Basic Tests to Measure Steel Strength and Formability
In evaluating a new sheet steel, there are a number of tests which are performed to understand how the material will react in both manufacturing and in use environments. A large number of these tests focus on formability, which determines the limits on the design complexity for vehicle components. Listing all tests required to prove out a steel would be difficult because each user has their own proprietary tests coupled with the industry standard tests. For reference, below are a few of the basic assessments used to evaluate the capability of a steel.
The tensile test is a key basic test used at NanoSteel for initial evaluation of material strength and ductility. In this test, a standard size ASTM specimen is pulled from either end and stretched until failure. The test provides an understanding of a material’s basic uniaxial mechanical properties.
The bend test takes a flat piece of steel and bends it into a U-shape using a tool / mandrel of varying radii in a free or tensioned state to determine the ability of the material to withstand the tension or compression placed on the material before failure occurs. In this test, the bending limit is determined when a piece is bent to such an extent that it cracks. It provides an understanding of the limitations on corners and edges.
The bulge test is used to understand bi-axial forming capability. In using steel for real world structures, material has to stretch in multiple directions. Bulge testing is a basic method of understanding a material's capability to be formed into useful shapes. For the test, the material sheet is held in place around its perimeter and a solid tool stretches the sheet into the bulge shape until failure occurs. In some test variants, hydraulic fluid pressure creates the bulge, eliminating tool surface to material friction.
The cup draw test is used to define the limiting draw ratio and drawing characteristics of sheet steel. The material is placed in a die and a punch is used to form the cup shape. This test gives engineers an understanding of how deep the material can be drawn before cracking. This is useful for designing parts with significant depth in the final shape.
The hole expansion test is used to evaluate the ability of the material to withstand lambda strain which defines a different formability condition than bending, drawing, or bi-axial forming. In this test, the material is held in a die and then punched to make a hole, which is subsequently expanded until the point of cracking. This test indicates a material's ability to accommodate hole and edge flange forming.