A new energy-based multiaxial parameter PART 2: Biaxial fatigue crack growth rate and life prediction of SAE 1045 steel specimens subjected to various biaxial strain ratios

Biaxial fatigue tests were performed on thin walled tubular 1045 steel specimens in a test fixture that applied internal and external pressure and axial load. There were two test series, one in which constant amplitude fully reversed strains (CAS) were applied and another in which large periodic compressive overstrain (PCO) cycles causing strains normal to the crack plane were inserted in a constant amplitude history of smaller strain cycles. Ratios of hoop strain to axial strain of lambda=-1, -0.625. - v, and +1 were used in each test series. When the magnitude of the compressive overstrains was increased, the height of the fracture surface irregularities was reduced since the increasing overstrain progressively flattened the fracture surface asperities near the crack lip. The reduced asperity height was accompanied by drastic increases in crack growth rate and decreases in fatigue life. An effective intensity factor was derived based on a new approach proposed in PART I (present proceedings). The components of this formulation consist of the shear energy and the corresponding normal energy calculated for the critical plane on which the stress and strain Mohr's circles are the largest during loading and unloading parts of a cycle. Predicted fatigue lifetimes based on a fatigue crack growth rate prediction program using the proposed parameter successfully correlated the closure-free crack growth rates for straining of various biaxial strain ratios.