Tests of five medium- and high-strength steels were used to investigate some prospective simplifications in JIc test and data analysis procedures. Three-point bend specimens with material strengths of 500 to 1500 MPa and a nominal thickness of 10 mm were tested at room temperature. The investigation included: (1) JIc tests using measured crack-mouth displacement to calculate load-line displacement based on an expression that relates the two displace ments; (2) a simple zero-point adjustment for J versus Δa curves, whereby certain intermediate Δa values are shifted to the blunting line to correct for errors at low values of Δa; (3) a comparison of Δa and JIc results from the load-drop method, which gives a simple measure of crack growth after maximum load, with results from the usual unloading compliance method.
The various results and comparisons are discussed in relation to their usefulness as a general JIc test procedure for a variety of materials or a limited use test for certain materials. Two specific test and data analysis procedures are proposed for general use in JIc testing, as follows: 1. A new expression is described that calculates load-line displacement for the bend specimen from measured crack-mouth displacement, for a range of a/W values and strain-hardening exponents. The expression makes possible a single-displacement unloading compliance JIc test for the bend specimen, using a single, standard, crack-mouth clip gage. 2. A simple zero-shift procedure is proposed for general use with JIc tests and for addition to ASTM Test Method for JIc, a Measure of Fracture Toughness (E 813). The procedure adjusts the zero point of Δa so that on average the Δa values lie on the blunting line over the J range of 20 to 60% of the provisional fracture toughness, JQ. The adjustment calculations can be done with a calculator or a few lines of computer code.