Stretch bending - the plane within the sheet where strains reach the forming limit curve

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.…”

“…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.…”

On the Use of Strain Path Independent Metrics and Critical Distance Rule for Predicting Failure of AA7075-O Stretch-Bend Sheets

“…The beneficial role of the punch curvature on the sheet failure has been extensively studied in the past, pointing out that formability limits under a stretch-bend deformation mode are higher than in stretching tests subjected to uniform strain conditions [ 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. From an experimental approach, Charpentier [ 13 ] revealed an enhancement of more than 50% in the limit strains when using an elliptical punch of 24 mm radius compared to a 95 mm punch radius over low carbon steel sheets of 1.85 mm thickness.…”

“…Within an analytical framework for predicting formability in presence of strain gradients, Tharret and Stoughton [20] found that necking at the convex surface (outer face) of stretch-bend mild steel sheets appeared when the strain on the concave side (inner surface) achieved a value consistent with the forming limit under in-plane stretching, i.e., the FLC. This criterion, known in the literature as the concave side rule (CSR), as well as the traditional mid-plane rule (MPR), which characterizes the material formability by using the mean value through the sheet thickness, may provide inaccurate predictions depending on the bending severity [18,[21][22][23]. Wu et al [24] developed a failure approach based on the bending-modified FLC (BFLC) to account for the stretch-bending condition, by making use of the stretch bendability index (SBI) concept, previously introduced by Sadagopan [25].…”