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In this paper, the modified Mohr-Coulomb (MMC) ductile fracture criterion was adopted to analyze the fracture behavior of the AA6061-T6 aluminum alloy sheet during the U-bending process. Appropriate calibration procedures were employed using various tension tests. A finite element (FE) model was built using the commercial FE code Abaqus/Explicit incorporating the calibrated MMC fracture criterion. It was found that the MMC criterion calibrated by the uniaxial, plane strain, notched, and modified in-plane shear tension tests can predict the onset of fracture in the U-bending process with reasonable accuracy. The error of the predicted fracture displacement was about 2% as compared to experiments. Due to the dependency of the fracture prediction accuracy on the stress state, the effects of process parameters on stress triaxiality and normalized Lode angle parameter were investigated. The negligible effect of process parameters on the stress state confirmed that the calibrated fracture criterion (MMC) was able to predict fracture in different forming conditions with equal accuracy. Also, the proportional stress and strain state in the U-bending process indicate that, besides the high fracture prediction accuracy of the MMC’s damage accumulation function, the fracture forming limit diagram (under proportional loading assumption) can serve as a reliable tool for estimating the onset of fracture in the U-bending process. Therefore, a series of studies were also conducted on damage evolution, crack propagation, principal strains, and fracture displacement during the U-bending process under various forming conditions.

In this study, the dimensional accuracy of metallic bipolar plates with stainless steel 316 was investigated in diagonal, longitudinal, and transverse direction (directional dimensional accuracy). The results showed that directional dimensional accuracy (uniformity of channel depth) would increase by raising the applied force. In addition, increasing the rubber hardness and rubber thickness leads to higher directional dimensional accuracy. After that, the dimensional accuracy in constant direction (total dimensional accuracy) was investigated. According to the result, central channels have lower channel’s depth than lateral channels. Difference between central and lateral channel’s depth was decreased when the applied force was increased. Also, total dimensional accuracy increased by increasing hardness and thickness of rubber. Most accurate bipolar plate is fabricated by 450 kN applied force and rubbers with hardness of Shore A 90 and 30 mm thickness. However, dimensional accuracy in conventional rubber forming process was not satisfying. Thus, a new method was used in order to improve the dimensional accuracy of fabricated bipolar plates named semi-stamp rubber forming. The results indicated that using semi-stamp rubber forming instead of conventional rubber forming would lead to 8.35, 3.72, and 3.3% improvement in directional dimensional accuracy and also 1.075% improvement in total dimensional accuracy.

Metal is considered to be a good material for a bipolar plate due to its good electrical conductivity, excellent mechanical properties, and low cost. In this study, fabrication of metallic bipolar plates with serpentine flow field microchannels was investigated by a novel rubber pad forming process. Stainless steel 316 with thickness of 0.1 mm was used. Polyurethane rubbers with the hardness of shore A40, A55, A65, and A90 and the thickness of 10, 20, and 30 mm were used in this study to manufacture bipolar plates. At first, convectional rubber pad forming process was used to produce bipolar plates by some experimental tests. In this step, the effect of applied force, rubber hardness, and rubber thickness on filling percentage, thickness distribution, and dimensional accuracy was investigated. The results showed that filling percentage would increase by rising applied force, but channel depth was not equal in different directions. Increases in rubber hardness led to improving uniformity in channel depth. In addition, decreases in rubber hardness and increases in rubber thickness caused higher filling volumes. However, maximum filling percentage, dimensional accuracy, and thickness distribution in convectional rubber forming process were not satisfying. Thus, a novel method was developed in this study in order to improve the quality of fabricated bipolar plates named semi-stamp rubber forming. The results indicated that using semi-stamp rubber forming instead of convectional rubber forming would lead to 11.7, 9, and 1.075% improvement in filling percentage, thinning percentage, and dimensional accuracy, respectively. According to the results, the developed model (semi-stamp rubber forming) could be a feasible technique in fabricating metallic bipolar plates.

Bipolar plates are some of the most important parts of a fuel cell that constitute a high percentage of weight and cost in a fuel cells collection. Furthermore, metals due to lesser weight and cost are suitable replacements for thick graphite plates which need high cost of machining. In recent years, among of sheet forming productions the applications of stamping process have become increased due to reduction of cost and increase production speed. The present study investigates the manufacturing process of metallic bipolar plates made of SS316L with a thickness of 0.1 mm using stamping process. In this respect, the effect of lubricant on improvement of fabricated plates has been examined. Also, two cases of forming process with and without lubricant on depth of filling flow channels, thickness distribution, thickness reduction and uniformity of channels have been evaluated. The results obtained from the experiments showed that effect of lubricant is so important. Moreover, by using of lubricant the thickness reduction, uniformity of channels, depth of filling flow channels and thickness distribution could be improved.

In this study, the modified Mohr-Coulomb (MMC) ductile fracture criterion was employed to investigate the ductile fracture during the roll forming process of the AA6061-T6 aluminum alloy. To this aim, the MMC fracture criterion was calibrated based on the tension tests and was integrated into the commercial finite element (FE) Abaqus/Explicit using an appropriate user subroutine. The experimental procedures were designed based on two distinct forming strategies, namely single-stage and multi-stage roll forming processes. The results indicate that the calibrated fracture model based on tension tests is capable to predict the fracture initiation in the single-stage roll forming process at an error rate of 4.47%. Meanwhile, the numerically predicted fracture initiation during the multi-stage roll forming process is inconsistent with the experiments. Hence, a nonlinear damage accumulation rule together with the additional test (multi-stage L-bending) was utilized to predict the onset of fracture throughout the multi-stage roll forming process, considering the effect of deformation mechanics and nonlinear loading path during the process. The results reveal that the fracture onset during the multi-stage roll forming process can be accurately predicted when the MMC fracture criterion with the nonlinear damage accumulation rule is used in the numerical fracture model (with an error of about 9%).