Research On Fatigue Crack Propagation Behavior Under A Single Tensile Overload

In practical engineering, metal components are mostly under variable amplitude cyclic loading, and very prone to fatigue damage. A single tensile overload occured in constant amplitude cyclic loading, is a common simple form of variable amplitude cyclic loading. Study on fatigue crack propagation behavior under a single tensile overload leads to accurate predictions of fatigue life with variable amplitude cyclic loading, and is helpful to the control of cyclic loading in engineering.Based on the theory of fracture mechanics, the changing rules of fatigue crack closure/opening loadings subjected to crack propagation after a single tenile overload, is obtained by the finite element model of CT specimen of A537steel. The concept of strengthening plastic zone is put forward to explain the crack propagation behavior under a tensile overload. With the crack propagation in the strengthening plastic zone, the crack closure effect is strengthened constantly, and crack propagation rate decreased continuously, until the crack gets through the whole strengthening plastic zone. So that the length of strengthening plastic zone determines the delayed retardation distance of crack propagation. The length of retardation zone is equal to the total length of plastic zones caused by constant amplitude cyclic loading and the tensile overload.The changing rules of fatigue crack opening loadings subjected to crack propagation after a single tenile overload, is obtained by the finite element model of CT specimens of Q345R steel with different stress ratios. Calculation models, considering stress ratio, of strengthening plastic zone and plastic zone caused by overload are established. To describe the changing rules of crack propagation rate, calculation model of crack propagation rate after a simple tensile overload is obtained by Paris formula and the effective stress intensity factorâ–³Keff. At last, a forecasting model of fatigue life under a single tensile overload is established. Results obtained by the calculation models above agree well with experimental data of A537and Q345R steel.