formability

Sheet metal formability: sheet metal formability can be evaluated by a variety of mechanical tests.
Publication Date: 01-AUG-02
Publication Title: Advanced Materials & Processes
Format: Online - approximately 2424 words
Author: Gedney, Richard
Article Price: $4.95
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NOTE: All illustrations and photos have been removed from this article.

Description Formability is a measure of the amount of deformation a material can withstand prior to fracture or excessive thinning. Sheet metal forming ranges from simple bending, to stretching, to deep drawing of complex parts (Fig. 1). Therefore, determining the extent to which a material can deform is necessary for designing a reproducible forming operation.

Because mechanical properties greatly influence formability, and forming properties may vary from coil to coil, it is essential to test incoming sheet material. However, the outcome of a forming process depends on both material characteristics and process variables such as strain, strain rate, and temperature. In fact, stress and strain fields are so diverse during a forming process that no single test can reliably predict the formability of materials in all situations.

Material properties that have a direct or indirect influence on formability and product quality are Ultimate Tensile Strength, Yield Strength, Young's Modulus, Ductility, Hardness, the Strain Hardening Exponent, and the Plastic Strain Ratio. All of these parameters can be determined by testing a specimen cut from the blank. This article explains the meaning of each of these tests, describes how they are carried out, and provides equations to calculate important values.

Tensile tests

A graphical description of the amount of deflection under load for a given material is the stress-train curve. The stress-strain curve is generated by pulling a metal specimen in uniaxial tension to failure. ASTM E8/E8M Standard Test Methods for Tension Testing Metallic Materials governs the methods for the determination of Yield Strength, Ultimate Tensile Strength, Percent Elongation at Fracture, and Reduction of Area, the latter two being measures of ductility.

Engineering stress S is found by dividing the load P at any given time by the original cross sectional area [A.sub.o] of the specimen.

S = P/[A.sub.o] Eq. 1

Engineering strain e is calculated by dividing the elongation of the gage length of the specimen L by the original gage length [L.sub.o].

E = [delta]L/[L.sub.o], = (L - [L.sub.o])/[L.sub.o] Eq. 2

Figure 2 depicts a typical stress-strain curve. The shape and magnitude of the curve depend on the type of metal. Point A represents the proportional limit of a material. A material loaded in tension beyond point A, when unloaded will exhibit permanent or plastic deformation. The proportional limit is often difficult to...