Fracture toughness examination of dual‐phase and interstitial free steel using essential work of fracture method

Abstract

Fracture toughness of automotive-grade dual-phase steel (DP450) and interstitial free (IF) steel is determined using essential work of fracture (EWF) methodology. Geometrical constraints of standard fracture toughness tests made the EWF method an alternative for thin sheets. Double edge notched tension (DENT) specimens are used for the EWF tests. The two steels have different mechanical and microstructural properties, which helps test the applicability of the EWF methodology to different materials. The effect of notch-tip radius on the EWF test results is analyzed for both the steels. The sensitivity of fracture toughness to notch-tip radius is not the same for both steels. During the EWF test, the digital image correlation (DIC) technique is used to determine the local strain variation of DENT specimens. The hole expansion ratio is determined for the two steels to compare against the EWF test results. The IF steel, despite lower strength, has better fracture toughness and hole expansion capability against the DP450 steel.

Microstructural Influence on Fracture Toughness of IF Steel and DP Steel

“…Table 2 shows the results of the standard tensile tests for the DP450 and IF steel in the transverse direction (TD) and at a strain rate of 0.002 s -1 . To define specific energy consumed in the fracture process zone, the essential work of fracture (EWF) was measured in the initial experimental study [5] in comparison IF vs. DP steel. The methodology was proposed by Cotterell and Reddel [6] based on Broberg's [7] concept of separating energy zones at the crack tip.…”

“…For the notched samples, the IF steel has about a 6% higher value of the specific essential work of fracture ("we") than the DP450 steel. For the pre-cracked samples, the IF steel has about 27% higher "we" value than the DP450 steel [5]. However, better plasticity distribution with increased strain hardening may not be true for fracture resistance.…”

“…In the previous part of the study [5], the essential work of fracture (EWF) methodology has been utilized to determine the fracture toughness of IF and DP steel. For both the steels, the values of plastic anisotropy and strain hardening are key parameters decisive for crack propagation.…”