Edge crack sensitivity versus tensile local ductility of AHSS sheet steels

Abstract

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.

Evaluation of hole expansion ratio of ultra-high strength martensitic steels produced with various processing routes

“…They claimed that the width to thickness ratio (w/t) of tensile specimens should be large enough to produce shear fracture across the specimen width and thus produce consistent fracture shape regarding all tested specimens. In our study showed that other values from tensile tests does not correlate with HER, as seen also in our studies [16]. However, the best HER values were achieved with steels (B steels in RQT condition) that had best overall formability based on formability maps, which indicates that formability map data can produce beneficial information regarding the HER.…”

“…Overall, the correlation is weak and true thickness strain does not explain the differences between HER values at least in our investigation. For example, Larour et al have found good correlation between HER and true fracture and thickness strain measurements for a wide selection of 600 -1200 MPa cold rolled steel sheets [16]. However, Wagner et al showed that the specimen geometry has a large role on determining the true thickness-or true fracture strain [17].…”

Tailoring the Ductility Characteristics of Lean‐Medium Mn Quenching and Partitioning Steels with Varying C Contents

“…Only a moderate RA stability enables the formation of the favored heterogeneities in the microstructure over a wide range of strain, leading to the desired good global ductility. [51][52][53] However, as well known from the literature, [54][55][56] microstructural heterogeneities have a negative effect on the local ductility. This was confirmed by the results of the present work, as shown in Figure 5b.…”

Edge fracture of the first and third-generation high-strength steels: DP1000 and QP1000

“…Among them, the hole expansion test (HET) is a commonly used method to effectively evaluate the stretchability of sheet metal and its edge fracture [12][13][14]. It is verified that HET gives good results in characterizing the edge crack sensitivity of AHSS steels [15]. Different influencing factors have been studied for DP and complex-phase (CP) steels to find out the most critical ones [16].…”

“…As reported in a large number of studies, the severe pre-damage introduced to the hole edge by the blanking process in the shear-affected zone (SAZ), places a dominating role in the edge stretch-flangeability. It is verified that HET gives good results in characterizing the edge crack sensitivity of AHSS steels [15]. Different influencing factors have been studied for DP and complex-phase (CP) steels to find out the most critical ones [16].…”