| Three dimensional braided composite T-beam incorporated many excellent features of composite,3D braided structure and T-beam,including high specific strength,high specific stiffness,high damage tolerance and fracture toughness,high delamination resistance and high bending stiffness.It has a great potential in the application of high-speed trains,new energy vehicles and unmanned aerial vehicles(UAV).This work aims to analysis the thermal-mechanical coupling response and damage mechanism of 3D braided composite T-beam under high temperature field and transverse impact loading.The main contents are as follows:(1)Quasi-static transverse loading tests of 3D braided composite T-beam with different rib height(0mm ? 5mm and 10mm)were carried out under different temperature(20 ?,50 ?,75 ? and 100 ?)with MTS 810.23 materials tester combined with a self made high temperature device.The loading responses were studied.(2)Transverse impact loading tests of 3D braided composite T-beam with different rib height(0mm,5mm and 10mm)were carried out under different temperature(20?,50?,75? and 100?)and different loading velocity(8.5 m/s,11 m/s and 13.5 m/s)with the modified split hopkinson bar(SHPB)combined with a self made high temperature device.The effects of test temperature,impact velocity and rib height on the impact loading responses of T-beam were analyzed.(3)The multi-scale structure geometry model of 3D braided composite was established.Taken into consideration the temperature effect and strain rate effect,the thermal-mechanical coupling constitutive material model was established.The constitutive material model combined with the adiabatic temperature rise equation constituted a closed thermal-mechanical coupling model.The user-defined material subroutine(VUMAT)was written in Fortran 90 code for finite element calculation.The 3D braided composite impact compression finite element model was established with software ABAQUS to calculate the thermal-mechanical coulping responses of 3D braided composite and verify the model's validity.(4)Based on the established thermal-mechanical coupling model,the transverse impact loading finite element model of 3D braided composite T-beam under elevated temperature was established with software ABAQUS to calculate the thermal-mechanical coulping responses of T-beam and analyze the damage mechanism.Major findings are as follows:(1)The effects of temperature and rib height on the quasi-static transverse loading responses of the 3D braided composite T-beam were obvious.The quasi-static load and energy absorption of T-beam decrease with increasing test temperature,and increase with increasing rib height.The displacement decreases with increasing test temperature and rib height.The effects of rib height on the quasi-static transverse loading responses of T-beam were more significant than those of temperature.The main failure mode of T-beam was yarn breakage and resin cracking.The failure mode was brittleness at lower temperature,while plasticity at higher temperature.(2)The effects of temperature,impact velocity and rib height on the transverse impact loading response of composite T-beam were significant.The higher temperature led to smaller transverse impact load,larger displacement and smaller impact absorption energy.The higher impact velocity led to larger transverse impact load,larger displacement,and larger impact absorption energy.The higher rib height led to larger transverse impact load,smaller displacement and larger impact absorption energy.The effects of impact velocity and rib height on the impact loading responses of T-beam were more significant than those of temperature.The main failure mode was shear failure,which manifested as yarn breakage,resin cracking and crack growth from flange to rib of T-beam.(3)The impact compression responses of 3D braided composite by finite element simulation verified the effectiveness of the thermal-mechanical coupling model.The impact compression deformation led to heat generation,resulting in the increase of composite temperature.The plastic deformation was the main source of temperature rise.The higher test temperature led to the smaller stress and modulus,resulting in less plastic work and smaller temperature rise.The higher impact loading pressure led to the higher strain rate,resulting in bigger failure stress and strain,and thus,the greater plastic work and higher temperature rise.The stress was affected by temperature and impact loading strain rate.In turn,the material temperature rise was affected by the stress and then affected the material properties,forming a closed thermal-mechanical coupled cycle.The stress distribution and temperature distribution of composite showed a "X" type shear distribution.It implied that the composite was mainly subjected to shear stress.The main failure mode was shear failure.(4)The thermal-mechanical coupled finite element model can effectively simulate the transverse impact response behaviors of 3D braided composite T-beam.At different test temperature,the peak stress of T-beam at the center point decreases with increasing stress wave period.The first cycle peak stress decreases with increasing test temperature.Corresponded to the stress,the periodic temperature rise gradually decreases,and the total temperature rise gradually accumulates.The temperature rise decreases with increasing test temperature.The greater the stress is,the higher the temperature rise is.The higher the test temperature is,the smaller the stress is,forming a thermal-mechanical coupling loop.The stress distribution and temperature distribution range of T-beam increase gradually with increasing stress wave,resulting in the increase of specimen damage,which was mainly concentrated in the loading position and the clamping position of T-beam.As test temperature increases,the stress and temperature rise decrease,and the stress distribution and temperature distribution range also decrease.The material performance weakens with increasing test temperature.Under different impact velocity,the peak stress of T-beam loading at the center point decreases with increasing stress wave period and increases with increasing loading rate.The corresponded temperature rise gradually decreases with the periodic loading of the stress wave,and the total temperature rise gradually accumulates.The temperature rise increases with increasing loading rate.The stress distribution and temperature distribution of T-beam increase gradually with increasing stress wave period,resulting in the increase of specimen damage,which was mainly concentrated in the central loading position and the clamping position.As the increase of impact velocity,the same period stress distribution and temperature distribution range increased,and the sample damage increased.Comparing the damage morphology between calculation model and test specimen,the stress concentration zone and damage area decrease with increasing test temperature.The damage was mainly concentrated in the central loading position and the clamping position.The stress distribution range and the damage area increase with increasing impact velocity.3D braided composite T-beam embodies a good impact resistance.The thermal-mechanical coupling model can be extended to the analysis and forecast of dynamic impact response of other textile structural composites under different temperature field and dynamic impact loading.It can be used to provides strength prediction and performance evaluation for the application of textile composite in high speed train,lightweight vehicle and aircraft. |
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