Advances in Engineering Plasticity XII

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Authors: Bing Ye Xu, Ying Hua Liu, Xian He Du, Gang Chen
Abstract: Local wall-thinning, which can be found frequently on the surfaces of pipelines, may not only reduce the load-carrying capacities of pipelines, but also cause serious industrial accidents. In this paper, through a large number of computational examples, the effects of axial, circumferential, small area and large area local wall-thinning with different sizes on load-carrying capacities and failure modes of pipelines under both internal pressure and bending moment were investigated and evaluated. By data fitting, an engineering computational formula for plastic limit loads of pipelines with local wall-thinning was presented.
Authors: Hyun Sik Choi, Heung Nam Han, Dong Nyung Lee
Abstract: The (001)[110] orientation of Al single crystal is known to be metastable with respect to plane strain compression to form the (112)[1 1-1] and (112)[-1-1 1] orientations, known as the copper orientations. The copper orientations did not transform into the cube texture {001}<100> after recrystallization (Rex), at variance with expectation. When Al single crystals were plane-strain compressed and recrystallized (Rexed), their Rex orientation varied with reduction in thickness. These results are discussed based on the strain-energy-release-maximization theory, in which the Rex texture is determined such that the absolute maximum stress direction (AMSD) due to dislocations in deformed materials is parallel to the minimum Young’s modulus direction in Rexed grains and other conditions, whereby strain energy release can be maximized. AMSD is obtained from slip systems activated during deformation and their activities, which were calculated from the VPSC code.
Authors: Rong Shean Lee, Ta Wei Chien
Abstract: In most situations, original Cockcroft criterion underestimates material formability in the first quadrant of FLD. So far, some modified Cockcroft criteria have been reported for different applications. This presentation will focus on the modified Cockcroft criterion which takes strain-path effect into consideration. This paper demonstrates the accuracy of this criterion through limiting dome height test, free bulge test, and the biaxial tensile test using cruciform specimen respectively. The results showed that the modified Cockcroft criterion with strain path effect has good agreement with experimental results.
Authors: Yi Luen Li, Tsung Yu Chou, Ming Yuan Shen, Wei Jen Chen, Chin Lung Chiang, Ming Chuen Yip
Abstract: The surface modification of carbon nanotubes (CNTs) has been recently observed to influence the distribution of CNTs in epoxy resin and the mechanical properties and electrical conductivities of these CNTs. Accordingly, the treatment of CNTs to with organic acids to oxidize them generates functional groups on the surface of CNTs. This investigation studies the consequent enhancement of the mechanical properties and electrical conductivities of CNTs. The influence of adding various proportions of CNTs to the epoxy resin on the mechanical properties and electrical conductivities of the composites thus formed is investigated, and the strength of the material is tested at different temperatures.The test results also indicate that mechanical strength and electrical conductivity increase with the amount of CNTs added to the composites. Different coefficients of expansion of the matrix, fiber and CNTs, are such that overexpansion of the matrix at high temperature results in cracking in it.Moreover, the creep behaviors of carbon fiber (CF) /epoxy resin thermosetting composites and CNTs/CF/ epoxy resin composites were tested and analyzed at different stresses, orientations of fiber, temperatures and humidities. The creep exhibits only two stages-primary creep and steady-state creep. The effects of creep stress, creep time, and humidity on the creep of composites that contain various proportion of CNTs were investigated at various temperatures.Additionally, increasing the number of cycles in cyclic creep tests at room temperature resulted in a decrease in creep strain even at a high temperature of 55°C. Possible room temperature creep mechanisms have been proposed and discussed. With increasing number of creep tests, the creep strain decreased due to strain hardening which occurred during creep. Creep strain is believed to increase with applied stress, creep time, humidity, temperature and degree of the angle θ between the orientation of fiber and the direction of the applied stress.Finally, the test results of creep strain of CF/epoxy resin composites and CNTs/CF/epoxy resin composites tested under various conditions can be smoothly fitted by the fitting curves of Findley power law.
Authors: Keita Goto, Tetsuya Matsuda, Naoto Kubota
Abstract: A fully-modeled unit cell analysis is performed to investigate the macroscopic and microscopic elastic-viscoplastic behaviors of a quasi-isotropic carbon fiber-reinforced plastic (CFRP) laminate. To this end, a quasi-isotropic CFRP laminate and its microstructure composed of carbon fibers and a matrix material are considered three-dimensionally. Then, a hexagonal prism-shaped unit cell fully modeled with fibers and a matrix including interlaminar areas is defined. For this quasi-isotropic laminate, a homogenization theory for nonlinear time-dependent composites with point-symmetric internal structures is applied, enabling us to analyze both the macroscopic and microscopic elastic-viscoplastic behaviors of the laminate. The substructure method is introduced into the theory to reduce computational costs. The present method is then applied to the elastic-viscoplastic analysis of a quasi-isotropic carbon fiber/epoxy laminate subjected to an in-plane uniaxial tensile load, to investigate the macroscopic elastic-viscoplastic behavior of the laminate and the microscopic stress and strain distributions in them.
Authors: C.P. Lai, Luen Chow Chan
Abstract: The titanium tailor-welded blanks (Ti-TWBs) are being developed in different industries such as automobile and aerospace, combining the advantages of both tailor-welded blanks technology and titanium alloys. In recent decades, computer simulation of sheet metal forming processes has been employed increasingly over conventional production test and adjustment methodology to achieve the optimum and cost-effective operation procedures. Recently, certain amounts of theoretical analysis for the sheet metal forming process have been developed. However, these analyses could not be applied directly to the material under multi-stage forming process. Thus, some researchers have developed a damage-based model to predict the instability and failure of sheet metals, particularly for the above Ti-TWBs, with consideration of material damage under discontinuous or proportional loading strain paths. So far this model has been used and proved to be successful to predict formability of selected sheets of steel and aluminium alloy. However, the application of the damage-coupled model has yet to be extended to the Ti-TWBs under thermal multi-stage forming operation.The main objective of this paper is to investigate numerically the formability of Ti-TWBs under multi-stage forming process with experimental verification. Titanium alloy sheets (Ti-6Al-4V) in thickness of 0.7mm and 1.0mm were selected and laser-welded the specimen of Ti-TWBs. The model based on the damage mechanics is introduced to predict the thermal formability of Ti-TWBs with change of strain paths. In this study, the anisotropic damage model incorporate with the finite element codes and user-define material subroutine were developed to predict the formability of Ti-TWBs with change of strain paths. The mechanical properties and damage parameters of Ti-TWBs for the simulation were measured experimentally. The simulation of Ti-TWB under multi-stage forming process were then conducted and validated experimentally at similar forming conditions. The predicted results have been found to agree well with those obtained from the experiments. This analysis can be applied readily to design and manufacture TWB components or structures so as to satisfy the need of such market demands.
Authors: Yeong-Maw Hwang, Shin Yan Hsieh, Ming Chung Chen
Abstract: Tube hydroforming is a relatively new approach to manufacture light weight metal structures. As the manufacturing technology became advanced, demands of lighter and stronger metal structures are also increasing. This research is to use tube hydroforming to expand the tube of a fuel filler for cars. The pipe material is JIS G3141. The product has a large expansion ratio at the edge of the pipe. Traditional tube hydroforming is difficult to achieve this large expansion ratio of a metal product. Tube hydroforming with movable dies is proposed to enhance the capacity of tube hydroforming technology. With movable die design, product with more uniform thickness can be obtained, and the forming pressure becomes lower than that needed in tube hydroforming without movable dies. A finite element code DEFORM-3D is used to analyze the plastic flow pattern of the tube. The loading paths of internal pressure, axial punches and movable dies for obtaining a sound product are determined by an adaptive simulation algorithm. The thickness distributions of the product for different loading paths are discussed.
Authors: Chao Chang Arthur Chen, Chun Chieh Chao, Kuo Wei Huang, Wei En Fu
Abstract: This paper is devoted to investigate an estimation methodology of micro hardness and Young’s modulus of reacted passivation layer of deposited copper thin film in dry and wet environment based a tip-grit atomic force microscope (TGAFM) scratch on copper thin film of silicon wafer. The TGAFM is a modification or attachment of a nanoor micron grit on the tip apex or cantilever beam of a closed-loop control AFM instrument. In this study, a diameter 800 nm SiO2 grit glued on the apex of the tip of AFM is used for experiment. Tip force model has been developed based on Hertzian model and Tresca criterion for stress-strain relationship from the geometries of scratch groove, depth and width to evaluate about microhardness and Young's modulus of copper thin film in regular air and DI-water. Experimental results show that the microhardness (H) is 1.62GPa and the Young's modulus (E) is 160.52GPa of copper thin film in DI-water environment. These mechanical properties of copper thin film is larger than the H= 1.52GPa and E= 126.04GPa for dry environment. Results of this study can be further explored to the grit force reaction on the passivation layer of copper film of chemical mechanical planarization (CMP) process development for semiconductor industry.
Authors: Chang Cheng Chen, Yi Xuan Qiu
Abstract: This paper proposes an integrated computer-aided design and computer-aided manufacturing procedure to produce the forging die for experiment, and the forging formability of micro stepped gear was explored. In the forging process, this paper proposes a novel hybrid forging method with a single die set which would execute two forging stages including the first stage of Upsetting and the second stage of Clamping type Gear forging. In the case study, the materials were fully filled into tooth cavity of small gear of module 0.12mm. On the other hand, the estimated unfilled rate of bigger gear is less 8 to 12%, showing this method is exactly feasible.
Authors: Cho Pei Jiang, Zong Han Huang
Abstract: The aim of this study is to investigate the effect of grain size on mechanical properties of commercially pure grade 2 (CP2) titanium bar with a diameter of 5 mm. The results reveal that the microstructure of β-phase forms when the annealing temperature exceeds 800oC. The formation of β-phase leads to reduce the ductility but increase hardness. The strength coefficient, yielding stress and hardness decrease with increasing of grain size when the microstructure of specimen is the α-phase.

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