Thermo-mechanical Coupled Analysis Of Thermo-oxidative Aging Effect And Structural Effect On 3-D Braided Composites Under High-speed Compressions

Carbon fiber reinforced composites,especially carbon fiber reinforced three-dimensional(3-D)braided composites,have excellent structural designability,overall stability,anti-delamination,anti-shear properties and other characteristics,which have great application space in aerospace and other engineering fields.The thermo-oxidative ageing in atmospheric environment and high-speed impact in the application of aerospace are unavoidable in the design of composites.For the Performance design and application prospect of composites,it is meaningful to reveal the influence of thermo-oxidative ageing effect and structural effect on the high-speed impact responses and damage failure mechanism of 3-D braided composites.By means of experimental characterization,theoretical analysis and finite element simulation,the purpose of this project is to explore the high-speed impact performance,temperature and thermal stress changes of carbon fiber reinforced epoxy resin 3-D braided composites with different braided structures before and after thermo-oxidative ageing,and to reveal the damage and failure mechanism of braided composites under different braided structures and different thermo-oxidative ageing conditions.The obtained results would provide a reference for the long-term design of the braided composites in the atmospheric environment.The main contents are as follows.(1)Thermo-oxidative ageing of braided composites and epoxy resins,and the changes of properties of composites and epoxy resin before and after ageing were characterized by experiments.According to the dynamic mechanical analyses(DMA)results of braided composites,three levels of thermo-oxidative ageing temperature were selected:110 ~oC,130?and 150?.Then all samples were isothermally aged at prescribed temperature for 2,4,8 and 16 days in an air circulating oven.The high-speed impact performances of composites and epoxy resins before and after aging were tested on a SHPB system.Meanwhile,we employed a high-speed camera to capture the damage and failure process of composites and resins.The interface change after ageing was recorded by using an electric scanning microscope.Microscope-infrared spectroscopy(Micro-FTIR)analysis was used to analyze the chemical change of resins after ageing.The damage and failure mechanism of composite materials was revealed by means of above experiments.(2)Plane strain analysis of transient thermoelastic response of braided composites.Based on the generalized thermoelastic theory,non-Fourier heat conduction model and fractional derivative model were used to study the internal thermal stress,strain field and temperature field response of braided composites with different braided structures when the external temperature field changes.The effect of the braided structure on the thermos-mechanical coupling behaviors and stress distribution will be revealed.(3)The high-speed impact compression responses of different braided composites under different ageing conditions are calculated using finite element simulation.Based on the real spatial configuration of braided yarn,the meso-structural finite element models were established.Moreover,we proposed a continuum finite element model based on continuum damage mechanics.The coupled effects of braided structure and thermo-oxidative ageing on the high-speed impact responses were revealed from the aspects of stress,failure,energy absorption,temperature,thermal stress,and interface damage during the impact.We found that:(1)The braiding structure has a significant effect on the high-speed responses of braided composites.The impact stress and impact resistance of the composite increase with the increase of braiding angle.When the braiding angle is 42°,the damage of the composite is not obvious.With the increase of braiding angle,the plastic deformation and temperature rise of the composites increase gradually during the impact process,and the corresponding thermal stress is also increasing.It is found that the thermal stress of 42~o braided composite is at least 5 times of that of 19~o braided composite when the temperature field changes in the direction of impact.Thermal stress has a great influence on the damage initiation and interface damage of braided composites.Although the temperature change of the composite with the braiding angle of 19 is not obvious and the thermal stress is small.The damage initiation of the model considering thermal stress is earlier and the interface damage is more serious by comparing WT and NT models.Therefore,thermal stress plays an important role in the damage of composite materials(2)The damage and failure of braided composites is the result of resin failure and interface failure.The damage and failure mechanism of braided composite will change after thermo-oxidative ageing.The impact performances of braided composites and epoxy resin decreased significantly after thermo-oxidative ageing,and the impact stress decreased gradually with the increase of ageing temperature and aging time.Unaged resin and the resin aged at 110?show brittle fracture after impact.After aging,the brittleness of the resin decreased and the toughness increased with the increase of ageing temperature.No obvious crack could be found for the epoxy resin aged at 130?and 150?during the impact.After ageing,the interfacial performances of the braided composites also decreased,and the interfacial performances gradually weakened with the increase of aging temperature.When the ageing time is 16 days and the aging temperature is 110?,the interface damage is not obvious,but for the composites aged at 130?and 150?,the interface crack can be observed obviously.In addition,due to the degradation of the properties of the aged resin,the plastic deformation and temperature rise of the composites decreased significantly,so the thermal stress also decreased.For the unaged composite and the composite aged at low ageing temperature(110?),the damage of resin plays a leading role in the damage of composites and the energy absorption of fiber tows.While for the composites at high ageing temperature(130?and 150?),due to the serious interface damage,the interface damage plays a leading role in the damage of composites and the energy absorption of fiber tows.(3)With the increase of braiding angle,the effect of thermal oxygen aging on the properties of composites will decrease gradually.For the braided composites,the stress,temperature rise,thermal strain and thermal stress will decrease after thermo-oxidative ageing,and these variables will increase with the increase of braiding angle.Increasing braiding angle can effectively reduce the negative effect of thermo-oxidative ageing.When the braiding angle is 42~o,the stress-strain and damage of the composite before and after ageing have no obvious difference,and the ageing has little effect on its impact performance and damage,but the temperature rise,thermal strain and thermal stress decrease significantly after ageing.Based on the high-speed impact performances of 3-D braided composites,this paper studies the influence of thermo-oxidative ageing and braided structure on the performance of braided composites.By using the method of theoretical analysis and finite element simulation,the influence of structural effect and thermo-oxidative ageing effect on the performances of braided composites was investigated.The obtained results were compared to experimental results to reveal the damage mechanism of braided composites.The results provide the theoretical basis for the long-term application service,structural design and engineering manufacture of braided composites in the thermo-oxidative environment.