Vibration Performances And Modal Experimental Techniques For Composite Sandwich Structures In High Temperature Environments

Structural vibration experiment in the thermal environment plays an important role in the ground simulation test for high-speed vehicles and their components.During the hypersonic flight in the atmosphere,the vehicle is subjected to an extremely harsh and complex loading environment.Aerodynamic heating leads to a sharp increase in the temperature on the windward.Vibration performances of the vehicle structure may be significantly affected by the elevated temperature and the temperature gradient inside the structure.Therefore,temperature influence must be taken into consideration in the ground simulation test.Moreover,vibration experiment in the thermal environment can also provide the essential foundation and reference for the theoretical research and simulation analysis of structural thermal vibration.However,high temperature brings many obstacles to the implement of vibration experiments.Both the exciting and the measuring are difficult to be realized on the hot surface of the structure under test.So far,many aspects of the related experimental techniques are required to be improved.Techniques for measuring the structural response in thermal environments are advanced and highly industrialized.Optical measurement techniques developing rapidly in recent years also provide diversified alternatives for the response measurement in thermal vibration experiments.In contrast,the specific research on the excitation technique for thermal modal testing is rarely reported.Target temperatures of thermal vibration experiments are getting higher,and test articles are also becoming more and more complicated.The limitation of existing excitation approaches is gradually highlighted.In this paper,existing excitation approaches that can be used for thermal modal experiments are tested at first.Application scope and limitations of each approach are discussed.Then,according to the characteristics of thermal modal tests,a novel excitation approach is proposed to apply the impact series load to the structure under test.This approach can effectively excite the structure without introducing any negative additional influence and yield high-quality test results.Also,it can realize the excitation in a narrow space.Two forms of excitation configurations are presented for the new excitation approach to adapt to different temperature conditions.The configuration suitable for the low temperature environment can measure the excitation force and meet the requirement of frequency response function measurement.The other configuration can be used under higher temperature conditions and provide the stable excitation in thermal environments.Also,it can prevent the shaker from overheated.Both excitation configurations are verified to be effective by experiments under different conditions.If the temperature distribution inside the structure significantly changes during the experiment,the measured structural response signals may be non-stationary.As a powerful tool for non-stationary signal processing,time-frequency analysis represents the signal in the time-frequency plane and visualized the frequency components of the signal directly.In order to analyze the result of the test under unsteady temperature condition,a new method is proposed to extract the ridges of time-frequency spectrogram based on the image morphology techniques.By this approach,ridges are accurately extracted from the spectrograms with the minimal computational cost during the processes of a simulation example and a set of thermal modal test data.Meanwhile,the 2D median filter is introduced to reduce the noise of the spectrogram of the experimental data.Then,thermal modal tests with single-side heating conditions are performed on two different forms of composite honeycomb structures,which are widely used in engineering.Each test article is composed of the carbon fiber reinforced resin matrix laminates and a piece of Nomex paper honeycomb core.During thermal modal testing,natural frequencies and modal damping ratios of the first set of composite honeycomb structures are dramatically changed with temperature rising.Though the experiments,the simulation analysis and the dynamic mechanical analysis test on the face sheet material,potential factors that influence the structural modal characteristics in the thermal environment are checked one by one.And the temperature-dependent properties of the face sheet material are identified as the main factor of the significant variation in modal parameters.Then the influences of different modulus components of the face sheet material on each natural frequency of the composite honeycomb structure are further explored by simulation to investigate the correlation between influence level and the corresponding mode shape.In contrast,the other set of the test article is reinforced at four edges by filling the hollow of the honeycomb core,which can simulate the operating conditions of the honeycomb sandwich plate mounted on a frame structure.During thermal modal testing,interfacial debonding occurred between the heated face sheet and the core layer.Modal characteristics of this structure did not change significantly until the debonding occurs and causes the sudden change.Combining the damage state of the debonding interface and the simulation of the structure under the effect of experimental thermal load,the reason for the debonding damage is analyzed from three aspects,including thermal stress,thermal buckling of the debonded face sheet and the increase of air pressure inside the honeycomb core.The contribution of each factor during the damage process is studied respectively.