Abstract
The hydraulic bulge test represents an effective experimental method to characterise sheet metals since the equivalent strains before failure are much larger than those measured during tensile testing and there is nearly no frictional effect on the results. Recently this test has been proposed not only for extracting data concerning the equi-biaxial strain condition, but to determine the forming limit diagram (FLD) in the range of positive minor strains. In the proposed methodology, different strain paths can be obtained by merely using a test blank having two holes with a suitable geometry and position to be tested, without the need of dies with elliptical apertures. However, a carrier sheet is necessary, thus implying results may be affected by friction effects. This paper proposes a new methodology for the determination of the right side of the Forming Limit Curve (FLC), based on the adoption of local heat treatments aimed at determining different strain paths on the blank to be tested while using the classical circular die for bulge tests. In particular, the formability of the alloy AA5754-H32 was investigated; 3D Finite Element simulations were conducted setting different laser strategies and monitoring the resulting strain path. Results revealed that the proposed methodology supports obtaining many additional points in the right side of the FLC, thus being effective and friction free.
Highlights
The Forming Limit Diagram (FLD) is often used as a failure criterion in sheet metal forming processes, to evaluate the formability of the material
Unlike the approaches based on ductile fracture criteria [1], through the forming limit diagram (FLD) safe, necked or failed points can be distinguished considering the level of major and minor strain which the point has reached: the transition from the sound/necking region to the failure one occurs when the critical values composing forming-limit curve (FLC) are overcome [2]
Sheet metal FLDs are widely obtained using Nakajima or Marciniak methods. These methods require at least five different sample geometries and punch to deform the material. Typical limit of these methods is the friction between the punch and the sample, which may affect the accuracy of the Forming Limit Curve (FLC) [4]
Summary
The Forming Limit Diagram (FLD) is often used as a failure criterion in sheet metal forming processes, to evaluate the formability of the material. To determine the FLC accurately, it is necessary to have a nearly frictionless state in the zone of evaluation [3]. Sheet metal FLDs are widely obtained using Nakajima or Marciniak methods These methods require at least five different sample geometries and punch (hemispherical and flat, respectively) to deform the material. Typical limit of these methods is the friction between the punch and the sample, which may affect the accuracy of the FLC [4]
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