Local formability assessment of AHSS steels with shear cut tensile tests

Abstract

As an alternative to the ISO16630 hole expansion test, the punched tensile test is increasingly popular for edge crack characterization of AHSS advanced high strength steels. In this investigation the reduction of area as well as thickness reduction at fracture in the vicinity of left/right sample edge fracture sides has been determined by means of light optical microscopy according to the Hance local formability test methodology for 10% to 40% cutting clearance in (both sided) sheared cut vs. spark eroded or milled edge conditions. An edge crack index has been defined based on the tensile sample fracture shape. Local formability tensile properties based on area reduction or average thickness reduction are more sensitive to edge condition than Axx fracture elongation values. The determination of the reduction of area at fracture is however challenging due to projection issues. The % thickness reduction at minimum thickness as well as at left/right thickness in cut edge vicinity may offer some additional information about edge crack initiation and final fracture. The shear punch edge quality (punch and die tool wear, target vs. actual clearance) should be closely monitored for accurate reproducible testing results.

Stretch flangeability of AHSS automotive grades versus cutting tool clearance, wear, angle and radial strain gradients

“…An optimum in HER is reached around 15% clearance. Those results are consistent with previous HER vs. clearance findings in[4]-[6] and AHSS processing guidelines for stretch flangeability optimization[7].…”

“…Nominal clearances  12% can be considered as long as burr formation is not occurring [4]. Own investigations on AHSS confirm an optimal clearance between 12% and 20% for optimum hole expansion ratio (HER) [5]- [6]. Current AHSS processing guidelines suggest adapted clearances according to sheet strength and thickness level [7].…”