Analyses On The Compression Property And Impact Damage Modes Of CF/PPS Composite At High Temperatueres

High-performance thermoplastic composites are regarded as the preferred materials for secondary aircraft structures due to their good heat resistance,high fracture toughness,excellent impact damage tolerance,improved processing technologies and recyclability over traditional composites.Thus,thermoplastic composites are often used as some components under special temperature environments and loadings,highly such as engine panel,which will bear the coupling effect of high temperature and mechanical loads.Although some researches have been carried out about the mechanical properties of thermoplastic composites in high temperature environments,limited by the insufficient theories and lack of experiences,the mechanical properties degradation mechanism and failure mechanism of thermoplastic composites at high temperature are not clear,especially for the compression and impact load conditions that can produce more complex failure modes.To this point,experimental,theoretical and numerical investigations were carried out on compressive and low velocity impact failure behaviors of carbon fiber fabric reinforced polyphenylene sulfide(CF/PPS)composite in high temperature environments.The weakening nature of mechanical properties and the evolution law of internal and external macro and micro damage under high temperature were emphatically analyzed,aiming at providing data accumulation and technical support for the applications of thermoplastic composites in special environments.In this paper,the thermal properties of CF/PPS composites were studied by differential scanning calorimetry(DSC)and dynamic thermomechanical analyzer(DMA),the transformation process and the degradation rule of modulus with temperature were analyzed,and the test temperature was determined according to the glass transition temperature.Compression tests were carried out on CF/PPS composite unnotched and notched specimens respectively,and the change rules of mechanical response and damage modes with temperature were analyzed.The failure mechanism and influence of temperature on the mechanical properties of composite were revealed.The results show that in the same temperature range,the degradation degree of the load capacity of the notched specimens was weaker than that of the unnoeched,and the failure modes of notched and unnotched specimens were affected by the structural characteristics of the woven fabric and the state of matrix at high temperatures,showing the transformation from shear fracture to micro buckling and from brooming to kink band respectively.Finally,based on the DMA test results,the compression strength and modulus at different temperatures were predicted by the thermodynamic theory prediction method,then validated by the test results.In order to further analyze the stress state,damage patterns and the influence of temperature on notched specimens under compression,a compression finite element analysis model of CF/PPS composite was established,in which the required parameters were obtained by the thermodynamics method.Compared with the test results,the applicability of the FEM in high-temperature compression property prediction of CF/PPS composite were verified,and the influence of diameter and laying sequence under the same D/W on the load capacity of notched specimens were analyzed.The results showed that the increase of temperature can affect the shear stress distribution of(± 45o)layers,but it had no significant effect on the damage patterns;when the D/W was constant,the increase of diameter can reduce the load capacity of notched specimens,and when D/W remained unchanged,the increase of clustering effect in the same direction can also reduce the load capacity.In order to clarify the low velocity impact damage mechanism of CF/PPS composite and lay a foundation for the research of low velocity impact behavior under high temperature,the low velocity impact tests of different energy levels were carried out at room temperature.The mechanical responses of low velocity impact were studied by impact force curve,punch speed curve and impact energy curve,and the impact damage patterns was analyzed by the dent depth,the surface damage morphology and the overall damage area.The results showed that at room temperature,when the impact energy was more than 25 J,the impact force did not change basically.When the impact energy was less than 15 J,the damage degree was low and the dent depth was still under the level of BVID.But when the impact energy was more than 25 J,the damage degree and damage range were significantly increased.Combined with the mechanical response and damage patterns analysis,15 J and 25 J can be used as the experimental impact energy for CF/PPS composite impacted at high temperatures.On the basis of the tests,the calculation method of impact force based on Hertz contact theory was established.It was found that the calculation results were in good agreement with the test results when considering the damage.Based on the analysis of thermal properties and the study of low velocity impact behavior at room temperature,with 15 J and 25 J as impact energy,the impact tests of CF/PPS composite at high temperatures were carried out to study the influence of temperature on the mechanical response and damage patterns The mechanical response analysis still mainly focused on the impact force,punch speed and impact energy curve,while the damage patterns analysis mainly involved the dent depth and surface damage morphology and overall damage area,and the failure mechanism of low velocity impact at high temperatures was also revealed based on the internal microscale damage morphology.The results suggested that the variation of impact responses with temperature were directly related to the changes of the damage modes.While the changes of damage modes were mainly reflected in the crossover location of warp and weft fiber bundles in the woven structure.When the impact energy was 15 J,the temperature had obvious effect on the impact damage,while the impact energy was 25 J,the subcritical damages,including matrix cracks,fiber bundles debonding and delaminations presented large-scale discontinuty distribution phenomenon and the inhibition of temperature was weak.In addition,the energy balance model based on Hertz contact theory was also effective on calculating impact force in the high temperatures.