Mode III fracture of 4340 steel: Effects of tempering temperature and fracture surface interference

Abstract

The effects of alloy strength, varied through temper treatment, and of fracture surface interference on Mode III fracture toughness of an AISI 4340 steel were investigated. The entire crack growth resistance curve, from quasi-static fracture initiation to final failure, was characterized by the linear elastic stress intensity factor,KIII, and by the plastic strain intensity parameter, ΓIII. The experimental trends on the role of microstructure in influencing fracture behavior are strongly dependent upon the characterizing parameter. In the linear elastic characterization, an increase in the alloy strength is found to cause an improvement in both the fracture initiation toughness and the resistance to stable Mode III crack growth. This approach, however, does not appear to be viable because small scale yielding conditions cannot be sustained in practice for all heat treatment conditions over the entire range of torsional crack growth. In the elastic-plastic analyses, however, an increase in the alloy strength leads to a marked reduction in the fracture initiation toughness, ΓIIIc, and the resistance to stable crack growth. Experiments on Mode HI failure were also conducted on circumferentiallynotched rods with different fatigue precrack lengths, but with the same uncracked ligament diameter, in an attempt to provide a quantitative description of the effects of crack face abrasion on Mode III fracture. Increases in the crack face contact area, due to Mode III crack extension, or an increase in the length of the fatigue precrack, result in a significant improvement in the apparent fracture resistance. Considerations in the choice of characterizing parameters and of crack-tip plasticity and crack-face abrasion for the interpretation of Mode III fracture in ductile solids are discussed.