Increases in Fatigue Crack Growth Rate and Reductions in Fatigue Strength due to Periodic Overstrains in Biaxial Fatigue Loading

Abstract

Fatigue crack growth under biaxial constant amplitude straining (CAS) and a strain history having periodic compressive overstrains (PCO) were investigated. A comparison of the growth of fatigue cracks under constant amplitude straining and under strain histories having periodic compressive overstrains revealed that the morphology of the fracture surface near the crack tip and the crack growth rate changed dramatically with the application of the compressive 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 asperities near the crack tip. The reduced asperity height was accompanied by drastic increases in crack growth rate and decreases in fatigue strength. Crack opening stress measurements for biaxial fatigue cracks made using confocal scanning laser microscopy (CSLM) image processing of the crack profile, showed that the biaxial cracks were fully open at zero internal pressure for block strain histories containing in-phase periodic compressive overstrains of yield point magnitude. Therefore, for the shear strained samples there was no crack face interference and the strain intensity range was fully effective. For PCO tests with biaxial strain ratios of -0.625, and +1, effective 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 parameters all involved the shear strain range, and the normal strain range acting on the maximum shear strain plane. For various biaxiality ratios, the ratios of the effective strain intensity factor range to the constant amplitude strain intensity factor range at the threshold were found to be close to the ratios of the closure free fatigue limit obtained from effective strain-life to the constant amplitude fatigue limit.