The Evaluation Method Of Mechanical Properties Of C/SiC Composite Thin-walled Structures Under Thermal-acoustic Loadings

Hypersonic vehicles suffer increasingly severe thermal-acoustic loadings during their service.The studies have shown that the surface temperature of the thermal protection system(TPS)can reach 1648 °C and the sound pressure level of acoustic can reach 180 d B during re-entry of hypersonic aircraft.In order to meet the requirements of the light weight and the high temperature resistance,the TPS of aircraft generally adopts the thin-walled structures of the fiber reinforced ceramic matrix composite material,which are sensitive to the thermal-acoustic loadings.The thermal loading results in the change of material properties and thermal stress,and the acoustic loading seriously affects the mechanical properties and bearing capacity of the structure.Therefore,the evaluation of mechanical properties of composite thin-walled structures under thermal-acoustic loadings has become an important reference basis for the design of aircraft structures,and the related theoretical and the experimental researches are still in its infancy.There are few research results that can be used for reference especially for the fiber woven composite materials,therefore in this paper the evaluation method of mechanical properties of composite thin-walled structures under thermal-acoustic loadings is studied experimentally and theoretically.The acoustic fatigue and residual strength experiments are carried out on the C/SiC composite plates at room temperature and high temperature,and the evaluation method of mechanical properties of composite thin-walled structures under thermal-acoustic loadings based on residual strength is proposed.The main contents are included as follows:First,the acoustic fatigue and residual strength experiments are performed at room temperature on the two-dimensional(2D)plain weave C/SiC composite plates with the traveling wave tube acoustic test system.And the fatigue failure time and failure mode of the plates under acoustic loading are studied,and the strength degradation law in the main direction of the composite is obtained.The experiments are the foundation of the follow-up study on the mechanical properties of C/SiC composite plate under acoustic loading.Second,this paper proposes the method for evaluating the mechanical properties of composite thin-walled structures under the acoustic loading based on residual strength by combining experiments and theory.Firstly a fatigue damage model of C/SiC composite based on Tsai-Wu strength criterion and residual strength evoluation is established.Combined with static strength,fatigue strength and residual strength experiments of 2D plain weave C/SiC composites,the residual strength evolution law of the material under acoustic loading is obtained.The residual strength of typical thin-walled structures is analyzed by the finite element model,then the prediction of residual strength is compared with the experimental results,and the comparision results indicate the method proposed is effective.Third,the acoustic fatigue and residual strength experiments are performed at 600℃ high temperature on the 2D plain weave C/SiC composite plates with the traveling wave tube thermal-acoustic test system.And the fatigue failure time and failure mode of the plates at 600 ℃ under acoustic loading are studied,and the strength degradation law in the main direction of the composite at 600 ℃ is obtained.The experiments are the foundation of the follow-up study on the mechanical properties of C/SiC composite plate under thermal-acoustic loadings.Finally,the mechanical model and analysis method considering the coupling effect of thermal-acoustic loadings are proposed.The static strength,the fatigue life and the residual strength experiments of 2D plain weave C/SiC composites are carried ou t at high temperature of 600 ℃,then the parameters of the residual strength model at high temperature are determined.Based on the residual strength model of C/SiC composites,the residual strength evolution law of C/SiC composites under the thermal-acoustic loadings is obtained.Combined with the structural finite element analysis,the residual strength evolution law and the Tsai-Wu failure criterion are used to obtain the residual strength.Then the prediction results of residual strength are compared with the test results,and the comparision results indicate the method proposed is effective.According to the characteristics of the loadings and the structure,the method comprehensively considers the factors that have a major influence on the structure residual strength.The method evaluates the structure strength under the thermalacoustic loadings effectively,and also is convenient for engineering applications.