Elastic-Plastic Fracture Toughness Tests with Single-Edge Notched Bend Specimens

Abstract

Fracture toughness tests have been performed on five geometries of single-edge notched bend (SENB) specimens machined from a 25.4-mm (1.0-in.) thick plate of ABS Grade EH36 steel, a normalized carbon-manganese steel. Critical values of the J integral and the crack-tip opening displacement (CTOD) were measured as a function of temperature. Test temperatures, which ranged from −196 to 25°C, covered the entire ductile-to-brittle transition range. On the upper shelf, critical values of J and CTOD at the onset of stable crack growth were insensitive to specimen geometry. However, in the ductile-to-brittle transition region, where fracture occurred by unstable cleavage, fracture toughness decreased with increasing specimen thickness and crack length. This effect of geometry on fracture toughness in the transition region is attributed to changes in cracktip region constraint with geometry. Various aspects of the SENB fracture toughness test have been examined. Experimental results indicate that both J and CTOD can be accurately measured with a single clip gage. Work hardening caused the ratio of J to CTOD to increase with strain. The m factor, which relates J to CTOD and flow stress, was directly proportional to the notch constraint factor. The η factor, which is a dimensionless constant used to relate the J integral to energy absorbed by the specimen, was independent of crack length for SENB specimens of ABS EH36 steel with a/W ranging from 0.19 to 0.75. The rotational factor rp was not a strong function of temperature or specimen geometry in the material studied.