**Abstract**

In sheet metal forming, the failure criteria is decided by the forming limit curve (FLC). The FLCs are mainly determined experimentally, but it is time consuming and not cost effective, and there are quite a few problems connected to such procedures. Therefore we need to produce FLCs using other methods. So discussed in this paper are some issues related to the use of the finite element method (FEM) for this purpose. This paper focuses on some procedures of limit strain prediction such as experimental methods, theoretical/analytical methods and FE-simulations. Herein the focus is on the use of the FEA. In the post processor of LS DYNA, the most widely used FEA package for metal forming analysis, the FLC is predicted by means of empirical correlation which depends on material properties such as strain hardening exponent (n) and sheet thickness (t). For verification and accuracy of the correlation, experimental tests were carried out for aluminium alloys for FLC.

**Keywords:** sheet metal, aluminium alloys, forming limit curve (FLC), finite element analysis (FEA)

#### **1. Introduction**

Sheet metal forming is a process in which flat thin blanks are deformed permanently to produce a wide range of products i.e. very simple sheet metal parts to complex three dimensional objects. Common parts made by sheet metal forming processes include automobile body panels, fuel tanks, aircraft parts and kitchen appliances, food and drink cans.

 Aluminium alloys are now-a-days replacing the steel in the automobile industry since they have lower weight, comparable strength and high corrosion resistance and they reduce the vehicle weight and hence are able to achieve better fuel consumption [15–18]. Hence the formability of aluminium alloys needs to be studied.

The ability of a sheet metal to be formed in a given process without failure is known as formability. The understanding of formability is essential for successful forming of sheet metals into desired shape without any defects. The formability depends on different factors such as material properties such as ductility, ultimate tensile strength, anisotropy, die and punch design and process variables such as lubrication, blank holding force etc. As the effect

**Figure 1.**  *Forming limit diagram.* 

of these parameters is complex, no single parameter can adequately measure formability [1–5].

Most of the formability tests give a rough indication of formability only in one single mode of deformation. The forming limit curve (FLC) (Keeler and Goodwin) is important for finding the limit stains where necking occurs in a sheet metal for all modes of deformation. Hecker in the 1960s developed some simplified techniques to evaluate the FLC. A typical FLC is shown in **Figure 1**. It is a pictorial representation of limit strains of a sheet metal which was subjected to different load paths, usually plotted as major principal engineering strain vs. minor principal engineering strain. The combination of major and minor strain points below this curve represents the safe region and this curve puts a limit up to which the forming operations can be done without necking or fracture [6–9].
