Structural Effect And Temperature Effect On Multiple Transverse Impact Damage Distributions In 3-D Braided Composite Beams

Three-dimensional braided composites are prepared with fiber tows braiding and matrix curing,which can realize the integration of material structures to manufacture complex shape structural parts and reduce the number of assembly connections in the way of near-net-shaping technology.Due to the high specific strength,high specific stiffness and high damage tolerance,3-D braided composites have obvious advantages over metal materials in high and low temperature environments in which the high ratio of strength/weight is required.Currently the 3-D braided composites have been widely applied to the design of high strength and lightweight engineering structures.The purpose of this investigation is to reveal the thermo-mechanical coupling damage mechanisms of 3-D carbon fiber reinforced epoxy resin braided composites with different braiding angles under multiple transverse impacts at room and high temperatures.A split Hopkinson pressure bar,X-ray micro-computed tomography and finite element analysis have been employed for studying the multiple transverse impact damage behaviors.We have conducted that:(1)An impact dynamics computational framework based on fully thermo-mechanical coupling has been developed for characterizing impact damage of fiber reinforced polymer composites such as the 3-D braided composites.We have established the material constitutive model and defined the stiffness matrix of constituents,cohesive zone model and thermo-mechanical coupling constitutive equation.The ductile and shear criteria have been introduced to establish the elastic-plastic damage constitutive relationship of composite materials.(2)A split Hopkinson pressure bar(SHPB)with self-designed heating device combined with high-speed camera was employed to test multiple transverse impact behaviors and to record the impact deformation.The ambient temperatures are: room temperature(25?),90?,150? and 210?.The braiding angles are 20°,30°(with or without axial yarns)and 40° respectively.The impact pressures are: 0.2MPa,0.4MPa and 0.6MPa.The dynamic thermo-mechanical(DMA)properties,Fourier transform infrared(FTIR)spectrometer and high strain rate impact compression properties of pure epoxy resin at different temperatures were tested to reveal the mechanical properties of epoxy resin at various temperatures.The influence of temperature,loading speed and braided structure on multiple impact damage was also found.(3)The full-scale microstructure model and macro/micro-structure hybrid structure model have been established.The thermal stress distribution and the structural effect of loading rate of 3-D multi-directional braided composites under transverse impact loading are analyzed with finite element method.The effects of microstructure,loading rate and ambient temperature on the local impact damage formation and propagation were studied.The thermo-mechanical coupling damage and impact energy absorption mechanisms were also revealed.(4)Combined with the X-ray micro-computed tomography(micro-CT)and finite element analysis,the spatial distribution of the impact damage in the 3-D braided composite beams was analyzed.And the relationship between the damage distribution and the microstructure was unveiled.We also compared the failure behaviors and damage modes of the 3-D multi-directional braided composites under different temperatures and impact velocities.The influence of temperature,loading rates and microstructure on multiple impact damages were also revealed.We have found that:(1)According to the multiple transverse impact damages of the 3-D braided composites with different braiding angles under various impact velocities,it is found that the impact peak load decreased gradually as the impact cycle continues.The samples with greater braided angle have higher resistance of transverse impact damages.The impact stress wave propagates along braided yarn path direction which leads to higher stress level at impact location in incident surface and the opposite location in backward surface.The braided composite with smaller braided angle has higher in-plane and transverse impact stiffness,while higher braided angle has higher transverse impact fracture resistance and impact stiffness.(2)We compared the transverse impact deformation and damage behaviors of the 3-D braided composites with various braiding angles under room and high temperatures.The results indicated that failure loads,initial modulus,and energy absorption decreased with the increase of temperature,whereas the deformation increased slightly with elevated temperatures.We have found that the impact brittle damages occurred earlier and the local adiabatic temperature raised higher when the temperature is lower than the glass transition temperature(Tg)of epoxy resin.While above the Tg,the impact ductile damages occurred later and the local temperature raised lower.The thermal stress distribution along the braiding yarn leads to cracks propagation in yarn direction.Part of the impact energy absorptions converted into thermal energy.In addition,the beam with larger braiding angle has high damage tolerance and crack propagation resistance.(3)The relationships among the ambient temperature,microstructure and the internal damage distribution of the 3-D braided composites under multiple transverse impacts was characterized with X-ray micro-computed tomography and finite element method.The results show that with the increase of temperature,the resin changed from brittle failure to ductile failure,the interfacial bond strength decreased,and the failure mode changed to fiber tows-resin interface cracking and resin debonding.With the increase of braiding angle,the failure mode changed from interfacial cracking to resin debonding and cracking simultaneously.The impact tolerance and the absorption energy were improved,which led to the increase of local temperature rise.As the impact cycle continued,the accumulation of impact damage resulted in deformation of the reinforcement,interface cracking and resin debonding,which had obvious cumulative effect.(4)According to the thermo-mechanical coupling damage behaviors of the 3-D braided composites during multiple transverse impacts,it is found that the adiabatic temperature rise and interfacial damage occurred in the crack and impact surface,where underwent local significant plastic deformation.The generated heat energy increased the local temperature,which further leaded to thermal expansion of the epoxy resin.In turn,the epoxy resin and the braided preform squeezed each other,resulting in adiabatic shear failure such as internal stress and local deformation.High stress concentration and interfacial debonding accelerated the development of damage in local regions,where the generated plastic deformation energy was converted into heat energy to improve local temperature rise.Therefore,a coupled thermal-stress closed loop has been formed.In summary,we have developed the completed thermo-mechanical coupling impact dynamics computational methodologies for investigating the multiple transverse impact damages in fiber reinforced composite materials.The methodologies could be extended to design other textile composite materials and engineering structures with higher impact damage tolerance under different temperatures.