Calibration of the SMCS Criterion for Ductile Fracture in Steels: Specimen Size Dependence and Parameter Assessment

Abstract

The stress modified critical strain (SMCS) criterion provides a local index for the initiation of ductile fracture in metals as a function of plastic strain and stress triaxiality. Previous research has confirmed the SMCS criterion to be an accurate index for fracture initiation in mild steels and demonstrated its application to civil/structural engineering. To facilitate practical implementation of the SMCS criterion, two key aspects of its calibration for steel materials are examined. The first pertains to the sensitivity of the measured SMCS material toughness parameter to the size of the test coupon. New results from 23 tests of cylindrically notched tension (CNT) specimens of various sizes and notch geometries indicate that the toughness parameter is relatively insensitive to calibration specimen size. This finding validates the use of miniature bar specimens to calibrate the SMCS model for thin plate steels and in-service structures, where extraction of larger coupons is impossible. The second aspect involves the development of closed-form expressions to determine directly the SMCS toughness parameter from CNT tests, thus avoiding the need for interpretation of the test data through finite-element simulations. Based on the results of 54 numerical simulations, encompassing a range of material constitutive properties, specimen geometries, and applied deformations, a semiempirical relationship (based in part on Bridgman’s solution for necked tension rods) is proposed to determine the toughness parameter directly from the CNT bar tests.