— An analysis of the influence of steady torsion loading on fatigue crack growth rates under rotating or reversed bending is presented. Mixed-mode (I + III) tests were carried out on cylindrical specimens in DIN Ck45k steel and results are compared for two different testing machines: rotary bending and reversed bending obtained by cyclic Mode I (ΔK1) with or without superimposed static Mode III (KIII) loading, simulating the real conditions on power rotor shafts where many failures occur. The growth and shape evolution of semi-elliptical surface cracks, starting from a chordal notch on the cylindrical specimen surface, was measured for several Mode III/ Mode I ratios. Results have shown that the steady Mode III loading superimposed on the cyclic mode I leads to a significant reduction in the crack growth rates. It is suggested that this retardation is related to an increase of plastic zone size near the cylindrical surface in association with the interlocking of rough fracture surfaces, friction and fretting debris, leading to a decrease of the ΔK effective at the crack tip profile due to the “crack closure effect”. This work provides a contribution to a better understanding of crack growth rates under mixed-mode load conditions thereby allowing one to predict remaining lifetimes and to estimate the risks of pre-cracked rotor shafts.

Semi‐elliptical fatigue crack growth under rotating or reversed bending combined with steady torsion

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,K
 III, 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.

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

The main purpose of this part of the book is to review new experimental methods which are useful and effective in determination of resistance to fracture under fast and impact conditions of loading In recent decade a substantial progress has been made in this domain

K.A. Gupte

Papers

Fracture of round bars loaded in mode III and a procedure for KIIIc determination