Random Fatigue Damage Model And Analysis For Composite Thin-wall Structures

The C/SiC composites are widely used in aerospace field as ideal thermal protection materials due to the excellent properties of high specific strength,high specific stiffness and good thermal stability,such as the thermal structure of spacecraft and the thin-walled components of large-area thermal protection system,etc.These applications in an elevated thermal environment will encounter severe random cyclic loadings(vibration loading and acoustic loading,etc)during the process of high-speed cruise and reentry.Different forms and degrees of vibration response will be generated,which may cause serious damage and even collapse.On the one hand,the integrity and reliability of structures will be affected seriously by the random vibration loadings.On the other hand,the C/SiC composite structures or components will involve very complex fatigue damage mechanisms.It is an important issue for life prediction and anti-fatigue design of C/SiC composite thin-wall structures under random fatigue loadings to establish an effective model to study the fatigue characteristics according to its failure modes.At the same time,fatigue failure of thin-walled structures under random fatigue loads is closely related to their own dynamic characteristics.It is necessary to develop a frequency domain,time domain or time-frequency interactive analysis method that can consider more factors and has higher computational efficiency in dealing with fatigue damage analysis and life prediction of large thin wall structures.In this dissertation,random vibration tests are conducted to investigate the random fatigue behavior of the 2D C/SiC composite thin wall structures.The residual stiffness analysis model was established in the time and frequency domain in combination with the constant amplitude fatigue test.The fatigue damage and life of 2D C/SiC composite thin wall structure under random loading are analyzed.The main contents are included as follows:Firstly,as for the 2D C/SiC thin-wall specimens with cantilevered clamped boundary,the narrow-band random vibration experiments are firstly conducted on shaker table.The dynamic strain time history of key position and the frequency of the structure are monitored during the tests.The random dynamic strain response exhibits typical narrow-band characteristics,and the strain amplitude probability density curve fits well with the Rayleigh distribution function.The fracture morphologies of the fracture specimens are investigated,and the main failure modes of the specimens with cantilevered clamped boundary are obtained.Secondly,the residual stiffness model of 2D C/SiC composite thin wall structure in time domain is established based on the study of constant amplitude fatigue.Combining with the random vibration test data,the proposed normalized residual stiffness model is used to analyze the change of stiffness in the thin wall structure under random loading.Furthermore,the random fatigue lives are predicted by using the normalized residual stiffness model.The maximum error between the predicted value and the experimental value is 29%,and the predictions are relatively conservative.Besides,the residual stiffness model based on power spectral density and probability density function is proposed to study the damage accumulation and fatigue life for 2D C/SiC specimens subjected to random fatigue loadings.The original process description in time domain is transformed into process description in frequency domain by the proposed model.Based on the stress power spectral density measured in random vibration tests,the variation law of residual stiffness and frequency are analyzed.The frequency decrease ratio of structure is investigated and used as the criterion of random fatigue failure of the structure.The random fatigue lives of the specimens are calculated based on the proposed criterion.The predictions show a good agreement with the experimental results.Finally,based on the residual stiffness reduction method of power spectral density method,a stochastic fatigue damage accumulation analysis method for 2D C/SiC composite thin-walled structures is established.The fatigue life of thin-walled structures under acoustic excitation is calculated and analyzed.The relationship between the structural dynamic characteristics,(such as the mode shape,natural frequency,acoustic loading frequency bandwidth and magnitude,structural vibration amplitude,etc.)and fatigue damage of thin wall structures is analyzed.The random fatigue damage and fatigue life prediction method of C/SiC composite thin-walled structure established in this paper would be helpful to guide the random fatigue analysis and anti-fatigue design of composite thin-wall structures subjected to random fatigue loadings under an elevated thermal environment in engineering.