Local formability of AHSS: Measurement technique, specimen types and robustness

Abstract

The recently proposed classification scheme for AHSS with global and local formability enables a material specific process development, allows a designation based on mechanical properties, as well as an assessment of various properties: deep drawability, edge-crack sensitivity, bendability, damage tolerance and fracture toughness. Local formability is measured by local fracture strains, e.g. area reduction at fracture or true thickness strain at fracture. Based on investigations of AHSS with strengths between 600 and 1000 MPa from different steel suppliers, this contribution addresses the influence of the measurement technique, of different specimen types and different sheet thicknesses on the measure for local formability. Furthermore, the correlation between true thickness strain at fracture and hole expansion ratio and also the hardness difference of the microstructure, as well as no correlation between local formability and bending angle according to VDA 238-100 are confirmed. It is shown that the influence of the measurement technique for true thickness strain at fracture is neglectable regarding the differentiation between the investigated grades. Using “uniaxial” tensile tests with specimen types 2 and 3 according to ISO 6892-1 one can obtain similar local fracture strains. Same states for the notched tensile tests with radii of 4 mm and 5 mm. The values gained by the smaller A30 mm specimen show slightly lower values compared to the “uniaxial” tensile specimen types. With increasing sheet thickness, a slight decrease in true thickness strain at fracture resulting from the combination of a changing width-to-thickness ratio of the tensile specimen and different microstructures can be seen. The large range of the measure for local formability using materials from different suppliers and different sheet thicknesses motivates the definition of requirements for material specifications and source approvals.