Advances in Engineering Plasticity XII

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Authors: Wei Dong Liu, Liang Chi Zhang
Abstract: Precision glass moulding is a technique that enables the production of optical lenses of complex geometries in a single step. However, it has been reported that the product quality highly depends on the properties of a raw material, the design of a die, and the selection of a processing program. This paper aims to reveal the formation mechanism of the residual stresses by optical lens moulding. To this end, a modulus-based constitutive model was developed to integrate with the deformation and stress analyses by the finite element method. The investigation showed that the residual stresses are caused by the variability and heterogeneity of thermal expansion in a lens, but that they can be reduced effectively by decreasing the rate of cooling within the stage from the molding temperature to the glass transition temperature.
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Authors: Takeshi Uemori, Hiroshi Miyake, Tetsuo Naka, Michihiro Takiguchi, Fusahito Yoshida
Abstract: The framework for the complex step derivative approximations (hereafter CDSA) to calculate the consistent tangent moduli is studied. The present methods is one of the most effective methods to implement any material constitutive equations to the commercial finite element codes and does not suffer from calculation conditions and errors. In order to confirm the efficiency of CDSA, we developed the user subroutine code based on the CDSA using associative J2 flow rules with general nonlinear isotropic hardening rules that is commonly and widely utilized in commercial finite element codes. In this study, the user material subroutine ‘Hypela2’ of MSC.Marc (ver.2013.0.0) was utilized. The finite element calculation result by the proposal method shows a good agreement with the corresponding result by the MSC.Marc default setting. Also we apply the Yoshida-Uemori back stress model to the CDSA and evaluate this new technique to predict the deformation behavior of high tensile strength steel sheet.
187
Authors: Hideyuki Hara, Kazuyuki Shizawa
Abstract: The fracture of ductile polymers occurs on the boundary between the molecular chain-oriented and non-oriented regions after the neck propagation. This behavior is caused by the concentration of craze that is a microscopic damage typically observed in polymers. In addition, it is known that the ductility of polymers decreases both at a high and a low strain rates in comparison with that at a middle one. In this paper, FE simulations are carried out for a crystalline polymer subjected to the tensile load at some strain rates by use of a homogenized molecular chain plasticity model and a craze evolution equation based on the chemical kinetics. Furthermore, failure criteria are proposed from an experiment on fibril strength. A fracture prediction based on the craze accumulation and the failure of fibrils is demonstrated applying the criteria to the numerical results. It is indicated that the fracture occurs at a smaller strain under a high and a low strain rate conditions than under a middle one.
193
Authors: Dyi Cheng Chen, Wen Hsuan Ku
Abstract: This study uses the three dimensional finite element code to examine the plastic deformation behavior of bicycle front fork forging. First the paper used Solid works 2010 3D graphics software to design the bicycle front fork die, and that used rigid-plastic model finite element analytical methods, and assuming mode to be rigid body. The front fork material is titanium alloy Ti-6Al-4V. A series of simulation analyses in which the variables depend on die temperature, billet temperature, forging speed, friction factors, die angle are reveal to effective stress, effective strain, die radial load distribution and damage value for bicycle front fork forming. The simulation combined Taguchi method to analysis optimization. The results of the analysis can be used to stabilize finite element software to forming front fork, and also confirm the suitability of bicycle front fork through experiment optimization.
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Authors: Jinn Jong Sheu, Chia Cheng Hsieh, Cheng Hsien Yu
Abstract: In this paper, the main focus is to demonstrate a systematic method of designing the roll flower and corresponding dies for a double-gutter frame applied to the drawer slider. The blank development method was proposed to calculate the width of blank for bending with small radii. The cold roll forming process design was based on the maximum longitudinal strain minimization and the interference of rolls with the double-gutter geometry of product profile. Extra pre-bending was designed to avoid the occurrence of blank collision during forming process. The FEM method was adopted to evaluate the process and die designs. Due to the geometry complexity of the product, strain distribution is uneven which results in more spring back and stress concentration. A geometry setting design was proposed to create local strain redistribution and smooth strain distribution of entire section profile after final forming step. Using geometry setting die design, the spring back at the end point and the gutter areas of final product section are 0.07 mm and 0.1 mm, respectively. Without the geometry setting die design, the spring back at the end point and the gutter areas of final product section are 0.11 mm and 0.15 mm, respectively. The simulation results demonstrate the proposed methods are able to improve the accuracy of cold roll forming products.
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Authors: Cheng Hsien Yu, Jinn Jong Sheu
Abstract: Cold forging die design and process simulation were studied in this paper for a disk with center boss and outer ring gear. The complexity of part geometry results in defects of under-filling and folding. The material flow interference in the radial and the axial directions at the corner areas is the main reason of the occurrence of defects. A multi-stage cold forging process was proposed to control the material flow and volume distribution simultaneously. FEM simulations were carried out to evaluate the designs of process and die. The proposed preform and web geometry designs were able to decrease the forging load and control the material flow. The simulation results showed the proposed methods were able to make this forged part without defects.
211
Authors: Liang Chi Zhang
Abstract: High speed cutting (HSC) has been used widely in the metal machining industry. However, applications and investigations have shown that to optimize an HSC process, many issues need to be understood, such as the fluctuations of cutting forces and residual stresses. Extensive studies have found that these fluctuations are originated from the shear banding during chip formation and from the work-material properties influenced by the coupled attack of high strain rate and high temperature rise during cutting. Due to the complexity of the material deformation mechanisms during HSC, both experimental examination and theoretical analysis are essential. This keynote presentation reviews an integral approach of the author’s team to establishing the investigation chain of experimental analysis, constitutive modelling and numerical simulation to tackle the intricacy of HSC-induced deformation. It points out that while an experimental examination can provide insightful understanding of the deformation mechanisms, it is often limited to a narrow range of testing conditions. A numerical simulation can overcome such experimental difficulties through large-scale parametric studies, but can also bring about erroneous results if the constitutive behavior of a workpiece material is improperly described.
217
Authors: Chu Han Wu, Liang Chi Zhang, Shan Qing Li, Zheng Lian Jiang, Pei Lei Qu
Abstract: This paper investigates the effect of the plastic deformation of surface asperities on the interface friction in metal forming involving multi-scale deformation with random surface topography. The equivalent interfacial layer (EIL) introduced by the authors previously was used to integrate the Reynolds equation with the plastic deformation of the randomly distributed surface asperities. The contributions of solid-lubricant interaction, lubricant viscosity and microscopic deformation were therefore included efficiently in a conventional macroscopic finite element analysis. The merit of the method was demonstrated by an investigation into the metal strip rolling, whose friction, lubrication and pressure distribution are otherwise hard to be characterized accurately.
222
Authors: Kazuki Fujita, Keizo Nishikori, Takeshi Iwamoto
Abstract: In various kinds of shape memory alloy (SMA), Fe-based SMA (Fe-SMA) shows smaller shape memory effect compared with the other SMAs. However, Fe-SMA shows huge advantages on the excellent formability, machinability, etc. Moreover, its production cost is cheaper than other SMAs; therefore, the alloy is attempted to be applied to structural members such as joints and dampers. Since bending deformation at higher deformation rate is generated in the members, especially the joints, due to impact force such as earthquake or wind, a clarification on the bending strength of the joints at various deformation rate is strongly required. In this study, at first, it is attempted that the bending strength and its rate sensitivity of the joints which consist of Fe-based SMA are experimentally estimated by the three-point bending test at various deformation rate. Then, the force balance equation is challenged to be derived to predict the bending strength.
228
Authors: Masaomi Nishimura, Yu Takagi, Masahiro Arai
Abstract: Axial compressive simulations are performed on defective and non-defective multiwalledcarbon nanotubes (MWCNTs) using the molecular dynamics method, and the effectof defects upon the buckling behavior is discussed. In our previous study, changes in atomicstresses in MWCNTs with three layers were evaluated until buckling occurred. That studysuggested that the transition from homogeneous stress distributions to inhomogeneous onesplays an important role in the occurrence of buckling in MWCNTs, though the critical stressesor strains relating to buckling are dependent upon the structure and location of defects. In thepresent study, the atomic elastic stiffness of each atom, B ij , is evaluated to discuss the onsetof local buckling in MWCNTs with five layers. The det(B ij) of all atoms is found to change toa negative value long before buckling occurs, while the second smallest eigenvalues of B ij forsome atoms change to a negative value just prior to buckling. The existence of dense regions ofatoms that have two negative eigenvalues of B ij are found to vary as a function of the defectlocation, and to correspond with onset points of local buckling.
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