Advances in Engineering Plasticity XII

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Authors: Sergei Alexandrov, Elena Lyamina, Hguyen Minh Tuan, Natalia Kalenova
Abstract: Solutions for many rigid/plastic models are singular in the vicinity of maximum friction surfaces. In particular, the magnitude of the equivalent strain rate near such surfaces is controlled by the strain rate intensity factor. This factor is the coefficient of the leading singular term is a series expansion of the equivalent strain rate in the vicinity of maximum friction surfaces. Since the equivalent strain rate has a great effect of material properties, it is of important to reveal the dependence of the strain rate intensity factor on parameters characterizing material models. In the present paper, quite a general model of anisotropic plasticity under plane strain conditions is adopted. Then, using an analytic solution for instantaneous compression of a layer of plastic material between two parallel plates the effect of the shape of the yield locus on the asymptotic behavior of the equivalent strain rate in the vicinity of the friction surface is demonstrated.
Authors: Ruho Kondo, Yuichi Tadano, Kazuyuki Shizawa
Abstract: A coupled model based on crystal plasticity and phase field theories that express both plastic anisotropy of HCP metals and expansion/shrinkage of twin-bands is proposed in the present study. In this model, the difference of the hardening rate in each slip system is expressed by changing their dislocation mobility as a numerical parameter defined in the crystal plasticity framework. The stress calculated via crystal plasticity analysis becomes to the driving force of multi-phase filed equations that express the evolution of twin bands of several variants, which include both the growth and shrinkage. Solving this equation set, the rate of twinning/detwinning and the mirror-transformed crystal basis in the twinned/detwinned phase are obtained and then, crystal plasticity analysis is carried out again. Using the present model, a uniaxial cyclic loading simulation along [0001] direction on the specimen including two variants of twin-bands is carried out by means of finite element method (FEM). The results show that the detwinning stress decreases with increase of the pre-tensioned strain. This is caused by a residual compression stress resulting from the twin shearing that occurs in the vicinity of two twin boundaries approaching each other.
Authors: Soo Sik Han
Abstract: The empirical approach was adopted to investigate the process parameters that effect on the quality of high strength steel sheet sheared edge. As a result of study, the quality of sheared edge that can be represented as the ratio of the shiny burnished surface to the sheared surface was not under influence of die clearance, lubrication and cutting line direction but under influence of the material strength and the usage of lower pad. The roll-over and burr height is highly affected by the die clearance and the usage of lower pad. The lower pad underneath of blank is of help to reduce the amount of roll-over and burr even at large die clearance.
Authors: Ling Ling Hu, Li Wen Xiong, T.X. Yu
Abstract: A numerical model of the aluminium foam with voronoi cells is built and uni-directionally crushed with various velocities from 1m/s to 110m/s. It is shown that the foam deforms homogeneously within the whole specimen and the stress in the foam increases gradually with the strain without an obvious plateau stage under the low-velocity compression, while the deformation is concentrated within a zone near the impact end and an obvious plateau stage can be found in the stress-strain curves of the foams under the high-velocity crushing. By analyzing the distribution of the density within the foams using the digital image processing technology, the densification strain of the foams under dynamic crushing can be determined. Then combining the foam’s stress-strain curve under the low-velocity compression, the dynamic plateau stress of the foams can be predicted. It is shown that both the densification strain and the plateau stress of the foams under the high-velocity crushing predicted by employing the digital image process technology are in good agreement with the numerical simulations. The results show that both the plateau stress and the densification strain of the foams increase with the impact velocity, which is essentially caused by the localization of the foam’s deformation under dynamic crushing.
Authors: Yelm Okuyama, Tetsuya Ohashi
Abstract: Plastic deformation and dislocations accumulation in a steel alloy dispersed with vanadium carbide particles is numerically analyzed by a crystal plasticity finite element technique and work hardening characteristics are discussed. Increment of dislocation density that contributes to work hardening is calculated from the mean free path of dislocations. The mean free path is defined by the spacing of forest dislocations and the average spacing of dispersed particles. Obtained yield stress and work hardening characteristics was close to that of experimental result, except that the value of work hardening rate was higher than that of experimental one.
Authors: Gow Yi Tzou, Yeong-Maw Hwang, Sai Chih Pan
Abstract: The study aims at exploring the effects of rotating compression forming on the material behaviors of cylinder. The experiments have been carried out under various angular velocities to realize the flow stress, the compression force, the rotating torque, the grain size, and the hardness. Furthermore, the FEM simulation is also performed to compare with the experiments. From the experimental results, as the angular velocity increases, the compression force and the grain size decrease, but the rotating torque and the hardness increase. The results can provide the industry as the reference.
Authors: Rong Shean Lee, Ta Wei Chien
Abstract: This paper presents a new method concerning testing formability in sheet metal forming, especially focuses on clarifying the divergence of the experiment and a variety of theoretical predictions on biaxial tensile state. Up to now, there are many different fracture criteria appeared. All researches have presented their experimental data which could justify the criterion they presented. However, the experimental results and predictions in the first quadrant of the forming limit diagram (FLD) often diverge. Today, limiting dome height test is commonly used for FLD experiment, but specimens are rubbed and bended during the test, both influencing the experimental results.In order to provide for convincible experimental data, this paper presents a new experimental method to establish the first quadrant of FLD. In this method, cruciform biaxial tensile specimen and biaxial tensile apparatus have been developed. The proposed specimen has the feature of thickness reduction and contour design to ensure the fracture location is in the central region, so that accurate biaxial tensile state can be obtained. Through this method, there is an opportunity to obtain the whole FLD using uniaxial tensile testing machine, which is a low-cost alternative in compared with limiting dome height test. Besides, the experimental results can be utilized to clarify the divergence between various theoretical predictions and experimental results in the first quadrant of the FLD.
Authors: Ryo Ueta, Kazuyuki Shizawa
Abstract: A three-dimensional compression analysis is performed by finite element method using a dislocation-based crystal plasticity model to clarify the formation mechanism of kink band in a polycrystalline Mg alloy with a long-period stacking ordered structure (LPSO) phase. The crystalline structure of LPSO phase is regarded as a HCP for simplicity, however, any deformation twinning is not taken into account. In addition, the activities of non-basal systems are considerably limited in the LPSO phase setting the values of their critical resolved shear stresses to large ones. We analyze a simple polycrystalline specimen composed of two α-Mg matrix phases and a LPSO phase both having a rectangular shape and twist grain boundaries are introduced into the interface. The obtained result shows that the kink band formation in the alloy is accomplished by the basal slips with different variants and the non-basal slips are activated on the grain boundary to maintain the continuity of deformation.
Authors: Gow Yi Tzou, Yeong-Maw Hwang, Hsiang Yu Teng
Abstract: This study establishes an analysis model based on the slab method assuming constant shear friction. The rotating compression forming of the double-layers bounded clad ring is derived by considering von Mises’ yield criterion with the shear stress. Effects of the frictional factor, shear yield stress ratio, rotating angular speed of the ring, frictional factor ratio, clad ring height ratio etc on compression characteristics are investigated effectively. Moreover, the compression pressure, radial stress for each layer, radial stress of the whole layer, and compression force are also predicted effectively. Finally, the FEM simulation is carried out to compare with the slab method to realize the variations of both models.
Authors: Hui Wen Hu, Ting Yu Chen, Sheng Yuan Wang, Tien Yo Ho
Abstract: This research aims at the development of faucet using the techniques of hydroforming and bending. In this study, a tube made from stainless steel SUS 304 is used. Finite element model, including tube, dies and punches, are established using a commercial code LS-DYNA. Tensile test is used to obtain the material properties especially in true stress-strain curve. Piecewise linear plasticity model is used to simulate the plastic deformation of material during the forming process. The initial internal pressure is designed by using the theory of thick-walled cylinder subjecting to internal pressure only. Simulation is first used to find the optimal loading conditions for hydroforming and bending forming. Experiment is then performed to fabricate the prototype of faucets using the simulated loading parameters. The results show that good correlation of the distributed thickness and profile dimension between simulation and experiment are obtained.

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