Fabricating Load Bearing/Heat Insulation/Thermal Protection Integral Composite By Co-injection RTM Process

The primary requirements for a hypersonic aerocraft structure are low density, high mechanical performance, thermal protection and heat insulation, respectively. Traditionally, the load bearing/thermal protection structure applied in the hypersonic aerocraft was manufactured with the step by step method and then jointing them with glue. It not only makes the manufacturing process more difficulty and complication and more fabrication cost, but also less reliability of the whole structure. A new load bearing/heat insulation/thermal protection integral composite structure which is manufactured by the Co-injection RTM (CIRTM) is developed in this dissertation, which could be used as primary load bearing and thermal protection compound for the hypersonic aerocraft. In the integral composite structure, the load bearing layer materials are carbon fiber reinforced epoxy (carbon/epoxy) composites, the heat insulation layer materials are low density and low thermal conductivity materials such as PMI foams or silica aerogel materials, the thermal protection layer or ablation layer materials are carbon/phenolic composites.In this dissertation, heat transfer analysis and design of multi-layer composites and integral composites with stitching thermal channels are carried out based on finite element analysis (FEA) method. The optimal phenolic resin for the matrix of thermal protection layer, and the optimal epoxy resin for the matrix of load bearing layer are chosen. The multi-layer composites with foam core sandwich and aerogel core sandwich, respectively, are manufactured by CIRTM process, mechanical properties of the multi-layer composites are investigated and compared to that of multi-layer composites manufactured by bonding step by step method. The stitching-CIRTM process is used to manufacture a multi-layer composite with stitching silica aerogel core materials, effects of stitching pillars on mechanical properties and stitching thermal channels on heat transfer properties of the multi-layer composites are investigated, respectively.Heat transfer FEA models for the integral multi-layer composites with and without stitching thermal channels are established, respectively. Applied the thermal load in trajectory which peak temperature is 912℃to outer surface of the thermal protection layer, the temperature distribution in the multi-layer composites is obtained. The results show that PMI foam is not suitable for using as heat insulation layer materials served at the thermal loading. For load bearing layer thickness (hl-b) of 2mm and thermal protection layer thickness (ht-p) 3mm, the least heat insulation layer thickness (hl-b) of silica aerogel core is 16mm. The wet stitching channels materials can be considered as silica aerogel/benzoxazine composites, the average diameter of the stitching is 3mm. For hl-b of 2mm, ht-p 3mm, and needle-distance×row-distance ( l×m) 8×8, the least thickness hh-i of silica aerogel core is 24mm. The dry stitching channels materials can be considered as quartz/benzoxazine composite, the average diameter of the stitching is 0.9mm. For hl-b of 2mm, ht-p 3mm and hh-i 17mm, the least l×m of silica aerogel core is 10×10. For hl-b of 2mm, ht-p 3mm and hh-i 18mm, the least l×m of silica aerogel core is 7×7.Thermal properties, mechanical properties and process parameters of ammonia-phenolic, barium-phenolic, benzoxazine, novolac cyanate ester resins were investigated systematically by experimental and mathematic model, and rheological models based on the dual-Arrhenius equation for the four phenolic resins are established. The curing characteristic temperatures for the four phenolic resins are studied by differential scanning calorimeter (DSC) technique at different heating rates. According to quality of the cured resins, as well as the curing characteristic temperatures, the optimal curing programmes were determined. Using carbon fiber plain cloth as reinforcement, the composite laminates with matrix of ammonia-phenolic, barium-phenolic, benzoxazine, respectively, were manufactured by VARTM process. Porosities, mechanical properties and ablation properties of the composite laminates are investigated. The results show that the benzoxazine matrix composite laminates have better performance which satisfied RTM requirement. Benzoxazine resin maintains the viscosity less than 800mPa·s above 76℃, and has a wide low-viscosity temperature range and a long low-viscosity keeping time. Cured benzoxazine resin has the best mechanical properties among four cured phenolic resin, char yield at 1000℃in nitrogen atmosphere reaches 47%. The features of the carbon/benzoxazine composite laminates can be summarized as following: porosity is 0.85%, tensile strength is 445MPa, mass ablation is 0.0435g.s^-1.According to the co-injection requirements for epoxy resin and benzoxazine, gel characters of the epoxy systems containing different curing agents are studied, and the mechanical properties of the cured products are also investigated. The rheological model based on the dual-Arrhenius equation for the chosen epoxy resin was established, co-injection temperature was determined combined with rheological model of benzoxazine. The curing characteristic temperature for the epoxy resin was studied by differential scanning calorimeter (DSC) technique at different heating rates. The tensile strength of the cured epoxy resin and cured benzoxazine manufactured by different curing programmes was measured, herewith, the optimal co-curing programmes were determined. The results show that E-44/GA327 has the best mechanical properties and thermal properties which is suitable for co-injection with benzoxazine. The weight ratio of E-44 to GA327 is 100:30. The mechanical properties are as follows: tensile strength is 80.6MPa, tensile modulus 2.98GPa, flexural strength 138.6MPa, flexural modulus 3.11GPa, elongation to fracture 3.64%. Glass transition temperature (Tg) is 164℃. The co-injection temperature of benzoxazine and E-44/GA327 system is 76～90℃, the optimal co-curing programme is 85℃/4h+130℃/4h+140℃/2h+160℃/1h+180℃/1h.Load bearing/PMI foam heat insulation/thermal protection multi-layer integral composites was manufactured by the CIRTM, the primary process parameters were investigated. Mechanical properties of the multi-layer composites with PMI foam core manufactured by CIRTM and bonding step by step, respectively, were investigated. The results show that the optimal process parameters for the CIRTM are as follows: fiber volume fraction of load bearing layer and thermal protection layer is about 45%, injection pressure is 1atm, injection temperature is 80℃, with vacuum assisted. Measured thickness of each layer of the multi-layer composites manufactured by CIRTM with the optimal process parameters is agree with designed thickness, and PMI foam wasn't infiltrated by epoxy resin or benzoxazine. The edgewise compressive,three point flexural and interfacial shear strength of multi-layer composites manufactured by CIRTM is better than that manufactured by bonding step by step, and that, coefficient of variance decreased significantly. The flatwise compressive properties of the multi-layer composites manufactured by two processes are identical. The multi-layer composites have better interface between each layer manufactured by CIRTM than by bonding step by step. The heat transfer experimental result agrees well with that of the FEA calculation. PMI foam is disabled under the thermal load of 350℃on the outer surface of thermal protection layer.Primary process parameters for load bearing/silica aerogel heat insulation/thermal protection multi-layer integral composites manufactured by CIRTM, were investigated. Mechanical properties and thermal properties of the multi-layer integral composites with silica aerogel core manufactured by the CIRTM and by the bonding step by step are studied, respectively. The results show that when the designed thickness of load bearing layer, heat insulation layer and thermal protection layer is 2mm, 18mm and 3mm, aimed at reducing the porosities of load bearing layer, thermal protection and the thickness of the resin layer infiltrated to the aerogel, the optimized process parameters for the CIRTM are as follows: fiber volume fractions of load bearing layer and thermal protection layer are about 45%, injection pressure is 1atm, injection temperature is 80℃, with vacuum assisted. The final thicknesses of load bearing layer and thermal protection layer of the multi-layer composites with silica aerogel core manufactured by the CIRTM are agree with the designed thickness, exception of the thickness of the heat insulation layer. The depth of the epoxy resin infiltrated to the aerogel core ( De poxy) is 3.4mm, and the depth of benzoxazine resin infiltrated to the aerogel core ( Db enzoxazine) is 2.5mm. The edgewise compressive, flatwise compressive, three point flexural and interfacial shear properties of the multi-layer composites with silica aerogel core manufactured by the CIRTM is better than that manufactured by bonding step by step. The multi-layer composites with silica aerogel core manufactured by the CIRTM has better interface than the one manufactured by bonding step by step. The heat transfer experimental results for of the multi-layer somposites with silica aerogel core under the thermal loading of 600℃on the outer surface of the thermal protection layer agrees well with FEA calculation.Wet stitching-pre-curing process and dry stitching-pre-curing process were used to manufacture stitched silica aerogel core, respectively, and then used as heat insulation layer materials, load bearing/stitched silica aerogel heat insulation/thermal protection multi-layer integral composites were manufactured by the CIRTM. The effects of stitching methods and l×m on inner configuration, densities, mechanical properties and thermal properties were investigated. The stitching thermal channels (stitching pillars) by wet stitching-CIRTM process include quartz/benzoxazine composites and silica aerogel/benzoxazine composites, the diameters of the channels are 0.9mm and 3mm, respectively. The stitching thermal channels by dry stitching-CIRTM process include quartz/benzoxazine composites and quartz/epoxy composites, the diameters of the channels are all 0.9mm. The edgewise compressive, flatwise compressive, three point flexural and interfacial shear properties of the sandwich composites manufactured by wet stitching-CIRTM process and dry stitching-CIRTM process increased significantly, compared to the composites by bonding step by step and CIRTM. However, mechanical properties of the sandwich composites by two processes increased with the decrease of l×m (the increase of stitching density). For the same l ? m, the mechanical properties of sandwich composite by wet stitching-CIRTM is much higher than those of the sandwich composite by dry stitching-CIRTM. The improvements of mechanical properties of the sandwich composites by stitching-CIRTM are due to the stitching pillars. The calculated results by FEA model show that the stitching thermal channels have significant effects of the heat transfer of, the multi-layer composites with stitched core, especially to the wet stitching channels.