Temperature Effect And Thermo-mechanical Coupling Mechanisms On Compressive Behaviors Of 3-D Braided Carbon Fiber/epoxy Composite Materials

Three-dimensional(3-D)braided composites have been widely applied to various engineering structures' design because of the preform structural integrity,delamination resistance,high-strength light-weight and versatile design ability.The 3-D braided composites have also been applied to various working conditions,such as high/low temperature condition.Under high/low temperature condition,it is inevitable that the compressive behaviors of 3-D braided composites are influenced by ambient temperature.This project was aimed at the temperature effect on the 3-D braided carbon/epoxy composites' quasi-static compressive behaviors.The effects of temperature and braided angle on the compressive behaviors were investigated.A full size microstructure model was established to predict the compressive properties,damage process and thermo-mechanical properties of 3-D braided composites in high/low temperature environment.The thermo-mechanical coupling mechanisms at microstructure level were found.The main works of this project are:(1)The quasi-static compressive properties of 3-D braided composites and epoxy resin were experimentally investigated in high/low temperature field.Stress-strain curves,energy absorption and damage morphologies were obtained.The effects of braided angle and temperature on the quasi-static compressive properties were studied.The selected braided angles are 26°,35° and 48° and the selected temperatures are-100?,-50?,0?,20?(room temperature),80?,110? and 140?,respectively.(2)Numerical simulation of the temperature effect on 3-D braided composites'compressive properties:A full size microstructural finite element model was established to reveal the microstructural mechanisms and failure mechanisms for the quasi-static compressive behaviors of 3-D braided composites.The effects of temperature and braided angle on the compressive behaviors were observed from the aspects of stress distribution,damage morphologies and damage process.(3)Thermo-mechanical coupling mechanism for compressive properties:The microstructural model was used to investigate the thermal stress and thermal expansion of 3-D braided composites,and to reveal the internal mechanisms.A multi-unit cell model was established to predict the variation of thermal expansion coefficients with braided angle.We found that:(1)Experimental:The temperature and braided angle influenced the quasi-static compressive properties and damage of 3-D braided composites significantly.The compression modulus,yield stress,failure stress and before-failure energy absorption were negatively associated with temperature and braided angle.Compared with the temperature,the braided angle affected the mechanical properties of 3-D braided composites more significantly.As the braided angle increased,the failure feature transformed from brittle to ductile,and the main bearing body shifted from yarn to resin matrix.For a big braided angle,the main damage modes of 3-D braided composite included matrix cracking and interfacial debonding.However,for a small braided angle,the main damage modes were matrix cracking and yarn breakage.The matrix cracking were mainly located at yarn edges and loading ends.The yarn breakage was mainly located in the bending area of surface yarn.Low temperature resulted in the severe damage with strong brittle behaviors.And high temperature resulted in the slight damage and highlighted the ductile characteristics.(2)Numerical results:The interfacial properties and fiber straightness affected the compressive performances of 3-D braided composites significantly.The model with imperfect fiber/resin interface could predict the compressive properties of 3-D braided composites more effectively.Temperatrue and braided angle affected the stress distribution and damage mechanism significantly.As the braided angle decreased,the stress distribution of matrix got more uniform,and the stress distribution of yarn got more nonuniform.Temperature variation increased the nonuniformity of stress distribution.As the braided angle decreased,the main damage body changed from matrix to yarn during the compression process.Low temperature increased damage and the matrix was mainly dominated by brittle failure.Conversely,high temperature decreased damage and the matrix was mainly dominated by ductile failure.(3)Thermo-mechanical coupling mechanism:Temperature variation,on the one hand,changed the constituent properties,and on the other hand,led to the inconsistent thermal deformation between constituents,which gave rise to the thermal stress difference.Virtually,under the quasi-static condition,the thermo-mechanical coupling mode could be found as unidirectional sequential coupling.The thermo-mechanical coupling of 3-D braided composites has obvious structural feature.The thermal stress slightly affected the compressive behaviors of 3-D braided composites.As the braided angle increased,the thermal stress level increased.Interfacial thermal stress differed in high and low temperature fields significantly.In high temperature field,the damage was dominated by compressive interfacial thermal stress.In low temperature,the compressive damage was domintated by the tensile interfacial thermal stress.As the braided angle increased,the interfaical thermal stress increased.(4)Thermal expansion behaviors:The axial/transverse thermal expansion coefficient was nonlinearly associated with braided angle.As the braided angle decreased,the transverse thermal expansion coefficient was always positive,while the axial thermal expansion coefficient diverted from negative to positive.For a certain braided angle(39°),the axial thermal expansion coefficient was nearly zero.It was found through numerical analysis that the yarn rotation and the constituent thermophysical properties were important internal mechanisms for the thermal expansion behaviors.The temperature effect on the quasi-static compression of 3-D braided composites is an important topic for the engineering application.This project investigated the temperature effect on the quasi-static compression of 3-D braided composites and the thermo-mechanical coupling properties by means of experiments and finite element analysis.A multi-unit cell model was established for the convenience of practical application.Such a methodology could be as a guide for the 3-D braided composite engineering design in different temperature fields,and also could be the basis for other 3-D textile structural composites' engineering design.