Research On Guided Wave Propagation And Damage Identification In Typical Structures Of Spacecraft

Nowadays,structural health monitoring(SHM)techniques based on guided waves have been one of the research hotspot.The SHM techniques provide the information of damages while predicting the development of damages to assess the health status of structures.With the development of manufacturing techniques,the honeycomb sandwich and variable-thickness structures are playing more significant roles due to their excellent mechanical performances in aerospace industries.When suffered from damages,these new structures will result in catastrophic failure during their service lives.Therefore,it is essential to study the physical mechanics of wave propagation in the honeycomb and variable-thickness structures.Applying the guided-wave-based SHM techniques on these two typical structures for damage identification is of great importance.This thesis reviews researches on guided-wave-based structural heath methods.Then the analytical methods about physical characteristics of wave propagation are demonstrated.The interactions between the guided wave and the different types of damage are introduced and the interactions between PZT(piezoelectric lead zirconate titanate)discs and the host structures are discussed.Signal processing techniques and damages identification methods are introduced emphasizing on the typical structures of spacecraft,e.g.the honeycomb and variable-thickness plates.Combination of the theoretical studies,numerical simulations and experimental validation is utilized to solve the key problems.Based on the Rayleigh-Lamb dispersion equation,fundamental theories about the Lamb wave propagation in plates and the dispersion characteristics are systematic demonstrated.The finite element models(FEM)for studying guided-wave propagation in structures are built.The stability of numerical algorithms of the FEM and the selectionof relative parameters are discussed.The models of interactions between PZT discs and the host structures are built based on the piezoelectric property of the PZT.The variety of FEM of distinct types of damages are introduced.The experimental system and devices are introduced.The selection of parameters for excitation guidedwave signals is analyzed.The general processing methods of experimental signal are introduced.To weak the reflected waves produced by the edges of structures and reduce the difficult of signal processing,the Caughey absorbing layer is setup outside the original models.The Caughey absorbing layer method uses absorbing layers by increasing damping to reduce the energy of reflected waves.Comparisons of absorption effects are made between the absorbing boundaries with uniform damping coefficient and with gradually increasing damping coefficient to find out the optimum type of absorbing boundaries.It is essential to further study the parameters,including the number,shape and length of absorbing layers,to obtain the most optimum result.An aluminum beam with the absorbing boundaries is used to verify the absorption effect.The wave signals obtained from the beam contain few reflected waves caused by the edges.For the various-thickness plates that are limited by their sizes or not suitable to set absorbing boundary,the structural weight function considering non-constant thickness effect is introduced.As two typical structures of various-thickness plates,the trapezoid section plates have symmetric sections and the ramp plates have anti-symmetric sections.To compare the accuracy of damage position identification,the FE modes of these two types of plates are set.The reconstruction algorithm for probabilistic inspection of defects(RAPID)with the linear elliptical weight function and the structural weight function are utilized to locate the damage,respectively.The results prove that the weight structural function is more applicable to locate the damages in the various-thickness plats,with higher identification accuracy.Then an experiment is set up to verify the effect of RAPID with the structural weight function by locating a hole in a ramp plate.The scanning laser vibrometer is used to acquire the wave fields in aluminum plates and the aluminum honeycomb sandwich plates for studying the guided-wave propagation mechanism.The experimental dispersion curves are obtained.Compare the experimental dispersion curves to the calculated ones from the Rayleigh-Lamb dispersion equation,there are acceptable errors between the dispersion curves,suggesting that the single processing technique to acquire dispersion curves is viable.Apply the same signal processing technique to the honeycomb plate,the dispersion characteristics including the group velocities and the phase velocities are obtained.In the frequency range between 55 kHz and 220 kHz,the guided-wave dispersion characteristics of the honeycomb plate are similar to the features of its skins.Therefore,the skins can be used to simplify the calculation of characteristics of honeycomb plate in this frequency range.When frequencies are higher than 220 kHz,the dispersion characteristics of honeycomb plates become obviously different due to the leaky lamb waves.Based on the properties of honeycomb geometry,the leaky Lamb waves are investigated through numerical method.The debondings between the skins and the honeycomb cores influence the leaky Lamb waves,the amplitudes of which change because of the debondings.The structural wave interactions(SWIs)are produced due to the complex geometry of honeycomb plates.The unique feature of interactions between the guided waves and the honeycomb plates is investigated.To study the relationship between the excitation frequency and the geometry parameters of honeycomb plates,the finite element method is utilized to simulate the wave propagation.A method based on the phenomenon of SWIs has been developed to detect skin-core debondings in the honeycomb plates.To better utilize the SWIs for debonding estimation,it is crucial to select an appropriate excitation frequency.An optimal excitation frequency is found to be slightly higher than the first order natural frequency of the hexagon.Scanning laser vibrometry experiments are performed to verify the simulation results of SWIs.Multiple debondings are identified experimentally.Base on the phenomenon of SWIs,the sizes and locations of debondings are assessed and the results are compared with the actual parameters to prove the reliability of the technique.This thesis studies the guided-wave propagation mechanism of various-thickness plates and honeycomb sandwich plates that are the typical structures in the spacecraft industry,proposing the method for damage identification based on guided wave techniques.The study in this thesis is instructive for comprehensively understanding the characteristics of wave propagation in various-thickness plates and honeycomb sandwich plates and the proposed guided-wave-based SHM technique is a significant contribution to the application in practice.