Indentation for fracture toughness estimation of high-strength rail steels based on a stress triaxiality-dependent ductile damage model

Abstract

Fracture toughness can be estimated using indentation test (KInd) based on continuum damage mechanics, usually with the assumption that the critical damage parameter (Dcr) of a standard tensile (smooth) specimen is equivalent to that at the crack tip. In view of the effect of stress triaxiality on damage development, KInd is determined in this work based on the Dcr at the crack tip. In addition, an adjusting parameter κ is introduced to reduce the Dcr value to reflect the potential difference in damage development between indentation compression and tensile fracture. The above approach is applied to three types of high-strength rail steels to determine their KInd values. Two types of notch-free (smooth and short-gauge) specimens are used to calibrate a ductile damage model for rail steels, and two additional types of round-notched specimens are used to establish the locus of plastic fracture strain versus stress triaxiality. The damage evolution and Dcr at the crack tip are extrapolated for the three rail steels. The so-determined Dcr is then applied to the indentation test, with κ as the adjusting parameter, to determine the critical contact depth for calculating KInd. Results show that Dcr is indeed dependent on stress triaxiality, increasing with stress triaxiality under tensile loading. Also, it is found that the change in KInd based on Dcr either at the crack tip or from the smooth specimen is generally consistent with the difference in the measured fracture toughness (KIc) among the three rail steels, thus, justifying the validity of using Dcr from the smooth specimen to estimate KIc of ductile materials by indentation test.