Ablation Mechanism and Thermo-mechanical Behavior of Silica/Phenolic Composites
|School||Harbin Institute of Technology|
|Keywords||thermoprotective composite silica/phenolic composite ablation proper-ties thermomechanical response high-temperature mechanical proper-ties digital image correlation method|
With increasing of demand of long time space vehicles, thermal protectionmaterials which used in long time low-medium heat flux environment have been paidmore and more attention. As a kind of resin-matrix thermal protective material,silica/phenolic composites have many advantages, such as low density and thermalconductivity, good endothermic ability, ablation resistant performance and thermalstability, as well as short processing cycle, cheap cost and excellent high-temperatureproperties. This kind of material can not only meet the thermal protectiverequirements of long time flying, medium level enthalpy and low-medium heat flux,but also resist long time ablation and shearing action of high-velocity gas flow.Additionally, there are good heat-insulating properties of silica/phenolic composites.For this reason, they are widely used in the ablation and thermal protection of longtime flight vehicles. The complex physico-chemical changes, ablation and phasetransition processes of silica/phenolic composite can take place when exposed to hightemperature. That is generally associated with pyrolysis reaction of phenolic resin,diffusion and flow of decomposition gases, pore pressure, thermal blockade effect,thermal expansion phenomenon and moving boundary. Furthermore, aerodynamicheating can cause thermal deformation of the thermal protective material and itsstructure. Strength and loading capacity of the material significantly degrade whensubjected to aerodynamic mechanical and thermal loading. Therefore, it is necessaryto study the ablation and thermal protective mechanism of silica/phenolic compositeunder long time low-medium heat flux environment and predict ablation propertiesand high-temperature thermomechanical behavior, and which is significant forstructure design, evaluation of integrity and reliability of heat shield for this kind ofcomposite.The mass loss mechanism and endothermic mechanism in ablation environmentwere studied in detail in this dissertation. Based on the surface ablation theory andboundary layer aerodynamics theory, a prediction method of surface ablationproperties was established. According to mass and energy conservation principles, amulti-physics field coupling model was proposed to predict the thermomechanicalbehavior of polymer composites. Prediction methods of high temperature stiffness and strength for thermal protective materials were presented. The above-mentionedprediction models were validated by relevant experiments.Firstly, the static and dynamic ablation experiments of silica/phenolic compositewere conducted. Based on thermo-gravimetric and endothermic (exothermic) curves,the pyrolysis reaction of the material with different temperature and heated rates werestudied. A thermal decomposition kinetics model was presented using one-orderArrhenius equation. According to the ablation feature on material surface andmorphology of post-heated specimen segment, the ablation and thermal protectivemechanism of silica/phenolic composite were analyzed. Considering the fused liquidlayer on ablation surface of silica-reinforced composites, surface ablation recessionrate and wall temperature were predicted under given aerodynamics thermalenvironment. The predicted results were in good agreement with the oxyacetyleneexperimental results. The proportions of the endothermic mechanisms including heatabsorption of heat capacity of ablator, thermal decomposition of the resin, heatradiation of surface material, thermal blockade effect and evaporation of fused silicafiber in the total heat absorption were derived by combining with surface ablationtheory and boundary layer aerodynamics relationships. Moreover, the effects of resincontent, thermal conductivity and specific heat on ablation properties of thermalprotective materials were discussed.Secondly, considering variation of the pore pressure and heat-mass transferprocesses within the materials, a one-dimensional thermal response model wasdeveloped using finite difference method when silica/phenolic composite wasexposed to one-sided radiant heat flux. The moving boundary caused by thermalexpansion and the thermal blockade effect were also considered in the model. Thetemperature distribution, pore pressure distribution, variation of volume content ofphase components, degree of decomposition and mass flux of gases of the thermalprotective material during volume ablation process were predicted. The sunlight wasused as the light source and the solar radiation heating experiment was carried out.The time-dependent temperature progressions at different material depths ofsilica/phenolic composite specimen were measured when exposed to one-sided heatflux. The experimental temperature profiles were in good agreement with thecalculated results. Furthermore, the accuracy of the thermal response model wasassessed. Thermochemical decomposition process of polymer composites can be regardedas a thermal-mechanical-chemical multi-physics field coupling problem. Based onmass and energy conservation principles, a multi-physics field coupling analysismodel was proposed to predict the thermomechanical behavior of porous elastomersunder high temperatures. The coupling actions of temperature, diffusion anddeformation were taken into account in the model. The governing differentialequations of thermomechanical response of silica/phenolic composite were modifiedusing the Bubnov-Galerkin finite element method to obtain the generalized effectiveelement stiffness equation. Finally, the global stiffness equations were assembled,and solved by the Gaussian elimination method. The high temperature deformation,thermal strain and thermal stress of the material were predicted when subjected toheat flux. Additionally, non-contact high temperature deformation measurementexperiment was conducted by applying digital image correlation technique and fullfield deformation and strain on measured surface of the specimen were obtained. Theexperimental strain results were in good agreement with the calculated values.Consequently, the accuracy of the multi-physics field coupling analysis model wasevaluated.Finally, the degradation in mechanical properties caused by the thermalsoftening effect and thermal decomposition of matrix material were studied. UsingMori-Tanaka method and Eshelby equivalent inclusion theory, a mesomechanicalmethod was proposed to predict high-temperature stiffness properties ofsilica/phenolic composite. Relationships between compression strength andtemperature of thermal protective material were obtained using high-temperaturecompression experiment. Based on thermal response analysis, the calculationformulas of high temperature compression strength for silica/phenolic compositewere presented. Degradation in strength properties with temperature and heated timewere predicted when the material subjected to one-sided radiant heat flux and staticcompression loading.