Vibrational dynamics

Modern gas turbine engines have routinely utilized thermal barrier coatings for the past three decades to coax greater thermodynamic efficiency out of jet engines. In service, it has been noted that these ceramic materials are also effective at suppressing high cycle fatigue in the engine components, increasing their respective fatigue life. Recent efforts have been made to mechanically characterize these materials; this has been a challenge as they are thin coatings, prone to history effects, and are materially nonlinear. Refinement of the apparatus have occurred and it is now desired to  characterize them across a range of likely service temperatures.
A free-free beam apparatus has been adapted to accomplish this. Important to achieving viable results is the design, analysis, and experimental validation of the chamber in regards to the free-free specimen being heat-able, modally detuned and free-hanging, and the preservation of a non-contacting excitation method. Critical to success is ensuring a near-isothermal heated beam specimen. After successful validation and calibration of the chamber, a common thermal barrier coating, 8-YSZ, was tested, primarily using the free-decay, logarithmic decrement method, using
the chamber from 70-900degF. Materially non-linear behavior was observed and findings of material storage modulus and loss coefficient that are consistent with previous studies were
obtained. The chamber was also used to characterize the bare metal beam blanks and bond-coat only specimens. The apparatus was found to be sensitive when determining the storage modulus and damping, more so than the baseline configuration. This was due to changes in design to proof
it against high temperature. Resulting, it clearly characterized the anisotropic modulus of titanium and determined that bond coating had a negligible contribution to the beam system. Design life criteria with the properties of the coating could now be adopted across a range of temperatures.

ABSTRACT: The general topic investigated in this thesis is “structural health monitoring”, with special care being devoted to damage detection and localization. The study is focused on methods which work in the space of the observed variables, where the global information measures can be used to detect the damage. In this context damage detection is demanded to global measures such as entropy or Lyapunov exponents. The comparison of these measures computed in subspaces of the observed variables or in their expansions, allows one to perform a sort of sensitivity analysis on the role of the variables involved in the problem.
Such global measures are able to detect but not to directly  localize damage. For this purpose, a new approach to detect and localize the damage is formulated and implemented. It exploits response surface techniques to approximate the relationship among the observed variables. Comparing the resulting models in different damaged and undamaged situations identifies the response differences and their causes.
The response surface procedure is described from its basic theoretical aspects up to the features of its numerical implementation. Additionally, the global measures introduced above drive the analyst in establishing whether or not the problem is well posed, i.e., whether or not the set of measured quantities are able to detect the specific damage to be localized.
The validation of the proposed methodology is first numerically pursued by applying it to the benchmark case set up by the Structural Health Monitoring Panel of the American Society of Civil Engineering (ASCE). The procedure is then tested on two different experimental situations. While the first study of a three-dimensional steel frame reproduces the features of the benchmark problem, the second investigation covers a monumental structure situation, where damage is represented by more or less extended cracks in the masonry. The potential expressed by the procedure also in this second application seems to promise interesting exploitation possibilities.

By becoming aware of awareness we can experience the innermost sense or the felt sense of spaciousness and self-illumination. Within this state of open awareness, the openness of spacious luminous awareness, we can enter the experience of the radiance of light as energy through the sublime, luminous vibrations. The experience of the unfolding of sublime awareness and the corresponding energetic manifestation of the self-arising of primordial awareness is self-liberation. These experiences of the sublime manifestation of luminous energy was described by the 14 th century Dzogchen master Longchenpa in his text Precious Treasury of The Genuine Meaning. This meditative experience often arises spontaneously in Dzogchen meditation practice. This experience of translucent energetic manifestation is also described in Chan Buddhism, Hindu Kashmir Shavism, and Yan Xin Qi Gong. This experience of energetic manifestation can be experienced within subtle forms of respiration. Sometimes the ongoing sense of subtle luminous respiration can be experienced even as there is a profound sense of breathlessness. Breathlessness used here means no breath or minimal breath. This experience of the ongoing sense of respiration without breath is very powerful and empowering. In this experience of vast stillness there is no breath and yet there arises from deep within, a most subtle respiration within the breathlessness. During this respiration of stillness, the body seems to breathe and be breathed by most subtle inner wind. The inner sense of stillness expands and extends throughout the entire body and beyond the body's boundaries. This stillness opens us into the dimension of potential space, the dharmakaya and pure awareness.

ABSTRACT. A full three-dimensional dynamic soil–foundation structure interaction (SFSI) analysis of a famous landmark in Luxor, Egypt, the South Memnon Colossus, is performed to investigate the response of this historical monument to seismic excitation. The analysis is carried out using the finite element (FE) method in time domain. The statue comprising the upper structure is modeled using 3D brick finite elements constructed from a photogrammetric representation that captures important details of the surface and allows the identification of probable zones of stress concentration. The modeling also takes into account the presence of a surface of discontinuity between the upper part of the statue and its fractured base. FE models of the foundation and the surrounding soil deposit are constructed and coupled with the statue model to analyze the seismic response of the entire system incorporating dynamic SFSI effects. These studies are useful for future conservation efforts of this historical landmark, and more specifically for designing possible retrofit measures for the fractured base to prevent potential collapse of the monument from overturning during an earthquake.

KEYWORDS: Soil–foundation–structure interaction; Multi-body deformable contact; Non-linear dynamic finite element analysis

ABSTRACT: The joints connecting vertical and horizontal elements are the “weak link” in structural systems assembled from wood panels. If they are too weak, local failures may occur, resulting in performance that is significantly below expectations. If they are too resistant, the joints may be unable to dissipate energy during vibrations, thus possibly initiating a fast progressive failure. This paper re-processes and re-elaborates the results of shaking table tests previously carried out by the author and other co-workers. The goal is to assess the feasibility of a joint which is able to dissipate energy during vibration, without degrading the connection performance.

KEYWORDS: dynamic excitation - energy dissipation - wood panels joint - monitoring system - shaking table tests

ABSTRACT. The structural behaviour of large, monolithic, ancient monuments consisting of heavy stone blocks connected one to the other by the gravity load alone, without the use of mortar, is investigated. The Memnon Colossi, which were built fourteen centuries before Christ at Luxor in Egypt, are the case study for this construction technique. This paper summarizes the results of studies carried out on this structure within the CHIME (Conservation of Historical Monuments in the Mediterranean Area) project, funded by the European Commission.
The modal frequencies of the two statues were identified experimentally from ambient vibration measurements. Analytical models using the finite element method were also elaborated to compare the numerical results with the experimental ones, and to perform dynamic analyses. The results indicated that the distributed cracks characterizing the current state of the structure, play a significant role in the static and dynamic response of the monument. In view of the monuments’ rehabilitation, an innovative technology, based on fastening the cracked blocks by pre-tensioned wires of shape memory alloy, is proposed and numerically simulated.

KEY WORDS: monument; modal identification; damage assessment; retrofit; shape memory alloy

Polyvinylidene fluoride (PVDF) is a polymer of industrial importance, mainly due to its piezoelectric and pyroelectric properties. A comprehensive study of the normal modes and their dispersion in PVDF (β-form) has been reported in the reduced zone scheme using Wilson’s GF matrix method as modified by Higgs. A Urey-Bradley force field has been used. The evaluation of normal modes and their dispersion has been taken to logical conclusion by calculating the heat capacity as a function of temperature. The extent of agreement with the experimental data supports the potential field. Characteristic features of the dispersion curves such as repulsion and exchange of character have also been discussed.

We investigate the vibrational properties of two kinds of single-wall ZnO nanotubes. The simulations are carried out for three types of zigzag nanotubes (5,0), (8,0), (10,0) and armchair nanotubes (3,3), (4,4), (6,6). The natural frequencies are determined by means of the molecular mechanics approach with the universal force field potential. The first four natural frequencies are obtained for length/diameter ratio of about 5–20. The vibration modes associated with these frequencies have been computed. Closed-form analytical expressions have been derived using the continuum shell theory for the physical explanations of the simulations results. We observe that the natural frequencies decrease as the aspect ratios increase. The results follow similar trends with results of previous studies for carbon nanotubes (CNT). However, the magnitudes of the frequencies are lower from the corresponding CNT counterparts, indicating that ZnO nanotubes are comparatively less stiff.

The near edge x-ray absorption spectra of CX4 (X = H, F, Cl) have been measured with high resolution at the 1s edges of carbon and fluorine, and the 2p edge of chlorine. The absolute photoabsorption cross section has been determined at the resonances and in the nearby continuum. In accord with previous work, the antibonding valence peaks of the halides do not show any vibrational structure but sharp Rydberg states are observed at the C 1s edge. We report the first detailed study of the CCl4 Rydberg states. The vibrational frequency of the CF4 Rydberg states is anomalously high, which we attribute to a Fermi resonance. The C 1s line widths were compared to theoretical predictions of ion state line widths and reasonable values are found for CH4 and CCl4, but for CF4 the measured value is higher than the theoretical value. The discrepancy is assigned to mixing of valence band character in the Rydberg states, which is especially strong in CF4, and to multi-centre autoionization. Finally the integrated oscillator strength below threshold is shown to vary in the order CF4 > CH4 > CCl4.

The torsional vibration of systems involving engines exhibits some phenomena not typical of vibration in general. These effects result from the geometry of the reciprocating mechanism and are revealed by detailed kinematic and dynamic analyses. Apart from the system's inertia varying with crank rotation, recent work by the authors has shown that it is also affected by piston-to-cylinder friction. In previous published work, that omitted friction, differences were observed between the measured and the predicted natural frequency function. This short paper revisits those differences and includes friction effects.