Study On Fatigue Crack Growth Behavior And Fracture Mechanism Of Al-alloy Subjected To Tensile Overloads

Numbers of previous study on overload induced crack retardation generally summarized as “three phase behavior”,however less researches further subdivided overload retardation effect for different overload condition.And they always explain the whole delay process with a single mechanism to describe OL effects,but it is often unclear.In this paper,2024-T4 aluminum alloy is used as the research object,and the influence and mechanism of the overload condition on the subsequent fatigue crack propagation rate(da/d N)are studied.The specific research is as follows:(1)By comparing Paris curves,fatigue fractures,branching of the crack tip and the hardness distribution along crack propagation path of th e overloaded and un-overloaded specimens,shows the effect of crack tip fracture zone,residual stress at the crack tip and crack closure on fatigue crack growth rate characteristics.This paper compares the Paris curve with the fatigue fracture to observe the fracture mechanism at different phase after the overload,and study the internal mechanism of the overload retardation effect under different overload conditions.Based on the above research,the overload hysteresis effect can be divided into three types.(2)The static crack with or without crack branching finite element model were established using ANSYS.The stress field distribution of crack tip is analyzed under fatigue load and single tensile overload.By comparing the stress field distribution of crack tip under fatigue load with single tensile overload,the effect of stretching overload on the stress field of crack tip is studied.By comparing the distribution of the stress field with crack branching,the influence of the crack bifurcation on the stress field of the main crack is studied.(3)In this paper,we use a heat treatment method to eliminate residual stress,which avoid plastic deformation of crack tip.Through comparing the fatigue crack growth rate before and after heat treatment,to study the contributions of residual compressive stress and crack closure to the overload retardation phenomenon.The mechanism of the different stage in the overload affected zone was studied by combining the Paris curve with the macroscopic feature of the fracture.The results show that the combined effects of OL-induced crack extension,enlarged compressive residual stress,crack closure and branching near the post-OL crack tip are responsible for subsequent fatigue crack growth.The da/d N variation(acceleration or deceleration)depends on the relationship between the increase rate in(35)Keff from fracture zone and the decrease rate from residual stress and crack branching.