Effect of strain gradient on stretch flange deformation limit of steel sheets

Effects of Testing Method on Stretch-Flangeability of Dual-Phase 980/1180 Steel Grades

“…Stanton et al 12 performed HET on various aluminum alloys using both a flat-bottom and conical punch with different edge conditions, concluding that the conical punch generally produced higher HER than the flat-bottom punch. 4,12,18,19 In HET, the flat-bottom punch causes the sheet edge to undergo stretching, while the conical punch causes both stretching and bending, and Neuhauser et al 20 explained that bending in a stretch bend test creates a material constraint that delays failure. The bending component creates a strain gradient in the material with the highest strains at the outer surface (at the burr).…”

“…4,[14][15][16] Studies have shown that materials tested with a conical punch generally have a higher hole expansion ratio (HER) than the same material tested with a flat-bottom punch. 4,12,[18][19][20] This difference has been attributed to the bending component introduced by the conical punch creating a material constraint that delays failure. 20 Edge conditions have also been shown to affect HER, with machined edges having a higher HER than sheared edges.…”

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

“…Some investigations also suggest that the strain gradient (radial curvature of hole edge during expansion) influences significantly positively the HER values [13]- [17]. The radial strain gradient can be influenced by varying the conical expansion tool angle and initial hole diameter [13].…”

“…This radial strain gradient is stronger for 60° conical and hemispherical punches compared to a flat punch [15]. A linear positive dependency of edge crack strain versus radial strain gradient is clearly shown in [16]- [17]. Different hole diameters from 10 to 50mm combined with 50-60° conical punches as well as Nakajima and Marciniak punches are varied to achieve a large range of strain gradient at hole edge in [16]- [17].…”

Measurement of edge fracture strain of dual-phase steels by in-plane bending test

“…It was widely shown that HEC results are sensitive to test parameters such as punch geometry [4], initial hole diameter [5], hole curvature variation [6], edge cutting method [7], direction of strain gradient i.e. perpendicular [8] or parallel [9] to edge direction. One reason for this observation can be the complexity of stress state at the edge of the test specimen that is impacted by friction, contact forces, and out of plane deformation.…”