Sheet metal forming operations consist of simple bending, to stretching to deep drawing of complex parts. The mechanical properties of the sheet material greatly influence its formability, which is a measure of the amount of deformation the material can withstand prior to fracture. This blog post covers important definitions determining sheet metal characteristics, the test methods, and the equipment used to quantify them.
Determining the extent to which a material can deform is necessary for designing a reproducible forming operation. Testing the incoming sheet material is also essential because material properties may vary from coil to coil and effect part quality and scrap rate.
The outcome of a forming process is dependent on both material characteristics and process variables such as strain, strain rate and temperature. Stress and strain fields are so diverse during a forming process that one test cannot be used to predict the formability of materials in all situations. Nevertheless, an understanding of material properties is necessary to determine the success of a forming process.
Material properties that have a direct or indirect influence on formability and product quality include:
- Ultimate Tensile Strength
- Yield Strength
- Young’s Modulus
- Strain Hardening Exponent
- Plastic Strain Ratio
Yield Strength and Ultimate Tensile Strength are not directly related to formability, however, the closer the magnitude of the two stresses, the more work hardened the metal. A work hardened metal exhibits lower ductility which reduces its ability to stretch.
Both elastic and plastic deformation occur during the forming process. Upon removal of the external forces, the internal elastic stresses relax. If the forming process is not designed properly, the stress relaxation or “springback” will cause the part to change shape or distort. A material with a lower value for Young’s Modulus, E, and/ or a higher value for Yield Strength will exhibit greater “springback” or shape distortion.
Ductility is defined as the ability of a material to deform plastically before fracturing. Two measures of ductility governed by ASTM E8/E8M are Total Elongation and Reduction of Area.
Total Elongation is the amount of uniaxial strain at fracture and is depicted as strain at point Z. It includes both elastic and plastic deformation and is commonly reported as Percent Elongation at Break (The gauge length used for measurement is reported with the result.).
Reduction of Area is calculated by measuring the cross-sectional area at the fracture point and is expressed as a percent value.
The Strain Hardening Exponent, n, is a measure of how rapidly a metal becomes stronger and harder due to plastic deformation. The deformation remaining after an applied load is removed is called plastic deformation. ASTM E646, a tensile test that measures the stress-strain response in the plastic region, governs the determination of the Strain Hardening Exponent. In the graph below, the plastic region is shown between point B, Yield Strength, and point D, Ultimate Strength. The n value is calculated by selecting five data pairs between the two points.
The higher the value of n, the more a piece of material can be stretched prior to necking. Said another way, the greater the value of n, the greater the difference between Yield and Ultimate and the further the material can be stretched before failure.
The Plastic Strain Ratio, r, indicates the ability of the sheet metal to resist thinning or thickening when being deep drawn into a cup for example. The r value is calculated from width and longitudinal strain and is a measure of sheet metal drawability. ASTM E517 Standard Test Method for Plastic Strain Ratio r for Sheet Metal governs its determination. Unlike many other materials with r values that remain constant over the range of plastic strains, the r value of sheet varies with the applied axial strain and as such should be reported at the tested strain level.
Sheet Metal Testing Tips
- The standard tensile test is used to measure the characteristic properties of sheet metals. More specialized tests such as the simple bend test, limited dome height test, cup test, hole expansion test or wrinkling test may be performed to simulate straining conditions found during the actual process.
- Strain-rate sensitivity or speed of testing is very important in sheet metal testing. Strength properties of materials increase at higher strain rates. ASTM E8 specifies an upper and lower limit on deformation rates based on strain rate, stress rate or crosshead separation. With some metals, the strain rate variation allowed in E8 may result in Yield Strength variations up to 20%. Variations in ductility and the strain hardening exponent may also occur. Therefore, a valid and constant strain rate should be used for comparing like tensile tests.
- For proper n value calculations, data pairs should be selected from a continuous portion of the plastic region on a stress-strain curve. If there is a discontinuity, then select a portion of the curve that is continuous or else discard the test.
- Plastic Strain Ratio is one of the more difficult formability parameters to measure because of the additional width strain measurement. R value errors as large as 40-50% can occur due to errors associated with measuring width strain. Careful inspection of the specimen after each test to check for curling and the use of proper extensometer knife edges are paramount to obtain accurate r values.
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