Study On High Temperature Impact Compression And Shear Mechanism Of Plain CF/PEEK Thermoplastic Composites

Compared with thermoset composites,thermoplastic composites have outstanding advantages for improving the service temperature range,impact damage tolerance,postimpact repair,and maintenance of structures.Thus,the application ratio of thermoplastic composites in aerospace,military and automotive fields has been increasing yearly.However,still a lack of research on the failure mechanism of thermoplastic composites under the coupled environment of high temperature and high strain rate.In this paper,the high-temperature impact compression and shear experiments of plain CF/PEEK composites were conducted by using a Split Hopkinson Pressure Bar(SHPB)and open hot airflow heating.And with the help of strain gauge,high-speed photography,Scanning Electron Microscopy(SEM),and other tools,it focuses on the mechanical properties and failure mechanism of composites under the effect of thermo-mechanical coupling.In addition,the JC plastic constitutive model and failure criterion of PEEK resin is also established according to the Johnson-Cook constitutive equation.The main contents and achievements of this paper are as follows:(1)The experimental temperatures of 23°C,120°C,and 295°C were determined using dynamic thermomechanical property analysis(DMA).Among them,the material is in the glassy state at 23°C and 120°C,23°C is the experimental temperature at room temperature,and 120°C is the experimental temperature close to Tg.The material is in a highly elastic state at 295°C,and this temperature is approximately the limit temperature for the transient use of PEEK resin.The creative employment of the open hot airflow heating method heats the specimen in real time during the experiment.This heating method ensures the uniform heating of the specimen while facilitating the recording of the progressive damage process of the specimen using a high-speed camera device.(2)The high-temperature impact compression behavior of CF/PEEK composites was investigated at different strain rates(400-4000/s),and the mechanical properties and damage morphology of the composites during out-of-plane and in-plane compression were obtained,as well as empirical laws for the compressive mechanical properties of the composites were given.The results show that the high-temperature softening effect of the composites is stronger than the strain-rate strengthening effect.And the peak stress is more sensitive to temperature and strain rate in the in-plane direction.In contrast,the elastic modulus and damage strain are more sensitive to temperature and strain rate in the out-of-plane direction.In terms of damage,for out-ofplane impact compression,the temperature effect changes the damage form from interfacial cracking and matrix cracking in the glassy state to "parallel" shear and fiber extrusion in the highly elastic state.As the strain rate increases,the specimens exhibit significant "fragmented" shear damage in the glassy state.And more severe "parallel" shear and fiber extrusion in the high-elastic state.For in-plane impact compression at lower strain rates(1500/s),the damage changes from delamination and local shear in the glassy state to a complex mixture of kinked bands,delamination,and multiple shear bands with significant "bulge" in the high-elastic state.At a higher strain rate(2800/s),catastrophic multi-shear zone damage and delamination failure occurred in the in-plane direction of the specimen.(3)The dynamic shear behavior of CF/PEEK composites subjected to impact loading at different temperatures(23°C,120°C,295°C)and strain rates(2500-6000/s)was investigated to obtain the mechanical properties and damage morphology of the composites during high-temperature impact shear,and the empirical laws of the shear mechanical properties of the composites were also given.The results show that with the increase of impact load,the effect of high temperature softening on the stiffness and strength of the composite is mitigated.In contrast,the effect of deformation is enhanced accordingly.The effect of strain rate strengthening on the composites’ stiffness,strength,and deformation was enhanced as temperature increased.In terms of damage,unlike the shear damage and yarn fracture that occurred after impact shear loading with a strain rate of 2759/s at 23°C(glassy state).The overall damage of the composite was significantly reduced after impact shear loading with a strain rate of 2834/s at 295°C(high elastic state).The failure in the shear region was manifested in the form of fiber buckling,fiber slippage,fiber pullout,and slight shear damage.When the specimens were subjected to a high strain rate impact load of 6000/s in the glassy state,the damage in the shear region of the specimen was significantly increased.The failure was in the form of catastrophic shear damage with some fiber pullout.Besides,after the composite specimens were subjected to the impact shear load in the above experimental environment,their internal fiber bundles would buckle under the load.Then a certain tensile stress would be generated in the fiber bundles.Which would lead to different degrees of interfacial cracking,matrix cracking,delamination,and other interlaminar failure forms in the non-shear region of the specimens.(4)The JC plastic constitutive model and failure criterion parameters of PEEK resin were derived using quasi-static compression and unidirectional tensile experimental data combined with Johnson-Cook constitutive equations.Then established the JC plastic constitutive model and failure criterion of PEEK resin.This work will provide great constitutive model support for the research of PEEK resinbased composites in finite element simulation.