Edge crack sensitivity versus tensile local ductility of AHSS sheet steels

• Published in: IOP conference series. Materials Science and Engineering Vol. 967; no. 1; pp. 12080 - 12088
• Main Authors: Larour, P, Freudenthaler, J, Kerschbaum, M, Dolzer, D
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 01.11.2020
• Subjects: Cold rolling; Crack sensitivity; Ductility; Ductility tests; Edge cracks; Elongation; Mechanical properties; Reduction; Reduction of area; Statistical correlation; Statistical methods; Tensile strength; Tensile tests; Thickness measurement
• ISSN: 1757-8981; 1757-899X
• DOI: 10.1088/1757-899X/967/1/012080

There is a growing interest in correlating usual tensile testing results with edge crack sensitivity testing from punched ISO16630 hole expansion ratio HER (10mm shear cut hole, 12% clearance, conical expansion tool). A new kind of tensile local ductility parameter has been developed lately based on broken sample surface of tensile specimens after testing. Reduction in area or thickness at fracture are more sensitive than conventional fracture elongation with a 50 to 80m gage length to characterize the local ductility potential of sheet steels. A representative amount (300 different sets of samples) of cold rolled sheet steels have been tested in the tensile strength range 600-1200MPa and thickness range 1-2mm with 3 replicates in the transverse and longitudinal direction with ISO 6892-1, type 2, A80 tensile samples. Correlation levels of ISO16630 HER values with conventional tensile mechanical properties such as uniform & fracture elongation, yield & tensile strength, n-& r-values or derivatives are disappointing low for the investigated AHSS grades. There is however a massive improvement in the empirical statistical correlation when using local ductility properties based on fracture area or thickness reduction measurements on broken tensile samples. Logarithmic local ductility strains correlate generally linearly with logarithmic hole expansion ratio. Logarithmic true local ductility values are proving more suitable than engineering strains for correlations. Transverse direction improves slightly the correlation quality vs. longitudinal direction. The correlation is also higher for thickness reduction in comparison to reduction of area based properties.