Mechanism And Experimental Study Of Crack Overload Retardation

Many engineering components may suddenly break after a certain number of fatigue cycle loads,causing serious safety accidents and economic losses.The fatigue cyclic load experienced by the component is mostly the variable-cycle load,and the single tensile overload is the simplest form of the variable-cycle load.At the same time,many load spectra can be approximated by a constant amplitude load cycle with multiple overloads.The study of single or multiple tensile fatigue crack propagation behaviors can reveal the fatigue crack growth mechanism under variable amplitude load more systematically.At the same time,the tensile overload can delay the crack propagation and provide a possible way to improve the fatigue crack growth life of the component.In recent years,people's understanding of overload has also made great progress,but it is not enough,especially in its overload retardation mechanism and micro analysis.This paper conducts experimental research from the macro and micro levels to analyze the internal mechanism of fatigue overload.The main work includes the following three aspects:(1)The effect of overload ratio and overload spacing on fatigue overload delay life was studied.Two overloads with different spacings were applied to the cracks.It was found that the longer the applied spacing,the longer the crack propagation delay life,indicating that the interaction between the two tensile overloads is not conducive to crack retardation.At the same time,the delay life of the two overloads is much lower than the delay life brought by the larger overload ratio.It can be seen that the effect of the overload ratio on the delay life is greater than the overload spacing.(2)Research on fatigue overload retardation based on crack closure and residual stress are conducted.The effects of two main influencing factors,crack closure and residual compressive stress,on the single tensile overload retardation are studied.Improved the attenuation of the stress intensity factor amplitude.A single tensile overload is applied in a constant-cycle load with a stress ratio of 0.7,The digital image correlation(DIC)technique is used to prove that there is no crack closure after overload,and the residual compressive stress is obtained from the overload attenuation curve.From its overload attenuation curve,the effect of residual compressive stress on the crack growth rate after overload is monotonously decreasing,and residual stress is the main cause of overload retardation.For the case of a stress ratio of 0.1,the overload retardation peak is high and the distance is long.At this time,the crack closure plays a greater role in the overload retardation than the crack tip residual stress.(3)Microscopic study of fatigue overload retardation considering crack blunting and branching.Scanning electron microscopy(SEM)was used to observe the microscopic morphology of crack tip before and after fatigue overload of 316 L steel.The microscopic mechanism of crack propagation path and tip morphology under overload was investigated.It was found that the tensile overload caused large plastic deformation in several grains in front of the crack tip.Crack blunting and branching were found after the overload.The crack branching consumes crack propagation energy,which delays the crack growth rate.