Springback in simple axisymmetric stampings
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I.
INTRODUCTION
IT is well recognized that elastic relaxation a f t e r forming (springback) occurs in all sheet metal c o m p o nents upon r e m o v a l from the stamping die. However, because low carbon sheet steel has been the material o f choice f o r automotive body applications, its springback has frequently been taken for granted, and only the additional springback encountered with the application o f materials o f higher strength-to-weight ratio (than low carbon steel) has been o f concern. Also, although springback occurs under all stress states, the necessity o f obtaining good fits between adjacent panels has focused much o f the activity on springback on the behavior o f flanges or rail sections. Significantly less attention has been devoted to the behavior o f stretched o r drawn components o f more general shape. Further, the published work on stretched or drawn parts confines itself to the elastic relaxation w h i c h occurs after the part is removed from the stamping die. However, this may not be a good descriptor o f the shape o f the final part since, during trimming, much o f the periphery o f the part will be removed, thus modifying the restraint applied to the part and resulting in additional relaxation. Also, the strains resulting from a flanging operation and the geometrical stiffening resulting from the flange will likewise influence the part shape--points which have also been raised in a recent article by Gerdeen et a l . m Thus, this study was initiated to make some preliminary assessment, both o f the extent o f springback in a stretch-formed part and the additional springback resulting from r e m o v a l o f any metal excess to the formed part. R O B I N S T E V E N S O N is with the Engineering Mechanics Department, General Motors Research, Warren, MI 48090-9055. Manuscript submitted April 1 7 , 1992. METALLURGICAL TRANSACTIONS A
II.
R E V I E W OF THE LITERATURE
Much o f the early interest in springback developed in the aircraft industry, driven by the need to form, by bending, aluminum panels o f predictable shape. An early analysis due to Schroeder [2] excellently captures the essence o f the issue, including the effects o f work hardening. Gardiner]3[ subsequently simplified the analysis by neglecting work hardening, thereby realizing a closedform solution and eliminating the graphical integration required in Reference 2. WQO and Marshall t4J reintroduced work hardening to the analysis (as did Queener and DeAngelis t51) and addressed the issue o f the effect o f a superimposed tensile l o a d , w h i c h they demonstrated could dramatically reduce springbackJ 41 A similar conclusion was reached by Baba and T o z a w a .tt] WennertT~ determined that a superimposed tensile load was also effective for reducing wall opening springback in channel sections, and a means o f implementing appropriate tensile loads during the forming process was described by Ayres. [8] Extensive data on material effects have been published, some in conjunction with the analyses described above (see, for
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