Increases in fatigue crack growth rate and reduction in fatigue strength due to periodic overstrains in biaxial fatigue loading

Fatigue crack growth under biaxial constant amplitude straining (CAS) and a strain history having periodic compressive overstrains (PCO) were investig ated A comparison of the grow th of fatigue cracks under constant amplitude stra ining and under strain histories having periodic compressive overstrains re vealed that the morphology of the fracture sur;face near the crack tip and the crack growth rate changed dramatically with the application of the comp ressive overstrains. When the:magnitude of the compressive overstrains was increased, the height of the fracture surface irregularities was reduced as the increasing overstrain progressively flattened fracture surface asperit ies near the crack tip. The reduced asperity height was accompanied by dras tic increases in crack growth rate and decreases in fatigue strength. Crack opening stress measurements for biaxial fatigue cracks made using con focal scanning laser microscopy (CSLM) image processing of the crack profil e, showed that the biaxial cracks were fully open at zero internal pressure for block strain histories containing in-phase periodic compressive overst rains of yield point-magnitude. Therefore, for the shear strained samples t here was no crack face interference and the strain intensity range was full y effective. Fcr PCO tests with biaxial strain ratios:of -0.625, and +1, ef fective strain intensity data were obtained from tests with positive stress ratios for which cracks did not close. The strain intensity factor ranges derived from popular fatigue life;parameters were used to correlate fatigue crack growth rates for the various strain ratios investigated. These param eters all involved the shear strain range, and the normal strain range;acti ng on the maximum shear strain plane. For various biaxiality ratios, the ra tios:of the effective strain intensity factor range to the constant amplitu de strain intensity factor range at the threshold were found to be close to the ratios of the closure free fatigue limit obtained from effective strai n-life to the constant amplitude fatigue limit.