The Investigation Of The Corrosion Fatigue And Crack Propagation Properties Of Magnesium Alloy

Magnesium alloy is a kind of biodegradable material with great potential for clinical application.It has good biocompatibility,cell compatibility,biodegradability and mechanical properties.Magnesium alloys,as implants in vivo,bearing the combined action of body fluid corrosion and cyclic loading,namely corrosion fatigue,which accelerates the failure of magnesium alloys.Therefore,it is of great significance to study the corrosion,corrosion fatigue and crack propagation behavior of magnesium alloy,so as to accelerate the clinical application of magnesium alloys.Based on the mechanical problems of magnesium alloys in clinical application,two parts of research were carried out in this dissertation.In the first part,the electrochemical corrosion and corrosion fatigue behavior of AM60 magnesium alloy were studied based on the actual possible changes of body fluid p H value.The following conclusions were obtained from corrosion and mechanical test and failure microscopic analysis:(1)The order of corrosion rate of AM60 magnesium alloy in simulated body fluid with different p H values is:p H 5.2>p H 7.4>p H 9.0;(2)The order of corrosion fatigue life of AM60 magnesium alloy under different p H simulated body fluids is:p H 5.2<p H 7.4<p H 9.0;(3)When the applied load is greater than 120 MPa,the mechanical factor is the main factor affecting the fatigue life of magnesium alloys;when the load is lower than120 MPa,the corrosion environment is the main factor affecting the fatigue life of magnesium alloys.(4)Microscopic analysis shows that fatigue crack is originate from oxide particle under air condition,while secondary cracks are at the crack initiation when test in corrosion environment,and hydrogen embrittlement is the main factor affecting the corrosion fatigue crack initiation of AM60.The second part is based on the fact that initial cracks are inclined to occur in the stress concentration area of implants in vivo.The biaxial fatigue crack propagation test of ZK60 magnesium alloy is carried out.The effects of load phase difference and stress ratio are investigated experimentally.By means of finite element calculation,mechanical test and failure microscopic analysis,it can be obtain the conclusions as follows:(1)The initial crack propagation directions of ZK60 magnesium alloy are 42 and54 respectively under loading phase difference of 90°and 180°,and the finite element simulation results are in good agreement with experimental results.(2)The crack growth rates of ZK60 magnesium alloy under different driving forces are quite different under out of phase loading conditions.The energy release rate range?G takes into account both the effects of K_?and K_?,so it can describe the fatigue crack growth rates more reasonably.(3)?K_effand?G can uniformly describe the biaxial fatigue crack growth rate of ZK60 magnesium alloy under different stress ratios.(4)Fracture analysis shows that under different stress ratios,the crack growth mechanism of ZK60 magnesium alloy is planar slip,while under phase difference loading,the crack growth mechanism is wavy slip.