Influence of Different Pre-Stretching Modes on the Forming Limit Diagram of AA6014

Abstract

In order to evaluate the formability of sheet materials forming limit diagrams (FLD) are recorded which represent the values of major and minor strain when necking occurs. FLDs are recorded based on the assumption that exclusively linear strain paths occur. In real forming parts, however, particularly in those with complex shapes, predominantly non-linear strain paths occur which reduce the accuracy of the failure prediction according to a conventional FLD. For this reason forming limits after loading with non-linear strain paths have to be investigated. In this contribution a systematic analysis of the forming limits of a conventional AA6014 alloy after loading with non-linear strain paths is presented. This material is pre-stretched in uniaxial, plane strain and biaxial direction up to several levels before performing Nakajima experiments in order to determine FLDs. During the pre-stretching process as well as during the Nakajima experiment the strain distribution can be measured online very precisely with the optical deformation measurement systems GOM Aramis or VIALUX. The gained curves are compared to the FLD of the as-received material. The results prove a significant influence of the pre-stretching condition on the forming limits of the used aluminum alloy. For a low pre-stretching in uniaxial as well as in biaxial direction the FLDs show a slightly reduced formability while after higher pre-stretching levels the forming limit can be improved such as for biaxial loading after uniaxial pre-stretching. The formability after pre-stretching in plane strain direction was changed. Also, a shift of the FLD depending on the direction of pre-stretching can be observed.

Path independent limiting criteria in sheet metal forming

“…Paul [27] showed that El-Magd, Swift-Voice and Hockett/Sherby equations are able to effectively extrapolate the true stress-strain curve beyond the uniform elongation and he validates it by bulge test data. Similarly, tensile and bulge test data are collected form Werber et al [24,25]. Up to uniform elongation both El-Magd and power law equations successfully predict the true stress-strain curve as shown in Fig.…”

“…FLD for as received condition of aluminium alloy AA6014 is collected from Werber et al [24,25] and depicted in Fig. 3(a).…”

“…Before going to in details predictive capability analysis of individual path independent limiting criteria, the effect of yield criterion and material strain-hardening law on the mapping process (like major and minor strain space to major and minor stress space) are discussed in this section. For this purpose, the material data are collected form Werber et al [24,25].…”

Study of the mechanical properties of sheet metals drawn through drawbeads

“…By Denninger et al [19], a so called Bending Limit Curve (BLC) was developed to describe failure in case of bending, what is also part of a drawbead passage. Werber et al [20] were able to show the effect of different linear pre-stretching methods on the forming limit diagram and therefore the failure behaviour. As the preloading in a drawbead is even more complex according to [6], a significant effect on the failure behaviour like fracture strains is also expected.…”

Experimental study on the formability of AA6016 sheets pre-strained by rolling

“…The conclusion was that the forming limit stresses are path-independent when the hardening is not affected by the change of load path and pathdependent otherwise. Werber et al [14] studied the effect of different pre-strains on the forming limit strains for AA6014 PX (pre-aged state), and later mapped these data into stress space [15]. They found that the forming limit stresses are not path-independent, but a systematic strain-path dependency was found.…”