M. Finn

The numerical simulation of stretch flange forming

Abstract Simulations of stretch flange forming operations are undertaken using explicit dynamic finite element calculations utilizing various quadratic and non-quadratic yield criteria. Both circular and square cut-out blanks are investigated with corresponding circular and square punches. Simple stretch flanges are considered, utilizing a single punch to expand the cut-out, as well as z-flanges, which employ a back-up punch to form the second bend needed in the z-flange profile. Results from a model of an automotive inner component incorporating a cut-out with stretch flange corner features are also presented. Predictions utilizing the Barlat-89 criterion are shown to accurately capture the effect of yield anisotropy ( R -value). The predicted strains from the corner regions of square cut-out stretch flange laboratory specimens are shown to be similar to those within the automotive inner panel, supporting the use of laboratory-scale stretch flange experiments to simulate the larger panels. Measured limit strains from the stretch flange formability experiments are compared to forming limit diagram (FLD) data from dome specimens. Stretch flange formability is shown to exceed allowable levels predicted using a classical FLD approach, particularly for simple stretch flanges, indicating that the FLD approach is overly conservative.

Use of a coupled explicit-implicit solver for calculating spring-back in automotive body panels

Abstract LS-DYNA3D, an explicit code, and LS-NIKE3D, an implicit code, have been coupled to facilitate the finite element (FE) modelling of sheet metal forming. The explicit FE code is used to model the forming process, in which the deformable blank contacts rigid tools. The implicit FE code is used to model the subsequent spring-back which occurs after the tooling is removed. In this way, the explicit code with its robust handling of contact during forming is combined with the implicit code and its large time steps during spring-back. The result is an efficient method for solving even very large (>20 000 deformable elements) sheet forming models. Three examples of the application of this method are given.

Study of damage initiation and fracture in aluminum tailor welded blanks made via different welding techniques

Abstract The present paper addresses microstructural evaluation of failure in multi-gauge aluminum alloy tailor welded blanks (TWBs), welded using non-vacuum electron beam and neodymium:yttrium–aluminum–garnet laser techniques. The limiting dome height test is used to evaluate formability of the TWBs. Three gauge combinations utilizing AA5754 sheets are considered (2 to 1 mm, 1.6 to 1 mm and 2 to 1.6 mm). Different weld orientations have been considered: transverse and longitudinal. In general, TWBs show two different types of fracture: weld failure and failure of the thinner aluminum sheet. Interaction of several factors determines the type of failure occurring in a TWB specimen. These factors are weld orientation, morphology and distribution of weld defects as well as the degree of constraint imposed by the thicker sheet on the thin sheet. The last factor usually depends on the difference in thickness of the sheets and is represented here in terms of gauge ratio.

Study of The Effect of Draw‐bead Geometry on Stretch Flange Formability

A fully instrumented stretch flange press equipped with a back‐up punch and draw‐beads near the specimen cutout area is simulated. The utilization of different draw‐bead geometries is examined numerically to determine the restraining forces, strains and amount of damage generated in stretch flanges during forming. Simulations of the forming process are conducted for 1mm AA5182 sheets with circular cutouts. The damage evolution with the deformed specimens is investigated using the explicit dynamic finite element code, LS‐DYNA, with a modified Gurson‐based material model. It was found that double draw‐beads can provide the same amount of restraining force as single draw‐beads, but at reduced levels of damage.