Mechanical Behavior And Failure Mechanism Of Short Fiber Reinforced PEEK Composites

Thermoplastic polymer reinforced with short fibers is widely used in the field ofmedical treatment,electronic industry,automobile industry,aeronautics and astronautics due to its excellent mechanical properties,easy processing,retrievability,and low production cost.As one of the best thermoplastic polymer,PEEK(Polyetheretherketone)and its composites have many advantages,like high strength,high modulus,good impact resistance and corrosion resistance.Hence,PEEK and its composites have significant applications in the field of aeronautics and astronautics.PEEK and its composites have been studied in terms of mechanical characterization,failure mechaism and constitutive models under quasi-static conditions,as reported in the literatures.However,the studies on the mechanical behavior and failure mechanism of PEEK composites under dynamic loading conditions are relatively scarce,which leads to the insufficient understanding of the rate-dependent mechanical behavior and failure mechanism of PEEK composites.Moreover,with the continuous pursuit of lightweight and high energy efficiency in aviation structure design,the nonlinear dynamics problem of structures becomes more and more prominent,and it is urgent to further investigate the dynamic mechanical properties of aviation materials.Therefore,this paper carried out a systematic experimental study on the mechanical behavior and failure mechanism of short fiber reinforced PEEK composites under dynamic loading conditions,and focused on investigating the effect of strain rate,reinforcing phase and temperature on the mechanical failure behavior of PEEK composites,and established the corresponding mechanical constitutive models.The main work and conclusions are drawn and listed as follows:(1)The quasi-static and dynamic compressive mechanical behaviors of PEEK composites are studied by electronic universal testing machine and split Hopkinson pressure bar.Utilizing the high-speed camera and scanning electron microscope(SEM),the deformation,failure process and fracture morphology of samples are characterized.Combined with the stress-strain response,the rate-dependent failure characteristics of the material are analyzed,and the rate sensitivity of the interfacial properties and its influence on the failure mechanisms of the material are revealed.The results show that the dynamic compressive strength of PEEK matrix shows certain sensitivity to strain rates,while PEEK composites show stronger strain rates sensitivity.There is an obvious enhancement of compressive strength through reinforcing PEEK with short fibers.The compressive failure strain of PEEK composites decreases with the increase of strain rates.The main failure modes of short carbon fiber reinforced PEEK(SCFR-PEEK)and short glass fiber reinforced PEEK(SGFR-PEEK)are fiber-matrix interfacial debonding and fiber pulled-out.The strain rate sensitivity of the fiber-matrix interface is the origin of strain rate sensitivity for the compressive response of short fiber reinforced PEEK(SFR-PEEK).The local heat generation causes matrix flowing at the fracture surface,which can further weaken the fiber-matrix interface,promote the crack propagation and fracture of the specimen.Hence,the interface debonding is one of the main damage modes controlling the failure of SFR-PEEK composites.Besides,the mechanical behavior and failure mechanism of SGFR-PEEK composites made from different processing technologies have been analyzed.The results show that the different angle distributions of the reinforced fibers caused by the processing methods affect the rate-sensitivity and ductility.And the injection specimens show better ductility than that of extrusion specimens.(2)A series of tests are conducted to investigate the quasi-static and dynamic tensile mechanical properties of PEEK composites at room temperature.During the test,digital image correlation(DIC)technology is used to obtain the displacement field and strain field for the surface of the samples,and the stress-strain curves at different strain rates are obtained.Besides,combined with energy dispersive X-ray spectroscopy(EDS)and SEM,the differences in deformation and failure mechanism of PEEK composites under quasi-static and dynamic tensile loads are analyzed,and the rate-dependent failure mechanisms of PEEK composites is revealed.The results show that the tensile strength of the PEEK matrix and PEEK composites show little sensitivity to strain rates.There is a noticeable enhancement of rate sensitivity for tensile strength through the addition of short fibers,and the failure strain of the PEEK matrix decreases with the increase of strain rate.The change of rate sensitivity is mainly caused by the different failure mechanisms of PEEK composites under quasi-static and dynamic conditions.Compared with the compressive tests,the strain-rate sensitivity of interfacial strength plays an essential role in the tensile failure mechanism of PEEK composites.In addition,the surface of the pulled-out fibers under dynamic tensile loading is rougher showing a higher interfacial fracture toughness,compared with that under quasi-static conditions.For the dynamic tensile tests,the main failure mode of SCFR-PEEK is fiber-matrix interfacial debonding and fiber pull-out,while for SGFR-PEEK fiber breakage is the main contributor of effective tensile strength.(3)A series of quasi-static and dynamic tensile tests are carried out under high(100~oC)and low(-30~oC)temperatures.Combined with the analysis of the surface morphology and the tensile stress-strain responses,the correlation between mechanical behaviors and strain rate and temperature for PEEK composites is described.The results show that the tensile strength and failure strain of PEEK and its composites present strong temperature sensitivity.At high temperature,the toughness enhancement and strength decrease of PEEK matrix and interface are the main factors affecting the failure mechanism of PEEK composites.On the contrary,at low temperature,the main factors are the decrease of toughness and the increase in strength.At high temperature,the main failure modes of PEEK composite are the interfacial debonding and fiber pulled-out,while at low temperature the fiber breakage plays a dominator role.On the other hand,the rate sensitivity of tensile strength and failure strain of PEEK composites are also affected by the testing temperature.(4)The modified phenomenological constitutive model and ZWT constitutive model with consideration of temperature and strain rate effect are established.Based on the systematic test data of stress-strain responses,the constitutive parameters of PEEK composites are determined under different loading conditions.Among them,the phenomenological constitutive model is modified by combining the Maxwell model with the WLF equation to take into consideration the time-temperature effect,while the ZWT constitutive equation is modified by introducing functional relationships between constitutive parameters and temperature.The constitutive models established in this paper show good agreements with the experimental tensile stress-strain curves,but slight deficiency in fitting the compressive stress-strain response.Comparing the two constitutive models,the modified ZWT model shows better accurate in describing the stress-strain response of the elastic deformation region,while the modified phenomenological constitutive model uses only one set of parameters to fit stress-strain curves from elastic region to flow stress region of the material at different strain rates and different temperatures.The obtained constitutive models can be applied in the numerical simulation study and structural design of short fiber reinforced thermoplastic composites structures.