Fatigue Evaluation Of Thin-walled Composite Structures Subjected To Thermo-acoustic-vibro Loads

The thin-walled composite structures in aerospace flight vehicles are subjected to severe thermal/mechanical environments,such as high temperature,wide-frequency noise and high-level random vibration during service.The vibration-fatigue evaluation and design of the thin-walled structures under thermo-acoustic-vibro loads is a key task in the structural reliability engineering of modern aerospace flight vehicles.The ceramic matrix composites are the main candidate materials for thin-walled structures in the thermal protection system.However,the vibration-fatigue durability of thin-walled ceramic matrix composite structures under thermo-acoustic-vibro loads still faces significant challenges,and hence its key problems in fatigue evaluation need further investigation.On the basis of thermo-acoustic-vibro loads and the mechanical characteristics of thin-walled ceramic matrix composite structures,this paper focuses on four aspects from theoretical analysis and numerical simulation by using frequency-based vibration-fatigue evaluation: the evaluation approach of vibration fatigue,acoustic fatigue in thermal environments,the frequency effect on acoustic fatigue,and the design and evaluation of thin-walled joints.The aim is to investigate the key problems of vibration-fatigue evaluation and design of thin-walled ceramic matrix composite structures under thermo-acoustic-vibro loads,and hence to provide a reference for structural safety and life evaluation of aircrafts.The detailed contents are as follows:(1)To improve the calculation efficiency,modal stress theory is applied to rapidly position hot spots of structural vibration fatigue in the frequency-domain method,and then to evaluate the structural fatigue life.Numerical simulation of stress mode shapes is carried out in the application of random-vibration fatigue.The feasibility and accuracy of predictions are verified by using the stress mode shapes for vibration-fatigue critical damage positions.(2)Acoustic fatigue evaluation of C/SiC thin-walled structures is analysed under the condition of variable temperatures,from both the temperature changing dynamic stress responses and temperature-dependent material S-N curves.The calculation accuracy of fatigue damage in typical flight time was increased by 2 orders of magnitude by comparing the process of temperature change calculated or not.(3)As for broad-band acoustic loads,the frequency-dependent random fatigue of the C/SiC thinwalled structures is analysed.The level crossing problem of stationary random signal is derived for the calculation of the positive-slope crossing rate,and is demonstrated its characterization of the statistical average frequency of a stationary random signal,thus it is proposed as the loading frequency for fatigue tests.In the acoustic fatigue evaluation model of C/SiC thin-walled structures,the calculation accuracy is improved by 5 times when frequency-dependent random fatigue considered.(4)As per the relationship of three aspects between the static strength,dynamic characteristics and dynamic strength,this paper uses topology optimization technology and the modal strain theory to study the dynamics approach for structural vibration-fatigue resistance.By means of the finite element model of composite materials,the influence of hole shapes of mechanical joints is evaluated on the stress concentration factor,and then an optimized hole design is proposed to reduce the stress concentration of thin-walled mechanical connections.An optimal Z-pin method is proposed for twodimensional C/SiC thin-walled composite structures.