Study On Compression Ratcheting And Biaxial Fatigue Crack Propagation Behavior Of PEEK Composites

Polyetheretherketone（PEEK）has been successfully used in the field of orthopedic implants due to its excellent comprehensive properties,and it has shown great potential in hip and knee replacements.However,it still faces the risk of excessive deformation under high load and fatigue fracture at stress concentrations during service.Therefore,pre-clinical mechanical performance evaluation is very necessary.Firstly,the tensile mechanical properties of nano-hydroxyapatite（HA）and short carbon fiber（CF）doped PEEK were compared.HA and CF were uniformly mixed with PEEK powder by ball milling and compression molding at the mass ratio of 10,20 and 30%,respectively.The results show that the young’s modulus of both composite materials increases with the increase of the amount of filled particles,but the enhancement effect of CF is more significant.The tensile strength of HA-PEEK decreases with the increase of the doping amount of particles,while PEEK material doped with carbon fiber（CFR-PEEK）shows the opposite.The elongation at break of the two composites decreases significantly with the increase of the doping amount of particles,while the hardness shows an opposite trend.Next,uniaxial compression experiments of different CFR-PEEK composites were carried out.It is found that when the CF content is less than 20wt%,the compression modulus and yield strength of the material increase with the increase of CF content,but when the CF content continues to increase to 30wt%,the above parameters hardly change.Microscopic analysis shows that excessive doping of CF will lead to agglomeration and introduce more defects,thus reducing the mechanical properties of the material.The compression creep and ratcheting behavior of CFR-PEEK composites were further investigated.The results show that CF significantly improves the creep resistance of the material,and 20wt%CFR-PEEK has the best performance.The ratcheting strain of the composite increases with the increase of stress amplitude and peak stress holding time,and decreases with the increase of stress rate and CF content.The material is significantly affected by the loading history of stress amplitude due to cyclic hardening,while the loading history of stress rate has little effect on the ratcheting strain of the material.In this thesis,the fatigue crack propagation（FCG）behavior of PEEK under biaxial loading was studied,mainly including the effects of stress ratio R,biaxial stress ratioλand loading phase differenceθon FCG.It is found that in the case of horizontal cracks,the FCG rate of PEEK increases with the increase of R value,and decreases with the increase ofλvalue.At the same time,the crack closure effect of PEEK was also investigated,and it is found thatΔK_effcan uniformly describe the FCG behavior of materials at different R values.In the case of slant cracks,ΔK_ⅠandΔG were used to describe the FCG behavior under different values ofθrespectively.It is found that the experimental results described by different fracture parameters differ greatly.In addition,the study on the crack propagation path shows that the crack does not bifurcate at a 90°phase difference,but a bifurcated crack appears at a 180°phase difference.Three methods were used to predict the crack deflection angle.Microscopic analysis of the fracture shows that under different experimental conditions,the fracture surfaces of the samples all show striation and parabolic characteristics.With the progress of FCG,these features gradually increase in number and size.As the R value increases,the roughness of the fracture surfaces increases.