proposed which permits the prediction of metal flow. The upper-bound extrusion pressure is found by using the upper-bound theorem. The grid distortion is computed tracing metal flow along the stream lines. The effective strains are computed to know the workhardening pattern of the extruded tube products. Experiments have been carried employing A12024 as working material. The effect of process parameters such as area reduction, die length, tubular shape ratio (thickness to outer diameter) and friction condition etc. on extrusion power and metal flow has been studied extensively. The theoretical predictions both in extrusion load and metal flow have been shown to be in good agreement with the experimental results. The suggested streamlined dies are shown to be effective for extrusion of round tubes rendering less extrusion power and desirable flow patterns.

AbstractIn this work, a novel forging technology has been developed to produce a magnesium alloy impeller with twisted blades of micro-thickness used in a fuel cell system. Due to the very complicated blades of the impeller, a specially designed split die was adopted for successful forging of the impeller. In this split die-set, the coherence of reinforcement ring and split dies is induced by forging pressure which increases with increasing forging load. Both the reinforcement ring and the split dies have appropriate tapers so that during the forging process they are consolidated to allow no gap between the split dies and are separated easily after forging. This novel technology can provide the near net-shaped impeller without the burr generation when split dies are used. Based on the newly developed technology, the impeller forging experiments were carried out using magnesium alloy AZ31 which had been microstructurally refined through equal channel angular pressing (ECAP). The tensile and thermal stability tests were carried out under various levels of temperature and strain rate to find optimum processing conditions for precision impeller forging. Finally, the changes in microstructure and microhardness at the various positions of the forged impeller were investigated to compare with those of the initial billet. It has thus been shown that the proposed new method is effective to achieve precision forging of a magnesium alloy with high precision.

AbstractAblation processes using various energy sources such as lasers, electrical power and ultrasonic sources have been widely used in industry to ablate workpieces made of hard-to-cut and temperature resistive materials. Even though the cutting mechanism of the ablation process is different from that of the mechanical cutting process, an identical toolpath of the mechanical cutting process has been applied to the ablation process. An inappropriate toolpath for the ablation process may result in a lower dimensional accuracy of the ablated part. Therefore, a new toolpath planning algorithm considering the characteristics of the energy source is required. In this paper, a toolpath planning algorithm for the ablation process is proposed. The proposed algorithm consists of three steps: (1) The generation of a valid toolpath element, (2) the storage of toolpath elements and the creation of sub-groups, (3) the linking of sub-groups. New guidelines on the toolpath demanded by the ablation process are studied. A new idea involving the use of a storage matrix is applied to the storage of toolpath elements and the creation of sub-groups. In order to verify the applicability of the proposed algorithm, the proposed toolpath planning algorithm has been implemented and tested with practical examples.

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