Research On Prediction Of Mechanical Properties And Thermal Physics Properties Of Braided Composites

Braided composite is composed by interlock fibers and matrix, its overall materialperformance is more excellent to overcome easier layering and lower intensity of thetraditional laminated composite material. Braided composite has more uniformmechanical properties which can be easy to be designed. It can satisfy high weight andhigh load carrying requirements for aerospace structural material. Because environment inaerospace is extreme hot and cold, predication for thermal properties of materials is alsorequired. Therefore it is very significant to predicate the mechanical and thermo-physicalproperties of braided composites and design associated prediction system softwareplatform.Firstly, the methods such as specimen dimension, clamping method and strainmeasurement of three-dimensional braided composite tensile test specimen are determinedrefer to the relational standard of other material tensile test specimen. The intensity andelastic modulus are obtained by combination material testing machine and datameasurement system. Microscope photos of fracture surface are obtained as reference forsubsequent simulation analysis. Using Tcl language based on ANSYS computing platformto achieve a braided composite property predication system, the experimental data isstored in the corresponding storage module which is developed for braided compositepredication system.Based on domain superimposed thinking, a direct meshing method is put forward forthe problem of meshing complicated entities composing of the reinforcement and matrix.The parametric cell meshed by hexahedral element is built with cutting method. In cuttingalgorithm, thirty-six conditions are included based on relative position between unit cellboundaries and elements, in which cutting conditions for hexahedral are fourteen, cuttingconditions for prism are nineteen, cutting conditions for tetrahedral are three. The code ofelement extracting algorithm is written in fortran language. The datas are transferred fromself-written code to ANSYS database efficiently. It is the foundation for developingcomposite mechanical and thermal prediction system software. Based on domain superimposed method, the relationships between degree of freedomof elements from reinforcement and matrix are given and the matching methods ofsuperimposed region mechanics and thermal properties are determined. In order to applyperiodic boundary condition in ANSYS and ABAQUS software, new mechanical andthermal periodic boundary conditions are given. The results mapping method is putforward for postprocess the results from gobal elements. The framework of predicationsystem software of braided composite is determinted based on parametric unit cell createdby domain superimposed method.The relationship between structural parameters of the unit-cell model are given,which are the basis to establish parameterized unit-cell model of triaxial braidedcomposites. The expression of cubic periodic spline can be obtained for introducingperiodic condition, and the calculation method of material principle directions at any pointwithin a given period cubic spline curve are described. The cross section torsional atdifferent positions are considered, so a more sophisticated parameterized unit-cell modelwhich are used to predict the mechanical and thermal physics properties of triaxial braidedcomposites can be established. The methods of calculating macroscopic elastic constants,thermal conductivities and thermal expansions are obtained basised on mechanical andthermal finite element theory. The laws of elastics and thermal physics of triaxial braidedcomposites are obtained.In order to investigate the progressive damage and failure process of composites, thedamage mode based on Murakami damage theory is established.3D Hashin damagecriterion with various damage modes and Mises criterion are considered for predictingdamage initiation of tows and matrix, respectively. The damage progression of tows andmatrix are controlled by equivalent displacements. ANSYS and ABAQUS software arecombined to analysis damage process through conversion process written by APDLlanguage can used to translate APDL model into ABAQUS keywords model. Interfaceelements are introduced in unit-cell model of triaxial braided composites. Theprogressive damage and failure strength of composites under different tensile loadingsare obtained by material subroutine which is developed by UMAT material subroutineinterface. The parameterized unit-cell model of3D four-direction braided composites isestablished by ANSYS software. The laws of elastic and thermal physics of3Dfour-direction braided composites are obtained. The progressive damage and failurestrength of composites with typical small angle and typical large angle under tensileloading are obtained by combination of ANSYS software and ABAQUS software. Theexperimental results are compared to verify the correctness of the method.