Large-area micro surficial features can realize different functions such as reducing the flow friction and improving the hydrophobicity. However, an evident challenge is how to realize the ...fabrication of thousands of micro features on large area in an efficient and low-cost manner. Roller microforming process is a satisfying method in that regard. Since the dimensions of micro features are in the same level with the grain size, the so-called size effect occurs during the microforming process. To understand how the size effect influences the roll-to-plate (R2P) micro imprinting process, experimental setup was established to form micro riblet features onto the surface of copper plate. The imprinted feature height is revealed to increase with the grain size. The flow stress reduction with the increase of grain size was identified to be the major reason for the increase of forming height at the smaller grain size condition. However, an evident underestimation was revealed for the case with the grain size of 180 μm. A further study was conducted by considering the crystalline microstructures via the Voronoi algorithm. The predicted forming heights were found to agree with the experiments, suggesting the inhomogeneous deformation of individual grains could improve the formability of material in the R2P microimprinting process. The uneven nature of individual grains actually lowers the difficulty of material flow since the deformation is easier to initialize and localize at weaker grains during imprinting.
A MATLAB algorithm was developed to insert cohesive elements at the interfaces between the matrix and precipitates in two dimensional (2D) representative volume elements (RVEs) of metals. The RVEs ...were created using OOF2 and imported into the “Complete ABAQUS Environment” (CAE) interface. These RVEs are based on actual images of the metal at the microscale, where the precipitates have irregular shapes. The RVEs contain precipitates that are dispersed into matrix materials. Commercial finite element (FE) ABAQUS software does not provide the option to automatically generate cohesive elements at the interfaces. The presented algorithm enables the insertion of cohesive elements at the interfaces between the matrix and precipitate in a convenient manner. This algorithm enables the simulation of the fracture process, including initiation at the interfaces and propagation at microscale, for metals that contain precipitates and/or particles. This algorithm extends the simulation capabilities of the FE solver ABAQUS.
The modified Gurson–Tvergaard–Needleman model, that considers shear decohesion as an increment in the total void volume fraction, is used to investigate the effect of superimposed hydrostatic ...pressure on the ductility of a round bar specimen. Experiments indicate that the fracture mode is not uniform across the cross section of a round bar specimen–no shear damage exists at the center while it is pronounced at the edge. The damage mechanisms of shear decohesion and void growth are delayed by applying superimposed hydrostatic pressure, which leads to an increase in the fracture strain and ductility. However, the effect of pressure on ductility depends on the location within the cross section. Void growth at both the center and the edge becomes increasingly delayed with increasing pressure, but this effect is more significant at the edge because of the existence of shear stress. Shear decohesion at the edge becomes the dominant damage mechanism, rather than void growth, when pressure is increased. The numerical results presented herein are used to explain and discuss this phenomenon in depth.
•Local stress-strain curves deviate from the traditional ones in warm tensile tests.•Two methods were proposed to correct the traditional rate-dependent tensile curves.•A modified Voce-type function ...was employed to verify the accuracy improvement.•Better agreement of the hardening behaviors in FE and experiments was observed.
The constitutive behavior of AA7075 aluminum sheet like other metallic sheet materials is highly temperature and strain rate dependent at elevated temperatures. The uniaxial tensile test is commonly employed to characterize the temperature and strain rate dependent deformation behavior. However, the traditional uniaxial tensile testing and data analysis procedure treats the gauge length of sample as a uniformly-deforming (or homogeneous) region. This may not be accurate due to early localized deformation that is initiated in the gauge region at elevated temperatures. Treating the post-necking hardening behavior of uniaxial tensile samples has been an important task to expand the effective range of true stress-true strain curves at elevated temperatures for metal and alloys. To explore this issue, uniaxial tensile tests on AA7075-F sheets (with as-fabricated temper) were conducted at 673o K and four different test speeds. The test procedure also utilized an on-line 2-D digital image correlation (DIC) system method to obtain full-field strain map from the deforming gauge region of the test specimen. Local stress-strain curves at different points along the gauge length were then calculated based on the recorded local in-plane principal stress and strain values from macroscopic force data and current cross-section of the specimen at the chosen points. In this manner, the DIC strain analysis not only enabled the determination of stress-strain response but also the determination of strain rates at specific chosen points along the gauge length. The local stress-strain curves revealed significant deviation at an earlier stage of deformation from the global (or macroscopic) stress-strain response based on the entire gauge region. To estimate the constitutive behavior at different strain rates, a correction method to traditional stress-strain curves was applied to obtain a family of stress-strain curves at different constant strain rates based on the local stress-strain curves. Additionally, the material deformation response in the stress, strain and strain rate space in the 3D Cartesian coordinate system was also constructed for tests conducted at different stretching speeds. Further, a verification test was conducted to see if the above methodology could provide an accurate estimation of local deformation behavior of AA7075 sheet. Furthermore, the above experimental method was also coupled with the Gurson-Tvergarrd-Needleman (GTN) ductile void damage model and incorporated into ABAQUS-Explicit general purpose finite element (FE) analysis code via a VUMAT material subroutine. Good agreement was obtained for both the local and global stress-strain curves between the FE test simulation and experimental stress-strain results. The proposed experimental methodology and the resulting stress-strain-strain rate behavior is believed to be applicable in describing the strain rate-dependent constitutive behavior of AA7075-F sheet metals at 673o K.
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It is generally accepted that the superimposed hydrostatic pressure increases fracture strain in sheet metal and mode of fracture changes with applying pressure. Void growth is delayed or completely ...eliminated under pressure and the shear damage mechanism becomes the dominant mode of fracture. In this study, the effect of superimposed hydrostatic pressure on the ductility of sheet metal under tension is investigated using the finite element (FE) method employing the modified Gurson–Tvergaard–Needleman (GTN) model. The shear damage mechanism is considered as an increment in the total void volume fraction and the model is implemented using the VUMAT subroutine in the ABAQUS/Explicit. It is shown that ductility and fracture strain increase significantly by imposing hydrostatic pressure as it suppresses the damage mechanisms of microvoid growth and shear damage. When hydrostatic pressure is applied, it is observed that although the shear damage mechanism is delayed, the shear damage mechanism is dominant over the growth of microvoids. These numerical findings are consistent with those experimental results published in the previous studies about the effect of superimposed hydrostatic pressure on fracture strain. The numerical results clearly show that the dominant mode of failure changes from microvoid growth to shear damage under pressure. Numerical studies in the literature explain the effect of pressure on fracture strain using the conventional GTN model available in the ABAQUS material behavior library when the mode of fracture does not change. However, in this study, the shear modified GTN model is used to understand the effect of pressure on the shear damage mechanism as one of the individual void volume fraction increments and change in mode of fracture is explained numerically.
Bipolar plates made of ultra-thin titanium sheet metals feature high corrosion resistance, electrical conductivity and strength-weight ratio. They are considered as a satisfying candidate for the ...proton exchange membrane fuel cell (PEMFC) used in vehicles. Bipolar plates have fine and complex sub-millimeter channel features which are employed to guide the flow of hydrogen, oxygen and cooling medium, to support the reaction space, to provide sealing structure, etc. Stamping process is an efficient method to fabricate those micro features with high precision and efficiency. However, the aspect ratio of those micro features is limited by the low forming limit of titanium sheet metals. Improving the aspect ratio of metallic bipolar plates could facilitate the uniform heat and mass transfer and further improve the efficiency and performance of PEMFC. Hence a challenging issue for the stamping process of bipolar plates is how to further improve the limit aspect ratio after forming. The multistage forming process is an efficient method to increase the ultimate forming depth and the accuracy by reducing the deformation localization tendency. This work aims to investigate the applicability of that process in fabricating high-aspect-ratio micro-channels on titanium sheet metals. Single stage forming experiments of the micro channels featured with fillet radius of 0.15 mm are first conducted. It is found that the ultimate forming depth of the channels parallel to the rolling direction is 12.67% higher than in the transverse direction. Furthermore, a series of punches and dies with three different fillet radii are employed in the multistage forming process. The ultimate depth of formed channels is revealed to increase from 438.1 to 621.0 μm, i.e. the aspect ratio from 0.46 to 0.67, comparing to the single stage forming process. Hence titanium bipolar plates with a higher aspect ratio could be achieved by utilizing the presented method. The contact resistances of the samples fabricated via single and three-stage forming were further tested to find out that the samples with a higher aspect ratio formed in a three-stage manner have an evident lower contact resistance.
•Three-stage forming of micro channels of titanium bipolar plates was investigated.•The limit forming height was improved for 42.4% comparing to single stage stamping.•The effect of multistage forming on the thinning of sheet was revealed.•The contact resistance with carbon paper decreases comparing to single stage samples.
The springback behavior has been revealed to affect the dimensional accuracy of sheet metal products significantly in the forming process. As the forming scale decreases to micro/meso level, the ...deformation behavior of sheet metals has been reported to be different from that at the macro level due to the so-called size effect. This study thus aims to investigate and characterize the springback behavior of sheet metals affected by the size effect at micro/meso scale. The copper specimens with different thicknesses and grain sizes were employed in the uniaxial tensile, nano-indentation and micro hardness tests to analyze the size effect on the material elastic/plastic deformation behavior at first. The flow stress, yield strength and hardness are revealed to decrease with the increase of grain size. Following that the bending experiments with three different punch angles were performed. The effects of both feature and grain sizes on the springback angle were investigated. The springback angle is found to decrease with the punch angle, the grain size, the ratio of thickness to grain size (t/d) and the reduction of thickness. To dig the fundamental mechanism behind the size-effect affected springback behavior, a composite model based on the surface layer theory was employed to describe the constitutive plastic deformation of material with different grain size and thicknesses. The model was further applied in the finite element (FE) simulations of bending process. Based on the experimental and FE results of sheet metals at micro/meso scale bending process, the interactive effect of feature and grain sizes is revealed to be critical in microforming processes. As a preliminary work, the flow stress reduction described by the composite model is found to be able to characterize the global springback behavior during the micro bending test.
Fabrication of miniaturized titanium thin-walled products directly using sheet metals is proven to be a promising approach with high productivity. In this process, however, there are many unknowns in ...terms of size effect and its affected fracture behavior. This study is thus aimed at investigating the effect of grain size on the fracture behavior of commercially pure titanium (CP–Ti) sheets with a thickness of 0.1 mm. Tensile tests combined with a digital image correlation (DIC) measurement system were conducted to determine the critical fracture stress of the specimens with different grain sizes. The fracture morphologies of the stretched specimens were further characterized by a scanning electron microscope (SEM). According to the experimental observation, an evident transition from ductile to brittle fracture as the grain size increases from 33.1 to 107.7 μm is revealed. Macroscopically, the elongation and critical fracture stress decrease with the increase of grain size. While according to the microscopic observations, the number of dimples decreases with the increase of grain size. Meanwhile the cleavage planes and river patterns gradually appear in the coarse grain fracture surface. To explore the fracture mechanism, transmission electron microscope (TEM) observations were made utilizing the specimens with fine and coarse grains. Significant dislocation emission from crack-tips was revealed at different grain sizes. Moreover, evident dislocation pile-up at grain boundaries was observed in the specimen with a grain size of 33.1 μm. Those intense dislocations reduce the effective stress at the crack tip resulting in higher crack propagation resistance. Nevertheless, the dislocation density at crack-tip decreases strongly with the increase of grain size leading to high crack-tip effective stress and less crack plasticity. Thus cleavage fracture was dominated in coarse grain CP-Ti sheets. A dislocation shielding model was developed to capture the dislocation observation in various grain sizes. The critical stress onset of CP-Ti sheets fracture during the tensile test was also predicted for validation.
The industrial applications of proton exchange membrane fuel cell (PEMFC) technology have attracted tremendous capitals and research attention in the last ten years. However, there are still many ...bottlenecking problems before the progress triumphs. Among them, a core challenge is the improvement of bipolar plates (BPs). As a key component of PEMFC, the BPs undertake the crucial functions of distributing reactant gases and cooling medium, collecting the electricity generated, exporting waste products, etc. Due to the unique advantages of ultra-thin, high strength, easy-to-mass-produce, etc., metallic BPs have been considered as the promising solution for the automotive and aerospace application scenarios with a strong requirement of power density. However, many challenges remain to be conquered in metallic BPs to fully meet all the requirements in those industrial applications such as corrosion, toxic ion emission and fabrication. The aim of this work is to provide an overview of the important researches in recent years dealing with the most critical problems of metallic BPs before industrial applications. The future studying directions in material and processing methods development are also given towards the wide application of metallic BP PEMFCs.
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The flow distribution in manifold has significant effect on the output performance of the stack according to the Cannikin Law, especially for multi-cell proton exchange membrane fuel cell (PEMFC) ...stacks. This study presents an analytical model which simultaneously considered local pressure losses, pressure recovery phenomenon, electrochemical reactions, and liquid water to calculate the flow distribution using the flow network method. Both U-shape and Z-shape flow configuration stack are investigated. The analytical model is simultaneously and quantitatively validated with three distinct variables (the dimensionless mass flow rates, the stack pressure drop and the pressure distribution in manifold) to ensure the accurateness of modelling results. Effects of reactant consumption and two-phase flow in unit cell on the flow distribution are quantitatively investigated. Results show that the flow maldistribution is overestimated when ignoring gas consumption and two-phase flow for both 200-cell U-shape and Z-shape stack. Balance the pressure drop of inlet and outlet manifold significantly improves the flow distribution for U-shape stack. Finally, the differentiated assembly strategy is first proposed to improve the flow distribution in manifold for multi-cell PEMFC stacks. Compared to the original design, the ratios of the improvements for 200-cell U-shape and Z-shape stack are 85% and 25%, respectively.
•New analytical model is developed to evaluate flow distribution in manifold.•Analytical model is simultaneously and quantitatively validated with three distinct variables.•Flow maldistribution is overestimated when ignoring gas consumption and two-phase flow.•Balance pressure drop of inlet and outlet manifold significantly improves flow distribution.•Uniform flow distribution cell to cell is obtained by differentiated assembly strategy.
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