Closure-free biaxial fatigue crack growth rate and life prediction under various biaxiality ratios in SAE 1045 steel

Biaxial fatigue tests were performed on thin-walled tubular 1045 steel spec imens in a test fixture that applied internal and external pressure and axi al load. There were two test series, one in which constant amplitude fully reversed strains (CAS) were applied and another in which large periodic com pressive overstrain (PCO) cycles causing strains normal to the crack plane were inserted in a constant amplitude history of smaller strain cycles. Rat ios of hoop strain to axial strain of lambda = -1, -0.625, -nu and +1 were used in each test series. Fatigue crack growth behaviours under CAS and PCO histories were compared, and revealed that the morphology of the fracture surface near the crack tip and the crack growth rate changed dramatically w ith the application of the compressive overstrains. When the magnitude of t he compressive overstrains was increased, the height of the fracture surfac e irregularities was reduced as the increasing overstrain progressively fla ttened the fracture surface asperities near the crack tip. The reduced aspe rity height was accompanied by drastic increases in crack growth rate and d ecreases in fatigue life. Using a pressurizing device attached to the confocal scanning laser microsc ope (CSLM), crack opening measurements were obtained. Crack opening measure ments showed that the biaxial cracks were fully open at zero internal press ure for block strain histories containing in-phase PCO cycles of yield stre ss 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 i ntensity data were obtained from tests with positive stress ratios for whic h cracks did not dose. A number of strain intensity parameters derived from well-known fatigue life parameters were used to correlate fatigue crack gr owth rates for the various strain ratios investigated. Predicted fatigue li fetimes based on a fatigue crack growth rate prediction program using criti cal shear plane parameters showed good agreement with the experimental fati gue life data.