362 research outputs found
Nonlinear Normal Modes For Damage Detection: Analytical Results and Experiments
No full textThe use of nonlinearities in damage detection is a potential avenue for developing accurate monitoring systems in a variety of engineering applications. In several situations the linear theory reveals its drawbacks, for example, when environmental disturbances affect the structural response. For instance, the thermal effects can induce frequency shifts of the same order of magnitude of the damage itself [1] and, for properly accounting them, accurate nonlinear models become necessary [2]. In this prospective, it is interesting to ascertain if the nonlinear response is more sensitive to damage than the linear one. The nonlinear normal modes concept (NNMs) lends itself naturally to extend the consolidated knowledge of the linear theory [3]. The sensitivity of the nonlinear dynamic response to damage is investigated considering a straight beam clamped at both ends with localized reductions of the cross-sectional area in different positions along the beam span. In the first stage, the problem is analytically tackled with the method of multiple scales for computing the NNMs and the backbones curves. The trends of the effective nonlinearity coefficient for the lowest bending mode as function of the damage position and severity are obtained and compared with those of the linear frequencies. In the second stage, an experimental campaign is carried out on a set of steel beams with different levels of area reduction within a few segments along the beam span. The analytical results are qualitatively corroborated by tailored experiments confirming the important role that the nonlinear structural response can play towards damage detection
A Multi-Bandgap Metamaterial With Multi-Frequency Resonators
No full textA 2D metamaterial cellular system inspired by lightweight honeycombs and spider webs is investigated. The hexagonal cells of the honeycomb act as hosting substructures for spider-web-like or cantilever resonators with added lumped masses which can vibrate, in principle, in any of the infinitely many modes. Contrary to traditional approaches utilizing discrete mass-spring resonators, here the infinite-dimensional (full spectrum) resonators are intentionally tailored to generate multiple, complete or incomplete, stop bands across which wave propagation is either totally or partially suppressed along preferential directions. The Plane Wave Expansion method is employed to obtain the dispersion curves and the bandgap sensitivity with respect to the design parameters. Experimental results based on laser scanning vibrometry corroborate the theoretical predictions and confirm the robustness of the stop band behavior with a wealth of results which pave the way towards suitable optimization strategies and a closer understanding of these formidable stop band cellular material systems
Sawan Sūrya temple inscription
No full textFigure 57 in
To engrave his virtues on the disc of the moon… Inscriptions of the Aulikaras and Their Associates
Dániel Balogh, 2019
Sawan Sūrya temple inscription. Photo by the author, 2017. Scale: 30 cm/12”. Courtesy of Yashodharman Museum, Mandsau
Design Development and CFD Simulation of a Variable Twist Wing
No full textWing twist is an aerodynamic feature added to aircraft wings to adjust lift distribution along the wing. Often, the purpose of lift redistribution is to ensure that load distribution is uniform from wing tip to root, it ensures that the effective angle of attack is always lower at the wing tip than at the root, meaning the root will stall before the tip. This is desirable because the aircraft's flight control surfaces are often located at the wingtip, and the variable stall characteristics of a twisted wing alert the pilot to the advancing stall while still allowing the control surfaces to remain effective, meaning the pilot can usually prevent the aircraft from stalling fully before control is completely lost. Twist that decreases the local chord's incidence from root to tip is sometimes referred to as washout. In this project the design is done in GAMBIT software, and CFD simulation of twisted wing is done in fluent software, the designed model is tested in software at different twist angles, and at different angles of attack to find the aerodynamic properties like CD, CL, and pressure
Determination of Fatigue Life of Surface Propeller by Using Finite Element Analysis
No full textPropeller design aims at achieving high propulsive efficiency at low levels of vibration and noise, usually with minimum cavitations. Achieving this aim is difficult with conventional propellers, as ships have become larger and faster propeller diameters have remained limited by draught and other factors. Surface piercing propeller offers an attractive alternative to high-speed crafts, which operate under limited draught. The performance of the vehicle depends upon the efficiency of the propeller. The geometric shape and its surface finish will decide the efficiency of the propeller. The material used is carbon UD and aluminum. The present project basically deals with the modeling, Analysis of the propeller using composite material of a marine vehicle having low draft. A propeller is complex 3D model geometry. CATIA modeling software is used for generating the blade model and tool path on the computer. Sectional data, pitch angle of the propeller are the inputs for the development of propeller model. Finite element analysis was carried out using ABAQUS. The propeller model developed in CATIA is converted in to IGES file and then imported to HYPERMESH for developing fine mesh of the model. As a part of the analysis static structural testing was conducted by varying material properties in pre-processing stage. Further fatigue analysis was performed to analyze the factor of safety. Based on the results obtained from both static analysis and dynamic analysis a better performing material is identified for the development of a propeller. The post processed results obtained from both analysis methods recommends carbon UD/ Epoxy for the fabrication of propeller
A scheduling policy experiment for lean implementation
No full textThesis (S.M.M.O.T.)--Massachusetts Institute of Technology, Sloan School of Management, Management of Technology Program, 1999.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (leaves 178-184).by Sawan P. Deshpande.S.M.M.O.T
A New Semi-Active Tensairity Structure Equipped With Shape Memory Cables: Experiments And Computations.
No full textThe concept of tensairity structure is interesting for numerous applications in several application fields according to the low ratio between self-weight and its loading capacity. The basic components of a tensairity are represented by a cylindrical pneumatic/inflatable element, a couple of cables wrapped around and attached to a slender beam positioned along the generatrix of the cylinder. When the tensairity is loaded, the beam is compressed while the cables are subject to tension thus realizing a very efficient structure. However, one of the main limitations for dynamic applications is due to the low damping ratio which makes the structure susceptible of continuous vibrations. This study explores a new concept of semi-active tensairity that provides a solution to this loss of performance. A reduced-scale prototype has been manufactured according to the tensairity concept in U.S. Patent No. 10,407,939. The main innovations are represented by the employment of NiTiNOL wires wrapped around the pneumatic element, the introduction of manual and automatic tensioning systems at the ends of the slender beams and the adoption of a design that provides the capability of sustaining loads in any direction. The load-displacement curves have been acquired with a MTS testing machine for the tensairity equipped with NiTiNOL and steel wires, respectively, and for different pretension levels. Dynamic experiments have been performed to measure the resonance frequencies of the lowest few modes as a function of the pretension levels. The results demonstrate the higher dissipation capacity of the tensairity equipped with shape memory wires whose tensions are semi-actively controlled. Finally, a nonlinear FEM model has been implemented in ABAQUS and parametrized in Python to simulate the static and dynamic tests
Nonlinear Damping Properties of Nanocomposite Beams via Frequency Responses and Laser Doppler Vibrometry
No full textThe nonlinear damping properties of nanocomposite samples, made of polybutylene terephthalate (PBT) hosting matrix, integrated by branched multi-walled carbon nanotubes (b-MWCNTs), are experimentally investigated by performing frequency sweep tests and Experimental Modal Analysis (EMA) by using the Siemens-TestLab acquisition system and the Polytec PSV-500-3D vibrometer, respectively. The frequency response curves (FRCs) for beams with different weight frac-tions of b-MWCNTs are obtained in cantilever con figuration by applying a base excitation with a shaker and measuring the free tip displacement with a triangulation laser (optoNCDT 1320 produced by Micro-Epsilon). The dependence of the damping ratio on the oscillation amplitude for the lowest mode is identifi ed with the half power bandwidth method applied to the FRCs (see. Fig. 1 (a)) acquired for increasing excitation levels. The observed trends of the damping ratio and resonance frequency can be associated to the geometric and material nonlinearities due to the cantielever con figuration (nonlinear curvature) and stick-slip interaction between the PBT matrix and b-MWCNTs. The PSV and the PolyWave software are employed to identify the lowest three modes (see. Fig. 1 (b)). Several identi fications are performed increasing the amplitude of the chirp excitation provided by the shaker with the aim of capturing the nonlinearities in the small oscillation range. This approach can be considered as an equivalent linearization of the response at di erent os-
cillation amplitudes. The identi cations performed with the two methods are in agreement and show the reliability of this approach to characterize the mechanical properties of nanocomposite materials
Nonlinear Normal Modes For Damage Detection: Theoretical Concepts And Preliminary Experimental Validation
No full textThe exploitation of nonlinearities in the eld of damage detection is a promising direction towards robust and reliable monitoring systems for engineering applications. In this perspective, the concept of nonlinear normal mode (NNM) is a natural candidate for extending the knowledge already established in the linear framework and for developing new approaches. A clamped-clamped beam with localized cross-sectional reductions across the beam span is considered. The problem is first analytically tackled via the method of multiple scales for computing the
NNMs and the backbone curves. The latter describes the dependence of the resonance frequencies on the oscillation amplitude and are governed by the so-called effective nonlinearity coefficient for each mode. Subsequently, an experimental campaign is carried out considering an undamaged and a damaged beam to validate the analytical results. The effective nonlinearity coeffcient is shown to exhibit a sensitivity to damage higher than that exhibited by the linear natural frequencies, whereas the backbones associated to each mode reveal an interesting dependence
on the sti ness reduction and damage location
Experimental nonlinear response of a new tensairity structure under cyclic loading
No full textPneumatic structures are recognized as promising thin-walled structures for their advantageous features such
as lightness, portability, versatile design, and ease of installation. Although their bearing capacity under
monotonic static loads can be formidable, their inherent dissipation capacity is low and thus entails significant
limitations when counteracting dynamic loads. A novel tensairity structure is here proposed to overcome
this drawback. The innovative design features a cylindrical inflatable element integrated with NiTiNOL
cables wrapped around and affixed to a slender beam positioned along its generatrix. A laboratory-scale
prototype is employed to assess how the structure behaves under cyclic loading in comparison to a standalone inflated beam and a conventional tensairity structure outfitted with steel cables. This experimental
study delves into the influence of internal pressure and pretension levels of the metallic cables. Experimental results unfold a smooth softening-type hysteretic behavior under cyclic loading, which is accompanied
by a slight stiffness degradation and a moderate pinching. The comparative analysis of the experimental
results also demonstrates the substantially improved and consistent dissipation capacity of the presented
novel concept of tensairity structure, which thus offers superior stability under cyclic loads. A parametric
identification based on a modified Bouc-Wen model is finally performed to simulate the hysteretic response
of the structure. A correlation is also established between the identified parameters of the phenomenological model and the internal pressure, type and cables pretension levels. The excellent agreement between
numerical predictions and experimental force-displacement cycles other than those used for the parametric
identification demonstrates the suitability of the adopted phenomenological modeling
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