Fatigue Failure Mechanism Of Metal Materials With Microdefects

Due to the effect of stress concentration,micro-defects on the materials’ surface and interior are more likely to cause the initiation of fatigue cracks and eventually lead to fatigue fracture.In the last decades,a large number of researchers have carried out extensive research on the effect of microdefects on the fatigue failure of traditional metal materials.With the increasing development of science and technologies,some novel material modification and preparation technologies have emerged in the modern industry,such as surface nanocrystallization(SNC)and additive manufacturing(AM).However,the effect of microdefects on the fatigue failure mechanism of materials processed or manufactured by these emerging technologies is still necessary to be studied.In the practical engineering application,due to the effect of external environment,the components after SNC treatment will inevitably suffer some external damage,forming micro-defects on the material surface.The AM technology will cause the appearance of some pores and inclusions in the interior of material due to the congenital deficiencies in the process of material forming.These micro-defects have critical effects on the fatigue properties of SNC and AM materials,and to some extent hinder their wider application in the field of engineering structures.Therefore,it is quite essential for SNC and AM technologies to study the fatigue failure mechanism of metal materials with micro-defects.In this paper,based on ultrasonic surface rolling processing(USRP)and selective laser melting(SLM)technology,the fatigue failure mechanism of metal materials with micro-defects was studied.It is found that USRP treatment can form a gradient nanostructured(GNS)surface layer of a certain thickness and introduce a certain depth of residual compressive stress on the material surface,both of which play a key role in improving the mechanical properties and fatigue properties of the material.Meanwhile,it is interesting to find that the predicted threshold stress intensity factor range based on Murakami’s model could agree well with the experimental result of not only coarse grain specimens but also USRP specimens if the surface HV of SNC materials is selected.This is because the fatigue limit of SNC materials is the fatigue crack initiation limit rather than the crack propagation limit.Hence,the fatigue limit of SNC materials is mainly dependent on the material properties of the nano-grain layer.In addition,the fatigue failure mechanism of AM Ti-6Al-4V alloy has also been studied.It is found that the fatigue failure modes of SLM Ti-6Al-4V alloy are different at high and low stress levels:the fatigue cracks usually generate on the surface when the stress is large,and the fatigue life of the alloy is short;however,the internal defects of the material become the main location of crack initiation when the stress is small,and the material has a higher fatigue life.It is note that,no matter the stress level is high or low,cracks will generate from the subsurface defects if there are obvious defects near the subsurface of the material,and the fatigue life will be significantly decreased.