Typical Stress-Strain Curve
For many years, steel producers and stamping plants have gathered the mechanical properties of sheet metal. Individuals recorded properties such as yield strength, n-value and R-value to name a few. As new materials are introduced into the stamping plants, new mechanical properties tests are being discussed. The current use of aluminum and Dual Phase steels has highlighted issues that were insufficiently described by the typical or standard values provided by the tensile test: edge cracking and variation in springback from run to run are two examples. Resulting outcomes can encompass excessive scrap, excessive re-working of parts to remove edge cracks, and even result in excessive downtime as die makers try to correct the issue. The introduction of increasingly complex and sophisticated materials will exacerbate the inefficiencies of the current stamping process. The learning curve can be frustrating.
Proper training for your workforce regarding material mechanical properties and the know-how to develop robust stamping recipes for Advanced High-Strength Steel (AHSS) will benefit the plant. The typical tensile test provides the yield strength, ultimate tensile strength, n-value, uniform and total elongation. And typically, a strain analysis is performed, producing a forming limit diagram to measure a material’s formability. But these tests are reflective of global forming behaviors and don’t adequately describe an advanced material’s performance in localized formability. We need to ask if typical mechanical properties provide adequate information to truly assess the impact to the stamping operation – and if not, what new tests will provide insight on how these materials react to certain forming loads.
Instantaneous n-value
A new output is instantaneous n-value. Instantaneous n-value identifies the strain gradient – how the material will work-harden at initial contact with the die geometry. If only the standard n-value (work hardening exponent) is observed, measured between 10% and 20% strain, the increase in work hardening that occurs during deformation will be missed. Hole expansion and three-point bend tests are now commonly performed. The hole expansion test identifies the materials ability to stretch at the sheared edge. The three-point bend test identifies the stretchablity of a material via the minimum bend radius that can be achieved for that specific material.
Focusing on AHSS, these materials are characterized by multiple phase structures designed to improve formability. These designer steels are being created to improve crash worthiness through higher strength and lighter gauge thicknesses. To build upon the current understanding of material mechanical properties and resulting forming behaviors, training on AHSS products, testing and stamping process countermeasures is encouraged.
