Curling of thin tubes: Analytical and experimental study
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Curling of Thin Tubes: Analytical and Experimental Study A. EI-Domiaty The curling of thin tubes under compressive loads produces double walled tubes for many engineering applications. Experiments have been performed on annealed copper, low carbon steel, stainless steel, and aluminum tubes to determine the relationship between the curling process parameters and the tube geometry, as well as tube material properties. Effects of friction and work hardening during forming are included in the analysis. The analytical and experimental results are compared with published results to determine the validity of the proposed analytical model. The model can be used to determine the curling load and the correct die radius for successful curling. For simple and quick determination of the die radius, the present work introduces design curves, which correlate the tube geometry with the radius of the forming die.
Keywords ductile failure, tube expansion, tube forming, metal fabrication
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theoretical predictions. Their analysis also aids in choosing the correct die fillet radius and the tube thickness for any set of tools.
1. Introduction Curling of tubes is one of the most popular processes in cold forming of tube ends, which is extensively used in many industrial applications. The process of tube curling has been used in the design and forming of energy-absorbing devices such as heat exchangers, collapsible steering columns, air drop cargo, and aircraft seat anchors. In a curling process, the tube end separates from the forming die surface and rolls up to curl in the radial direction, while the other end of the tube is under a compression load. When the tube curls inside out, the process is called "outward curling." Conversely, inward curling is obtained when the tube end curls outside in. Figures 1 and 2 show the outward and inward curling processes. In the inside-out or outward curling, the circumference of the tube is enlarged, and in the outside-in or inward curling, the circumference of the tube is reduced. The stress states in both curling processes are different. In the outward curling the hoop stress is positive, and in the inward curling the hoop stress is negative. A number of experimental and theoretical studies were performed on the curling processes. The experimental studies were aimed at verifying the efficiency of the energy absorption technique. The theoretical studies were performed to explain the mechanics of the curling process for obtaining correlation with experimental results to establish more precise estimates for curling process parameters. The basic theoretical approach in the mechanics of external or inside out inversion of circular tubes was used by Kinkead (Ref 1) to calculate the required load. In this work, the effects of friction and work hardening on the curling load are included. The results obtained by AI-Qureshi and DeMorais (Ref 2) demonstrate that tubes in a partially work-hardened condition are more amenable to this type of process than annealed tubes, which exhibit
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