Experimental Characterization And Multi-scale Finite Element Analysis Of Thermal Conduction And Thermal Expansion Properties Of Textile Structural Composites

Compared with traditional metal materials,textile structural composites(TSCs)have the advantages of high specific strength,high specific stiffness,light weight,and corrosion resistance,with potential applications in the fields of aircraft,high speed vehicles,ship,engineering construction,sports equipments,and etc.In practical applications,textile structural composites will be always subjected to complex or long-term thermal stimuli.The concentration of thermal stress or the increase of thermal strain induced from intense temperature variations are one of the main forms of fatigue and even failure of TSCs.In order to avoid potential safety hazards resulting from thermal fatigue as well as to deepen the understanding of internal heat transfer process and thermal deformation trend of TSCs,it is important to study their thermal conduction and thermal expansion properties.In this paper,the thermal conduction and thermal expansion behaviors of different TSCs are studied by integrating experimental measurement with multi-scale finite element analysis.Meanwhile,the influences of structural factor,temperature effect,material properties,interfacial thermal resistance,and porosity on the thermal conduction and thermal expansion performances are also discussed.There are mainly four kinds of TSCs involved in this paper,i.e.,unidirectional laminated composite(UD),plain woven laminated composite(PW),3D angle-interlocking woven composite(3DAWC),and 3D rectangular braided composite(3DRBC).The main contents are as follows:(1)Thermodynamic performance tests of epoxy resin: The dynamic thermo-mechanical properties,initial and complete degradation temperatures,and specific heat capacity of epoxy resin were measured by dynamic mechanical analysis(DMA),thermogravimetric analysis(TG),and differential scanning calorimeter(DSC),respectively.(2)Thermal conductivity tests: A self-made device was established to measure the in-plane and out-of-plane thermal conductivities of TSCs,which was first calibrated with standard aluminum samples.The transient hot-wire method was used to investigate the in-plane thermal conduction behaviors of PW and 3DRBC.The hot plate method under steady state condition was adopted to characterize the in-plane thermal conduction behavior of 3DAWC and out-of-plane thermal conduction behaviors of all TSCs.A standard thermal conductive device(DZDR-S)was selected to measure the out-of-plane thermal conductivities of UD,PW,and 3DAWC in order to validate the results of the self-made device.The interface contact resistance and air convection coefficient are also measured by the self-made device.The infrared thermography was used to observe the heat transfer processes as well as compare thermal conductive capabilities between different TSCs.(3)Coefficient of thermal expansion(CTE)tests: A classic high temperature and horizontal dilatometer,which was first calibrated by a standard quartz glass sample,was used to measure the in-plane and out-of-plane CTEs of UD,PW,and 3DAWC.(4)Multi-scale finite element models: Based on the method of equal porosity,the resin-voids unit cell(UC)was established to predict the thermo-mechanical properties of resin containing voids.According to the hexagonal distribution of fiber in matrix and the principle of equal volume fraction,a micro-scale fiber bundle UC was introduced to analyze the thermo-mechanical properties of matrix impregnated fiber yarns.In order to reduce the model size and improve the computational efficiency,meso-scale UC was established with representative volume elements(RVEs)of composites to calculate the thermal conductivities and CTEs of TSCs.In order to observe internal temperature distribution and heat flow transmission path,full-scale composite structural models were also established based on the practical dimensions.The results obtained from the resin-voids UC and micro-scale fiber bundle UC,regarded as thermo-mechanical properties of matrix and yarn,were further used for calculation of the meso-scale UC and full-scale composite model.The periodical displacement and temperature boundary conditions(PBCs)were applied to the UCs based on master node to slave nodes technology.According to the practical temperature loading methods,the macroscopic temperature boundary conditions were applied to the full-scale composite structural models.(5)Analysis of thermal conduction behavior: The thermal conductivities of carbon fiber and epoxy resin present an approximate linear correlation with temperature,leading to obvious temperature dependence of the thermal conductivities of TSCs.Compared with meso-scale UC,the thermal conductivities obtained from full-scale structural model are closer to those of experimental.The thermal conductivities of the TSCs increase with the increase of applied temperature or fiber volume fraction,the decrease of the angle between fiber and heat flow,interfacial contact resistance,or porosity.The thermal flows mainly distributed on fibers and preferentially selected the direction with high thermal conductivity.The temperature distribution and thermal flow transmission path depended on the yarn's orientation.Yarn with high thermal conductivity would also conduct some heat to the surrounding resin with low thermal conductivity.And the closer the resin to yarn was,the higher temperature of the resin would be.The temperature of interface between carbon fiber and epoxy resin was between the upper limit temperature of carbon fiber and the lower limit temperature of epoxy resin.In addition to the 3DRBC,the in-plane thermal conductive capability was superior to that of out-of-plane direction.In the in-plane direction,temperature and thermal flow transmitted fastest along the direction of yarn perpendicular to the heat source.In the out-of-plane direction,temperature and thermal flow gradually decreased from the bottom heat source to the top heat sink.As for the 3DRBC,its axial thermal conductivity was higher than that of radial direction.In its axial direction,temperature diffused from the central heat source and presented a “concave” distribution.Thermal flow transmitted from the heat source along the orientated direction of braided yarn in a “×” path.In its radial direction,the temperature distributions of inner,surface,and corner regions depended on the orientation path of braided yarn.When the braided yarns moved to the surface and corner regions,their temperatures would be higher than those of inner region.Since the fiber volume fraction of inner region was largest,“convex” temperature distribution would appear on the surface of its axial direction.(6)Analysis of thermal expansion behavior: The thermo-mechanical properties of epoxy resin played a key role in the temperature-dependent thermal expansion behavior of the TSCs.After the glass transition temperature of epoxy resin,the in-plane CTEs of the TSCs gradually decreased to negative,while their out-of-plane CTEs increased rapidly.The axial negative CTEs of carbon fiber led to the in-plane thermal shrinkage of TSCs.The free thermal expansion of epoxy resin was constrained by the textile preforms,resulting in weaker out-of-plane thermal expansion capability of TSCs than that of pure epoxy resin.However,due to the existence of interface,the opposite phenomenon occurred.The CTEs of the TSCs decreased with the decrease of angle between fiber and thermal flow,the increase of fiber volume fraction,or porosity.Compared with traditional numerical prediction models of UD composite,the results obtained from finite element method(FEM)were closer to those of experimental.The interfacial thermal stress was maximum along the in-plane loading and minimum along the out-of-plane loading.For the in-plane loading of PW and 3DAWC,yarns in the loading direction experienced first expansion and then shrinkage.The radial position of interweaving yarns would also change,due to the constraints from the yarn in the loading direction.Both of warp and weft yarns would appear a slightly lateral expansion.For their out-of-plane loading,the textile preforms could effectively reduce the thermal displacement and thermal strain,but greatly increase the thermal stress.Due to the heterogeneity of their internal structure,the resin region near higher fiber volume fraction had larger thermal stress and thermal strain.After the glass transition temperature of epoxy resin,the thermal stress on epoxy resin gradually fell close to zero,but its thermal strain rose rapidly.The experimental method and multi-scale finite element analysis can be successively extended to investigate the thermal conduction,thermal expansion,and thermal-mechanical coupling properties of other more complex TSCs.We hope such a research could be applied to predict thermal stability as well as to evaluate thermal damage resistance performance of spacecraft,automobile hood,integrated circuit board et al in a variable temperature environment.