Research On Ablative Behavior And Tensile Mechanical Properties Of Carbon/phenolic 2.5D Woven Composites

In recent years,the new generation of ultra-high Mach number aircraft has put forward strict service requirements for the integration of ultra-high load and low ablation of thermal protection material systems.The carbon fiber 2.5D woven prefabricated has the spatial configuration of layer-by-layer angle-interlock connection.The phenolic composite reinforced by carbon fiber has shown excellent external performance,high carbon residue rate,and good ablative stability,which provides a practical way to realize the thermal protection material with both load-bearing efficiency and ablative performance.However,2.5D woven composites have complex meso structures,which makes it difficult to elucidate the mechanical properties and damage mechanism in the force-thermal coupling environment.Based on this,the carbon/phenolic 2.5D woven composites were designed and prepared in this paper.The model reconstruction,ablation behavior,and post-ablative tensile properties were studied by combining experiment and numerical calculation,in order to provide data and method support for the materials with high load and ablative resistance required by aerospace engines.The main research contents and conclusions of this paper are as follows:(1)Using T700-12 K carbon fiber and boron-modified phenolic resin as raw materials,two kinds of carbon/phenolic 2.5D woven composites with different densities were prepared by resin transfer molding process(RTM)with 2/2-angle interlocking structure(2.5D woven).The internal images of carbon/dense phenolic 2.5D woven composites were collected by microcomputed tomography(Micro-CT),and the yarn shape parameters were extracted by Volume Graphics(VG)processing.On this basis,the correlation theory of process and structural parameters is established,and the mesoscopic model is reconstructed.The results show that the established meso-model of the carbon/phenolic 2.5D woven composites is in good agreement with the Micro-CT images,which provides strong support for the subsequent numerical simulation of quasi-static tensile mechanical properties and damage evolution.(2)The oxy-acetylene ablative test was carried out around two kinds of carbon phenolic2.5D woven composites with different densities.The ablative rate,apparent appearance,and internal damage morphology of the samples were obtained by combining Micro-CT and FESEM characterization techniques.The results show that the carbon/dense phenolic 2.5D woven composites have a lower line ablation rate(0.029-0.030 mm/s),while the carbon/light porous phenolic 2.5D woven composites exhibit a lower mass ablation rate(0.019-0.025 g/s).The thermal insulation performance of the two samples with different densities is also different.The thermal conductivity of carbon/dense phenolic 2.5D woven composites and carbon/light porous phenolic 2.5D woven composites are 0.617-0.856 W/(m·K)and 0.404-0.697 W/(m·K),respectively,at 25-350℃.In addition,the carbon/dense phenolic 2.5D woven composites show large cracks and small cracks,but the carbon/light porous phenolic 2.5D woven composites only show large cracks after the ablation.(3)Taking the carbon/phenolic 2.5D woven composites before and after the oxy-acetylene ablation as the research object,the quasi-static in-plane tensile properties were tested by acoustic emission(AE)technology.The mechanical characteristic parameters,loaddisplacement curves and macro and meso damage modes were obtained,and the failure mechanism was illustrated.The results show that the tensile strength and modulus of the carbon/dense phenolic 2.5D woven composites are 775.1 MPa and 60.4 GPa,respectively,while that of the carbon/light porous phenolic 2.5D woven composites are 556.1 MPa and 42.4GPa,respectively.After oxy-acetylene ablation for 30 s and 60 s,the tensile strength and modulus retention rates of carbon/dense phenolic 2.5D woven composites are 19.93-23.60%and 43.30-45.18%,respectively.However,the carbon/light porous phenolic 2.5D woven composites were 38.82-43.12% and 50.34-57.84%,respectively.In addition,the damage morphologies of the samples show that the in-plane tensile mechanical properties are mainly affected by the microstructure of the matrix,the initiation of pyrolysis crack,two crack propagation modes(type I and type II),and the degree of oxidative damage of the fibers.(4)By introducing process defects and ablation test cracks into the geometric model,a mechanical analysis method based on the meso-scale single-cell model and the macro-mesoscale hybrid model was constructed,and a UMAT subroutine containing the Hashin criterion,the maximum stress criterion,and the damage evolution intrinsic model was written to carry out the quasi-static tensile mechanical properties and progressive damage numerical calculations of carbon/phenolic 2.5D woven composites.The accuracy of the developed mesoscale model containing cracks was verified by comparing the load-displacement curves,damage morphology,and crack extension.The simulation results show that the ablated specimens have smaller fracture displacements and damage accumulation stages under tensile loading due to the effects of carbonization and cracking compared to the unablative specimens.In addition,the pre-ablative carbon/light porous phenolic 2.5D woven composite exhibits more damage due to the low mechanical properties of its matrix;the post-ablative carbon/dense phenolic 2.5D woven composite exhibits more crack damage,making it susceptible to stress concentration in the damage defect region,which in turn leads to a wide distribution of damage.