Key Engineering Materials Vols. 340-341

Abstract: An instrumented-drop weight impact test carried out at room temperature under a range of loading rates was applied to study the deformation and failure behaviors of PC/ABS (50/50) blends. Actually, these blends have different type of ABS, one grade of the blends is the blend containing small-sized particles of rubber and another grade is the blend containing larger-sized particles of rubber in the ABS systems. Testing results showed that both of the blends generally exhibited similar behaviors but they were totally different under 3 m/sec. A weld line-like formation captured on the fracture surface was found in each of the blends. Preliminary study using scanning electron microscope (SEM) indicated that crack also propagated along the weld line. Fracture of the blends might be initially induced due to fracture of the weld lines. Hence, it has been suspected as a factor affecting behavior of the blends. Since existence of the weld line-like formation has not been found in fractured ABS materials, it is necessary to point out the complex relationship among of the ABS composition, the existence of the weld line and the weld line quality in the blends. A finite element (FE) simulation of the testing was carried in order to determine whether the weld line strength is significant enough affects the behaviors. Although the model was generated using estimated failure criterion for the weld line, the simulation results showed that weld line strength might influence the blends behavior.

Abstract: For a quantitative analysis and better understanding of the stress-strain response and their effects on interface debonding and crack initiation and propagation of alternative hard and soft multi-layers under circumscription loading, to explore methodologies of optimum design of surface multi-layers, large strain elasto-plastic finite element method is used to evaluate the mechanical behaviors of TiN/Ti multi-layer films on rigid substrate under hard asperity indentation. The results show that with the same total thickness of the film, the alternate hard and soft multi-layers with larger ratio could endure larger circumscription loading, and the different total thickness of multi-layers could affect the circumscription loading greatly. So there must be a circumscription loading in this multi-layer film structures. Generally speaking, the circumscription loading has something to do with the material modulus, the radius of the press-head, the thickness ratio of soft and hard layers, and the total thickness of alternate hard and soft multi-layers. For a quantitative analysis of this problem, finite element method is utilized to analyze the phenomena of the plastic indentation. According to analysis of the film deformation, the maximum bending stress, and the surface bending stress, and those parameters that have influence on different alternative films are studied. The study of this paper is expected to provide some theoretical basis for the optimum design of multi-layer film structures.

Abstract: In this study, a general simulation code is developed to analyze the shrinkage effect of polymerization and optimize the fabrication parameters including the scanning path, exposure time and scanning speed for the stereolithography system. The code is based on the dynamic finite element method. Liquid element is preconstructed without curing properties till the absorption energy exceeds the critical value of dynamic stiffness matrix assembling. A unit element block is utilized with a weight coefficient for expressing a laser Gaussian energy distribution during the discretizing of the scanning path into increments. A fan blade is proposed to validate the agreement between the simulation and experimental results. The prototype is a fabrication and the surface of blade was measured by the digitizing system ATOS (Advanced TOpometric Sensor) for comparing the deformation with analysis prediction. Consequently, the simulated result closely conforms to the experimental result.

Abstract: A new elasto-plastic and geometrically nonlinear finite element model of space beam considering restraint torsion and the coupling effect of deformations is presented in this paper. The warping restraint torsion and the coupling effect of deformation are considered in the displacement formulation of arbitrary point on the space beam. The geometrical relationship of arbitrary point is derived according to the definition of Green strain. The elasto-plastic and geometrically nonlinear finite element model of space beam is derived using Updated Lagrange description. The effect of axial force, shearing force, biaxial bending moment, moment of torsion and bimoment is involved in the geometrical stiffness matrix of element. The yielding developments both across the section and along the axis of the member are taken into consideration by selecting Gauss points. The full historical nonlinear analysis is achieved using the method of load increment and modified Newton-Raphson method. The validity of the new model derived in this paper is proved by numerical example. This new model can be used in the elasto-plastic and geometrically nonlinear analysis of space beam structures constructed by the members of arbitrary cross section.

Abstract: This study aims to clarify the process conditions of the UO-tube of a sheet metal of steel. It provides a model that predicts not only the correct punch load for drawing, but also the precise final shape of products after unloading, based on the tensile properties of the material and the geometry of the tools used. An elasto-plastic incremental finite-element computer code, based on an updated Lagrangian formulation, was developed to simulate the UO-tube process of sheet metal; the results are compared with corresponding experimental results. Special care was taken to formulate accurate boundary conditions of penetration, separation and alternation of the sliding-sticking state of friction, as the contact conditions between the tools and the sheet varied throughout the entire processes of U-bending and successive O-bending. Calculated sheet geometries and forming force agree well with experimental data. In particular, selective reduced integration was adopted to formulate the stiffness matrix. The extended r-minimum technique was used to deal with the elasto-plastic state and contact problems at the tool-metal interface. A series of simulations were performed to validate the formulation in the theory, leading to the development of the computer codes. The whole deformation history, the distribution of stress and the distribution of strain during the forming process were obtained by carefully considering the moving boundary condition in the finite-element method. The simulation demonstrates clearly the efficiency of the code to simulate various bending processes that proceed under complicated deformation- and contact-history.

Abstract: Solid shell element models which possess only translational degrees of freedom and are applicable to thin structure analyses has drawn much attention in recent years and presented good prospect in sheet metal forming. In this study, a solid shell element model is introduced into the dynamic explicit elastic-plastic finite element method. The plane stress constitutive relation is assumed to relieve the thickness locking and the selected reduced integration method is used to overcome volumetric locking. The assumed natural strain method is adopted to resolve shear locking and trapezoidal locking problem. Two benchmark examples and a stage of roll forming process are calculated, and the calculating results are compared with those by solid element model, which demonstrates the effectiveness of the element.

Abstract: The effect of oscillation of internal pressure on the formability and shape accuracy of the products in a pulsating hydroforming process of T-shaped parts was examined by finite element simulation. The local thinning was prevented by oscillating the internal pressure. The filling ratio of the die cavity and the symmetrical degree of the filling was increased by the oscillation of pressure. The calculated deforming shape and the wall thickness are in good agreement with the experimental ones. It was found that pulsating hydroforming is useful in improving the formability and shape accuracy in the T-shape hydroforming operation.

Abstract: Loading path is one of the most influential parameter in tube hydroforming(THF) process. Load history has a major effect on failures such as buckling, necking, bursting, and so on. Because loading conditions that consist of axial feeding and internal pressure are imposed simultaneously. Therefore suitable loading path should be determined to prevent onset of failures i.e. bursting on final products. This paper deals with the procedure on determination of the loading path in order to ensure the robustness of the final products after the THF. In order to verify the availability and feasibility of the proposed methodology a subframe model of engine cradle module in automotive is implemented. In this study, thinning ratio and forming limit stress diagram is used to demonstrate the improvement of the finished product. The result shows that the developed algorithm has successfully promoted the effectiveness and feasibility in the THF. Consequently, it is shown that the automatic approach on the determination of loading condition which is proposed in this paper will provide a valuable method to satisfy the increasing practical demands for designing process condition in THF.

Abstract: In this paper, three-dimensional(3D) Numerical Manifold Method (NNM) based on hexahedron element cover with full first-order cover function is proposed and the shape function of C8 isoparametric element in FEM is used as the cover weight function. All sub-matrices in equilibrium equations, including stiffness matrix, initial stress matrix, point force matrix, surface force matrix, body force matrix, inertia force matrix, contact matrix and friction matrix, are derived. Different with 2D contact, the direction of shear stiffness and friction force can not be easily defined in 3D contact. A new iterative method based on vector theory to detect the contact direction is developed. The application of 3D NMM in underground excavation is also presented and show good agreement with real engineering.

Abstract: The 3-D rigid-plastic finite element method using a diagonal matrix was applied to parallel processing using a distributed memory type PC cluster. The cluster composed of cheap PCs becomes common as a low-cost system in the parallel processing. Since the computers in the distributed memory type PC cluster have individual memory units, the transfer of date among computers during the computation is required, and thus the time for the data transfer is taken into consideration. The renewal of data in each computation is limited because of the time of data transfer unlike the shared memory type workstation. This brings about the delay of data renewal. A data transfer scheme was investigated to optimize the total computational time in the parallel processing. The effect of the delay of date renewal on the convergence of the solution was examined in the simulation of upsetting of rectangular block with an inclined tool by means of a cluster composed of 4 PCs and 100MBit/s Ethernet.