Abstract
Research indicates that high-speed forming can significantly enhance the forming limit of plates; however, studies on welded tubes remain relatively scarce. The forming limit diagram (FLD), which is composed of ultimate strain, serves as a critical tool for assessing the forming performance of metallic materials. Consequently, this study investigates the differences in forming limits of welded tubes under varying loading speeds using the FLD. Hydraulic bulging tests were conducted on SUS304 stainless steel welded tubes at various loading speeds. The forming limits of welded tubes at various loading speeds were determined using digital image correlation (DIC) technology. A finite element model consistent with the numerical simulation test conditions was established, and the applicability of three failure criteria—the thickness gradient method, the strain increment ratio method, and the Oyane criterion—was explored by considering the relative deviation as the limit point. The results demonstrate that the relative offsets of the limit values under medium- and high-speed loading compared to low-speed test values are 11.063% and 16.448%, respectively. An increase in loading speed does not enhance the forming limit of the welded tube; instead, as the loading speed increases, the forming limit of the welded tube decreases. In finite element simulations conducted at three loading speeds, the average relative offset was used as an index to evaluate the applicability of the failure criteria. The average relative offsets of the simulation data obtained by combining the thickness gradient method, the strain increment ratio method, and the Oyane criterion from the test points were 4.838%, 2.882%, and 2.013%, respectively. Therefore, the Oyane criterion exhibits the best applicability, whereas the strain increment ratio method is suitable for simulating welded tubes at both low and high loading speeds.
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