Research On Thermal Vibration Characteristics Identification Of Typical Composite Structure Of Spacecraft And Model Updating Methods

As the flying speed of the aircraft is getting faster and the endurance time is getting longer,the aircraft is facing an extremely harsh high-temperature environment.The high temperature will change the inherent characteristics of the aircraft and cause changes in dynamic characteristics,which will affect the aircraft’s performance.Therefore,it is of great significance to accurately predict the dynamic characteristics of the aircraft in a high-temperature environment.At present,the commonly used prediction methods are thermal modal test and finite element analysis.The two prediction methods are complementary to each other.The former improves data support for the latter.In contrast,the latter can efficiently predict the structure’s dynamic characteristics in a hightemperature environment and guide the former.In order to ensure the reliability of finite element analysis,it is necessary to have an accurate finite element model.However,modeling based on theory and experience is often not accurate enough.Then,the finite element model updating method was born to improve the finite element model accuracy efficiently.Since the composite structure is difficult to process after forming,which can easily cause structural damage,the commonly used bolt-connected excitation method is not suitable for the composite structure.The metal structure can directly use the welding method to fix the thermocouple,while the composite material structure cannot use this method.However,the current high-temperature glue can easily fall off in a highly hightemperature environment,causing measurement failure.In order to overcome the above difficulties,this thesis designed a high-temperature thermal modal test system suitable for composite structure.Based on the high-temperature thermal modal test system proposed in this thesis,the corresponding adjustments are made according to the different working conditions and structures.Based on the high-temperature thermal modal test system proposed in this thesis,the corresponding adjustments were carried out according to the different working conditions and structures.The high-temperature thermal modal tests of C/Si C box structure(bilateral clamped boundary,maximum temperature of 1000 ℃),carbon fiber reinforced aluminum matrix composite laminates(free boundary,maximum temperature of 250 ℃),and carbon fiber reinforced bismaleimide resin matrix composite conical shell(free boundary,maximum temperature of 250 ℃)were completed.The high-temperature thermal vibration characteristics(natural frequency,vibration mode)were obtained,and the variation law with temperature was summarized.Through the parametric modeling method,the high-temperature dynamic models of three typical spacecraft composite structures are established.Due to the complexity of composite materials,some high-temperature material parameters are not obtained through standard tests but are assigned based on engineering experience.In addition,the properties of the composites are highly dispersed due to the processing technology,and the measurement results of the standard test can not characterize the properties of all composites of the same kind.Therefore,this thesis used a sensitivity-based model updating method to update the high-temperature dynamic models of three typical spacecraft composite structures.On this basis,the influence mechanism of temperature on the thermal vibration characteristics of three typical spacecraft composite structures was studied by finite element analysis.For model updating,since most of the current methods are proposed based on optimization theory,it is easy to fall into local optimization in the face of the nonlinear coupling relationship between parameter and response(generally modal parameters such as natural frequency and vibration mode).Local optimization results in good agreement between the updated response and the measured response,but there is an error between the updated parameter and the measured parameter,and even the updated parameter does not have physical meaning.In order to solve the local optimization problem,this thesis proposes a finite element model updating method based on parameter decoupling sensitivity analysis.By introducing the concepts of conversion matrix and virtual response,this method converts the nonlinear coupling relationship between parameter and response into a linear decoupling relationship between parameter and virtual response to realize parameter decoupling.Then the objective function is established on the basis,and the objective function is minimized to obtain the convergent updated result.With the development of the model updating method,researchers found it unreasonable to believe that the test results are accurate.Uncertainty is common in practical engineering,and ignoring uncertainty will lead to an inaccurate model.Therefore,it is necessary to consider uncertainty in modeling.The particularity of a high-temperature environment not only increases uncertainty but also leads to fewer test data.Therefore,this thesis studies interval model updating method with low requirements for test data.Considering that the interval algorithm cannot be directly applied to model updating,this paper uses the interval analysis ability of universal grey mathematics to transform the interval problem into the framework of universal grey mathematics.Then,the objective function of interval model updating is constructed using universal grey distance.Then,the interval model updating problem is transformed into the deterministic model updating problem about the upper bound and the interval diameter.Finally,the deterministic model updating method is used to obtain the updated interval results.In summary,this thesis research the thermal vibration characteristics of typical spacecraft composite structures and the finite element model updating method.A hightemperature thermal modal test system suitable for composite structures was designed.With finite element model updating,the high-temperature vibration characteristics of C/Si C box structure,carbon fiber reinforced aluminum matrix composite laminates,and carbon fiber reinforced bismaleimide resin matrix composite conical shell were identified.A finite element model updating method avoiding local optimization is proposed.The interval model correction method considering uncertainty is developed.