The present research investigates the complex phenomena of wave scattering in bolted joint. The goal is to develop an understanding of the attenuation behavior of propagating waves, through the structure, as the bolt is subjected to different torques. This is a first step towards developing a structural health monitoring technique for detecting torque loss at a bolted joint. To simulate the local effects of the bolt, a micromechanics based model has been developed to model the scattering and attenuation behavior due to a single fiber in a matrix with a circumferential interface crack. A slicing approach is used to account for the effect of multiple interfacial cracks at different orientations through the depth
of the structure, to simulate the global effects of the bolt. The change in wave attenuation as a function of bolt location, at different depths in a plate, is studied. Next, the variation in wave scattering as the bolt, which is now fully embedded in the plate, is subjected to different torque is investigated. The local stress fields that develop in the plate due to the torque are treated as a pre-stress condition and their effect on the resultant wave scattering is investigated using the developed model. The resultant attenuation accounts for the combined effect of the geometrical attenuation and the attenuation due to the pre-stress. Numerical results obtained show small but steady increase in the attenuation with the applied torque. Experiments conducted to validate the developed model show similar trends.
In structural health monitoring, energy dissipation of wave propagation is a key factor to determine optimal placement of sensors and quantify damage. This paper focuses on the study of wave scattering and attenuation in fiber-reinforced composite laminates with damage. In order to obtain the overall attenuation coefficient, the propagation of elastic shear wave in fiber-matrix medium is investigated starting from the Helmholtz equation. The wave attenuation due to interfacial damage is considered. The attenuation due to cracks of varying sizes and the effect of frequency on the attenuation value has been examined. It can be shown that a critical frequency exists at a given crack size for which the attenuation in the composite medium is at its highest value. Furthermore, the wave attenuation in composite laminates is investigated by incorporating energy transfer in layerwise medium. The overall attenuation coefficient for the laminate is obtained. Experiments are also conducted to evaluate some of the observations obtained from the model.
In structural health monitoring, the fundamental goal is to address the problem of damage identification, localization and quantification. Using the wave based approach, the presence of damage is visualized in terms of the changes in the signature of the resultant wave that propagates through the structure. Since surface mounted piezoelectric transducers have been used for monitoring, the voltage output of each sensor is used for signature characterization. Due to the time-varying nature of these signals, performance of some existing analyzing tools may not be satisfactory. In the present study, the use of the matching pursuit decomposition has been investigated as a signal processing technique to compare signals from healthy and damaged structures.
In this paper, experimental and analytical investigations are conducted on the energy dissipation of polymeric composites containing carbon nanotube fillers. The specimens are fabricated by directly mixing single-walled carbon nanotubes into two types of representative polymers, a stiff resin system (Epon 9405/Epodil 749/Ancamine 9470) and a soft resin system (D.E.R. 383/Epodil 748/Ancarez 2364/Ancamine 2384), with high-power ultrasonic agitation. To characterize loss factors, uniaxial testing with harmonic loading is conducted to directly measure the phase lag between stress and strain. The loss factors are measured using different material deformations. The present paper also presents an energy loss model by addressing nanotube/resin interfacial friction. The concept of “stick-slip” motion caused by frictional contacts between nanotubes and resin is proposed to describe the load transfer behavior. Energy dissipation and loss factor are characterized through the work done by interfacial friction. The trends of strain-dependent loss factors obtained from experiments and analysis are compared.
The concept of smart structures, such as piezoelectric laminates, has received a great deal of attention recently as an alternative to conventional techniques. These advanced structures can be designed to actively react to disturbance forces in order to maintain structural integrity while maintaining, or even improving, the level of performance. Great potential can be found in advanced aerospace structural applications. However, the introduction of smart devices inevitably perturb the local values of the field variables and nucleate damage such as debonding and delamination at the interface of piezoelectric devices and the host structure due to stress concentration. The layerwise characteristics of the laminates make the determination of stress and strain distribution a challenging problem. Conventionally, classical lamination theory has been extended to smart laminated structures which ignores transverse shear effects'3. A higher order theory was proposed and applied by Chattopadhyay et al.4'5 in the analysis oflaminated structures to address transverse shear effects without shear correction factors. The theory proved to be successful in global analysis for thick structures and smart structures. However, it fails to provide continuous distribution of transverse shear stresses. This implies that the theory is not sufficient in predicting local information regarding stress and strain distributions which is critical in the analysis of structural failure. The multifield characteristics of piezoelectric structures make the analysis even more complex, particularly in the presence of thermal effects as dictated by specific missions. A typical environment is represented by a solar flux of 1350W/m2 as vehicles move from shadow to sunlight. Some research in the field of smart structural modeling in the presence of thermal effects has been reported610. However, oneway coupling that only considers the effect of a known field on another field is used in these works. The bi-way coupling between piezoelectric and mechanical fields was included in the hybrid plate theory developed by Mitchell and Reddy3. A coupled thermal-piezoelectric-mechanical (t-p-m) model was developed by Chattopadhyay et al.1113 to address the bi-way coupling issues associated with smart composites under thermal loads. Their work indicates that the effects of bi-way coupling on structural deformation increase with the thickness of piezoelectric device. However, an equivalent single layer approach is used, and therefore the localized interlaminar characteristics cannot be addressed accurately by this theory. The present paper aims at the investigation of interlaminar stress distribution in laminated shell structures using coupled thermal-piezoelectric-mechanical model. The goal is to develop a theory that is capable of providing sufficient accuracy while guaranteeing computational efficiency compared to other layerwise theories. To maintain local accuracy of stress and strain distributions, the trial displacement field is assumed using zigzag functions and C0 continuity through the entire laminate thickness accommodating zigzag in-plane warping and interlaminar transverse shear stress continuity. The continuity conditions of inplane displacement and transverse shear stress fields as well as traction free boundary conditions are applied to reduce the number of primary structural variables. The temperature and electrical fields are assumed using higher order functions. These descriptions can satisfy surface boundary conditions of heat flux and electrical potential. The mathematical model is implemented using finite element technique. The case of cylindrical bending and spherical composite shell structures with piezoelectric patches are investigated. The analysis of stress distributions under electrical and thermoelectrical loading is performed and numerical results are presented.
A new theory is developed to model the hysteresis relation between polarization and electric field of piezoceramics. An explicit formulation governing the hysteresis is obtained by using a saturation polarization, remnant polarization and coercive electric field. A new form of elastic Gibbs energy is proposed to address the coupling relations between electrical field and mechanical field. The nonlinear constitutive relations are derived from the elastic Gibbs energy and are applicable in the case of high stroke actuation. The hysteresis relations obtained using the current model are correlated with experimental results. The static deflection of a cantilever beam with surface-bonded piezoelectric actuators is analyzed by implementing the current constitutive relations. Numerical results reveal that hysteresis is an important issue in the application of piezoceramics.
High resolution photoelectron spectroscopy is applied to the study of molecular clusters. The primary species studied are fluorene-Arn complexes. Spectroscopy of the neutral S1 state has been performed on clusters as large as n equals 30. In order to study the photoelectron spectra of the clusters size selectively mass analyzed threshold ionization (MATI) is used which is a mass resolved version of the ZEKE technique. MATI spectroscopy has been applied to clusters up to n equals 5. The spectral shifts in the S1 origin and ion threshold are used as a measure of the relative stability of the different clusters. Using previous experimental and theoretical work on related clusters the structures of the clusters are inferred from the observed spectral shifts. In some cases multiple conformations of a particular cluster size are identified.
Time resolved zero electron kinetic energy (ZEKE) photoelectron spectroscopy is applied to the study of molecular dynamics on the picosecond time scale. The dynamics processes studied include intramolecular vibrational redistribution (p-difluorobenzene, acenaphthene) and vibrational predissociation of van der Waals complexes (aniline-CH4). The technique offers excellent time resolution as well as sensitivity to the detailed nature of the vibronic state undergoing dynamics.