Research On Compression Properties Of Low Density Glass/Phenolic Composites At Room And High Temperature

Low-density glass/phenolic(LDGP)ablation materials have good density design ability and heat insulation performance,and are key candidate materials for based heat insulation and sidewalls of spacecraft during high-enthalpy,medium-low heat flux density,long-time flight.Firstly,due to the complexity of molding the process of LDGP composites,accurately predicting the mechanical properties of such materials is a challenge.Secondly,in the process of real service,this kind of thermal protection material is subjected to aerodynamic heat and loads for a long time,and in need of certain requirements on the compression performance of the material,especially high-temperature compression performance,so as to ensure the structural integrity and bearing capacity of the entire thermal protection structure.However,the material will undergo pyrolysis reaction under high temperature heat flow load,forming carbide layer,pyrolysis layer and original material layer from the surface to the inside,the corresponding environment will gradually changing from the air environment to the vacuum environment.There is no effective test method to get the high temperature mechanical performance of the material in the corresponding environment,resulting in the lack of high-temperature mechanical properties data.In view of the above two problems,the paper considered the microstructural features of the original materials,predicted the room-temperature mechanical properties of the materials by hierarchical prediction methods,and carried out hightemperature compression experiments in the corresponding environment for the gas environment in which the original materials and carbon layers are located.Combined with the structural evolution of the material at high temperature,the paper analyzed the mechanical behavior and failure mechanism of the original material and the carbon layer at high temperature.Firstly,using SEM and Micro-CT to observe the microstructure of the original material and carbon layer systematically,the high-temperature evolution law of the carbon layer structure was analyzed,and the geometric parameters required for the prediction of the mechanical properties of the intrinsic material were statistically analyzed.The material was found to be 2D braided structure,with preform containing three fiber bundles of glass fiber,quartz fiber and organic fiber and matrix containing a large number of randomly distributed glass microspheres.The image processing method was used to count the diameter of the glass microspheres,the fiber bundle cross-section dimensions and the fiber bundle amplitude characteristics,finding that the outer diameter of the glass microspheres followed the Gamma distribution law,and the geometric model parameters of the composite materials were obtained.The material was fully pyrolyzed at 500 ℃ ~ 1500 ℃ to form a porous carbon layer to analysis high-temperature evolution of the carbon layer structure.It was found that as the pyrolysis temperature increases,the material undergoes pyrolysis of the matrix,the heat of the organic fiber,the melting of glass microspheres and glass fibers and the melting of quartz fibers.And the higher the temperature is,the more cracks and holes are produced in the carbon layer.Secondly,a method for predicting the mechanical properties of layers of LDGP materials was established,and the stiffness and strength properties of the glass microsphere matrix,the three fiber bundles,and the composite materials were predicted.According to the distribution law of glass microspheres in the matrix,an RVE model of glass microspheres was established and the equivalent stiffness was predicted by finite element methods.Glass microspheres were regarded as multiple inclusions in the matrix,according to the simple cube strength theory,the strength of matrix under the inclusion of multiple glass microspheres was calculated.The stiffness and strength of the three fiber bundles were predicted by empirical formulas.Based on the mechanical properties of the matrix and fiber bundles,an RVE model of the composite material was established,by applying periodic boundary conditions,and the stiffness properties of the material were predicted using the finite element method.Considering the damage of the component materials,the UMAT subroutine was used to analyze the progressive damage process of the material under the out-ofplane compressive load.It was found that the damage started at the maximum bend of the interface between the matrix and the fiber bundle,it expanded along the interface of the matrix until into the matrix,however,fiber bundles have less damage.The matrix and composite compression experiments were carried out at room temperature to verify the feasibility of the model.Finally,the high-temperature compression experiments of the original material and its carbon layer in the corresponding environment were carried out.The high-temperature mechanical behaviors of the original material and the carbon layer were analyzed.Combined with the structural evolution of the carbon layer,the strengthening mechanism of the material properties at 700 ℃ was proposed.Through experimental environment design,oxidation temperature point design and holding time design,the high-temperature compression experiments of LDGP materials in atmospheric and vacuum environments were conducted.In the atmospheric environment,the compressive strength of the material is insensitive to fiber direction,compressive properties can be treated as isotropic,and the compressive strength of the material increase with the increase of temperature,but the abrupt change occurs at 700 ℃.Under the vacuum environmental of 700 ℃,1100 ℃ and 1500 ℃,the mechanical behavior of the original and the carbon layer at high temperature is similar to the high temperature mechanical behavior of the material in the atmospheric environment,and the strength is significantly increased at 700 ℃.The compression test of the carbon layer at room temperature was carried out,combining the high temperature structural evolution of carbon layer,the strengthening mechanism of the soft-phase carbon layer filling brittle crack and fiber melt tougheni ng at 700 ℃ was proposed.