Modal Analysis and Experimental Testing of Air-Inflated Drop-Stitch Fabric Structures Used in Marine Applications

2017 ◽  
Author(s):  
Andrew W. Hulton ◽  
Paul V. Cavallaro ◽  
Christopher J. Hart
Keyword(s):  
Natural Frequencies ◽  
Numerical Models ◽  
Experimental Testing ◽  
Ideal Gas ◽  
Mode Shapes ◽  
Unit Weight ◽  
Material System ◽  
Inflatable Structures ◽  
Technical Textiles ◽  
Load Carrying

Rapid deployment of marine structures is of growing importance to U.S. naval forces. Surface-based inflatable structures including Rigid Inflatable Boats (RIBs), inflatable causeways and bridging, and launch and recovery systems provide unique solutions for temporary structures during sea-based missions. When performance specifications demand minimal weight and stowage, rapid deployability and temporary rigidity, solutions are limited to inflatable structures constructed of flexible materials. Driven by air pressure, today’s inflatables provide significant load-carrying capacities per unit weight (or stowed volume) utilizing technical textiles, elastomers or “soft” composites. Overloading of inflatable structures produces unique fail-safe behaviors (reversible wrinkling) that allow the structures to assume rigidity and load-carrying capacity upon load removal. Design standards are virtually nonexistent for inflatable structures involving shapes constructed of spheres, beams, arches and most recently flat panels using 3D woven drop-stitch panels. Predictive performance tools (analytical and numerical) for static applications lag significantly behind those for conventional structures. Nonlinear system behaviors (material and geometric), thermo-mechanical coupling and fluid-structure interactions (FSI’s) pose significant challenges when applying existing design tools to inflatable structures. This gap is further exacerbated for dynamic applications as inflatable structures exhibit pressure-dependent natural frequencies and mode shapes. Surface-based structures must be designed with consideration given to operational sea state frequencies and wave periods so that the onset of structural instabilities (wrinkling, buckling) and loss of load-carrying capacities can be prevented. The present research establishes the validity of physics based models using the Ideal Gas Law as an equation of state (EOS) to predict the natural frequencies and corresponding mode shapes of air-inflated drop-stitch fabric panels as functions of inflation pressure. Particular concern is given to the breathing modes for inflation pressures ranging from 5.0 to 30.0 psig. The presence of breathing modes can negatively impact the riding performance of RIBs vessels constructed with drop-stitch fabric hulls by amplification of the panel’s skin separation displacements and vertical accelerations, and are not seen in this material system for the pressures considered. Both numerical and experimental methods are pursued; the results of laboratory modal experiments are used to validate the numerical models. Predicted and experimental natural frequencies and mode shapes are compared and excellent correlation was observed. Increasing inflation pressures produced increasing in-plane and through-thickness normal stresses and modal frequencies of the drop-stitch fabric panels.

scholarly journals Damage Evaluation of Free-Free Beam Based on Vibration Testing

2020 ◽  
Vol 1 (2) ◽  
pp. 142-152 ◽  
Author(s):  
Duong Huong Nguyen ◽  
Long Viet Ho ◽  
Thanh Bui-Tien ◽  
Guido De Roeck ◽  
Magd Abdel Wahab
Keyword(s):  
Natural Frequency ◽  
Natural Frequencies ◽  
Numerical Models ◽  
Element Model ◽  
3D Models ◽  
Mode Shapes ◽  
Structure Damage ◽  
Modal Properties ◽  
Vibration Responses ◽  
Free Beam

Damage can be detected by vibration responses of a structure. Damage changes the modal properties such as natural frequencies, mode shapes, and damping ratios. Natural frequency is one of the most frequently used damage indicators. In this paper, the natural frequency is used to monitor damage in a free-free beam. The modal properties of the intact free-free beam are identified based on a setup of 15 accelerometers. A finite element model is used to model the free-free beam. Three models are considered: beam (1D), shell (2D), and solid (3D). The numerical models are updated based on the first five bending natural frequencies. The free-free beam is damaged by a rectangle cut. The experiment is re-setup and the model properties of the damaged beam are re-identified. The cuttings are modeled in the numerical simulations. The first five numerical bending natural frequencies of the damaged beam are compared with the experimental ones. The results showed that the 1D beam element model has the highest errors, while the 2D and 3D models have approximately the same results. Therefore, the 2D representation can be used to model the damaged beam for fast computation.

A Calculation Model for Deteriorated Members of 3D Frame Structures in the Static and Dynamic Analyses

2010 ◽  
Author(s):  
H. Karadeniz
Keyword(s):  
Structural Analysis ◽  
Degrees Of Freedom ◽  
Natural Frequencies ◽  
Calculation Model ◽  
Mode Shapes ◽  
Special Treatment ◽  
Structural System ◽  
Mass Matrices ◽  
Load Carrying ◽  
Load Vector

Jacket type structures in offshore structural industry consist of a large number of tubular members with various dimensions, which are joined to each other by welding that makes connections to be rigid. Diagonal members have relatively small dimensions, legs or chords have larger dimensions in general. Although the connections at joints are made rigidly, the actual joint behaviors under wave loadings are not fully rigid in the vicinity of connections due to local deformations of members having large diameters. In the short term, due to ultimate wave and earthquake loadings, some plastic deformations can also occur in members at some critical joints so that related members cannot be behave as rigidly connected and some releases of member forces occur. In the long term, fatigue damages can be observed at some joints that damaged members loose their functionality partly or fully as depending on damage rates. All these phenomena can be considered as member deterioration. A special treatment of deteriorated members can be used in the structural analysis by using a computation model that allows flexibility of damaged members at joints. The solution of this problem can be achieved by introducing a fictitious member concept, which can be derived as depending on actual member dimensions and joint configurations. The technique of using fictitious members introduces additional degrees of freedom that are not desirable in the analysis. A procedure which uses modified stiffness and mass matrices for flexibly connected members are more practical and attractive since a) no additional degrees of freedom are introduced, b) member-release and fixed-connection conditions can be directly obtained, c) a general member-end condition in any direction can be easily specified, d) a failure mechanism can be easily determined, e) in the fatigue damage calculation the load carrying capacity of the member can be used until the whole member cross-section is damaged and f) natural frequencies and mode shapes of damaged structural system can be estimated in terms of the natural frequencies and mode shapes of the undamaged structural system. The paper introduces a general formulation of a partly connected member to be used in structural analysis. For this purpose, a spring-beam element is defined using massless spring systems at member ends. An algorithmic procedure is presented to update member stiffness and mass matrices as well as member consistent load vector.

Investigation on Natural Modes of Floating Structures Considering Design Strength

2009 ◽  
Author(s):  
B. W. Kim ◽  
H. G. Sung ◽  
S. Y. Hong
Keyword(s):  
Natural Frequencies ◽  
Numerical Models ◽  
Strength Distribution ◽  
Mode Shapes ◽  
Floating Bodies ◽  
Design Strength ◽  
Floating Structures ◽  
Subspace Iteration ◽  
Natural Modes ◽  
Stiffness Distribution

In this paper, influences of design strength on natural modes of floating structures are examined by analyzing natural frequencies and mode shapes of floating bodies with nonuniform stiffness. A FSRU (Floating Storage and Regasification Unit) is considered as an example structure. Natural modes such as natural frequencies and modes shapes are calculated by the subspace iteration method which is widely used as a solver of eigenvalue problem. Various numerical models of FSRU are considered. One is a simplified model of which stiffness is uniformly distributed. The other is a model with stepped distribution of stiffness. The third model has a triangular stiffness distribution. By comparing the results of each model, the influences of nonuniform strength distribution on natural modes are investigated.

scholarly journals Model Updating of a Freight Wagon Based on Dynamic Tests under Different Loading Scenarios

2021 ◽  
Vol 11 (22) ◽  
pp. 10691
Author(s):  
Rúben Silva ◽  
Diogo Ribeiro ◽  
Cássio Bragança ◽  
Cristina Costa ◽  
António Arêde ◽  
...  
Keyword(s):  
Natural Frequencies ◽  
Model Updating ◽  
Numerical Models ◽  
Vertical Direction ◽  
Modal Identification ◽  
Mode Shapes ◽  
Elastic Behaviour ◽  
Test Duration ◽  
Fe Model ◽  
Dynamic Tests

This article presents an efficient methodology for the calibration of a numerical model of a Sgnss freight railway wagon based on experimental modal parameters, namely natural frequencies and mode shapes. Dynamic tests were performed for two distinct static loading configurations, tare weight and current operational overload, under demanding test conditions, particularly during an unloading operation of the train and without disturbing its tight operational schedule. These conditions impose restrictions to the tests, especially regarding the test duration, sensor positioning and system excitation. The experimental setups involve the use of several high-sensitivity accelerometers strategically distributed along with the vehicle platform and bogies in the vertical direction. The modal identification was performed with the application of the enhanced frequency-domain decomposition (EFDD) method, allowing the estimation of 10 natural frequencies and mode shapes associated with structural movements of the wagon platform, which in some cases are coupled with rigid body movements. A detailed 3D FE model of the freight wagon was developed including the platform, bogies, wheelsets, primary suspensions and wheel–rail interface. The model calibration was performed sequentially, first with the unloaded wagon model and then with the loaded wagon model, resorting to an iterative method based on a genetic algorithm. The calibration process allowed the obtainment of the optimal values of eight numerical parameters, including a double estimation of the vertical stiffness of the primary suspensions under the unloaded and loaded static configurations. The results demonstrate that the primary suspensions present an elastic/almost elastic behaviour. The comparison of experimental and numerical responses before and after calibration revealed significant improvements in the numerical models and a very good correlation between the experimental and numerical responses after calibration.

scholarly journals Free Vibration and Buckling Analysis of the Laminated Composite Beams by Using GDQM

2009 ◽  
Vol 18 (2) ◽  
pp. 096369350901800
Author(s):  
Gökmen Atlıhan ◽  
Ersin Demir ◽  
Zekeriya Girgin ◽  
Hasan Çallıoğlu
Keyword(s):  
Free Vibration ◽  
Natural Frequencies ◽  
Differential Quadrature Method ◽  
Laminated Composite ◽  
Mode Shapes ◽  
Load Carrying Capacity ◽  
Load Carrying ◽  
Generalized Differential ◽  
Laminated Composite Beam ◽  
Laminated Composite Beams

In this study, effects of stacking sequences of composite laminated beams on natural frequencies and buckling behaviour have been analyzed by Generalized Differential Quadrature Method (GDQM) and Finite Element Method (FEM). Mode shapes were also investigated for one mode of buckling and three modes of free vibration analyses. In addition, variations of mode shapes for different boundary conditions were presented in details. Numerical results show that the effective stiffness of the laminated composite beam can be altered through an adjustment in the stacking sequence. Thus, such an adjustment in stacking sequences allows operations in desired natural frequencies and load carrying capacity without changing its geometry drastically or without changing its weight.

scholarly journals Investigation and Validation of Numerical Models for Composite Wind Turbine Blades

2021 ◽  
Vol 9 (5) ◽  
pp. 525
Author(s):  
William Finnegan ◽  
Yadong Jiang ◽  
Nicolas Dumergue ◽  
Peter Davies ◽  
Jamie Goggins
Keyword(s):  
Wind Energy ◽  
Wind Turbine ◽  
Natural Frequencies ◽  
Numerical Models ◽  
Experimental Testing ◽  
Turbine Blades ◽  
Wind Turbine Blades ◽  
Static Testing ◽  
Design Life ◽  
Design And Manufacture

As the world shifts to using renewable sources of energy, wind energy has been established as one of the leading forms of renewable energy. As the requirement for wind energy increases, so too does the size of the turbines themselves, where the latest turbines are 10 MW with a turbine diameter in excess of 190 m. The design and manufacture of the blades for these turbines will be critical if they are to last for the design life, where the accuracy of the numerical models used in the design process is paramount. Therefore, in this paper, three independent numerical models have been created using three available finite element method packages—ABAQUS, ANSYS, and CalculiX—and the results were compiled. Following this, the accuracy of the models has been evaluated and validated against the results from an experimental testing campaign. In order to complete the study, a 13 m full-scale wind turbine blade has been used, which has been subjected to static testing in both the edgewise and flapwise directions. The results from this testing campaign, along with the blade mass and natural frequencies, have been compared to the results from the independent numerical models. The differences in the models, along with other sources of error, have been discussed, which includes recommendations on the development of accurate numerical models.

Wave characteristics of nonsymmetric cross-ply laminated composite beams

2021 ◽  
pp. 146442072097755
Author(s):  
Richard Bachoo
Keyword(s):  
Free Vibration ◽  
Composite Beam ◽  
Power Flow ◽  
Natural Frequencies ◽  
Numerical Models ◽  
Laminated Composite ◽  
Free Vibration Analysis ◽  
Mode Shapes ◽  
Wave Approach ◽  
Laminated Composite Beam

The free vibration characteristics of a nonsymmetric cross-ply laminated composite beam coupled in bending and longitudinal deformation is studied using a wave approach. The effects of shear deformation and rotary inertia are included in the analysis. Exact analytical expressions are derived for the natural frequencies, mode shapes, and the power flow of the propagating waves. The derived expressions are validated using the results from past literature and provide a benchmark for numerical models. The advantages of the wave approach over conventional free vibration analysis methods are highlighted. Specifically, the wave approach is used to derive a simplified expression for the mode count function of the composite beam. Additionally, the wave approach is also used to investigate the power flow and cross-conversion of the propagating wavetypes across various classical boundary conditions. The influence of the number of cross-ply layers on the natural frequencies and power flow are also investigated. The efficacy of the wave analysis is illustrated through several numerical examples.

scholarly journals Experimental Modal Test of the Laboratory Model of Steel Truss Structure

2016 ◽  
Vol 12 (2) ◽  
pp. 116-121 ◽  
Author(s):  
Ján Kortiš ◽  
Ľuboš Daniel ◽  
Milan Škarupa ◽  
Maroš Ďuratný
Keyword(s):  
Natural Frequencies ◽  
Numerical Models ◽  
Steel Structure ◽  
Modal Testing ◽  
Mode Shapes ◽  
Laboratory Model ◽  
Residual Lifetime ◽  
Good Tool ◽  
Modal Test ◽  
Proper Estimation

Abstract The experimental modal analysis is often used to validate the accuracy of dynamic numerical models. It is also a good tool to obtain valuable information about current condition of the structures that could help to determine residual lifetime. The quality of modal testing results is highly dependent on the proper estimation of the natural frequencies from the frequency response function. This article presents the experimental modal test of the laboratory steel structure in which the natural frequencies and mode shapes are determined.

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