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

摘要

Fracture toughness tests have been performed on five geometries of single-edge notch bend (SENB) specimens machined from a 25.4-mm (1.0-in) thick plate for normalized 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 degrees C to 25 degrees 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 cleave, a 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 crack-tip region constrain 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 n 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 with a/W ranging from 0.19 to 0.75. the rotational factor, r(sub p), was not a strong function of temperature or specimen geometry.