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Über dieses Buch

Sensors, Instrumentation and Special Topics, Volume 6. Proceedings of the 29th IMAC, A Conference and Exposition on Structural Dynamics, 2011, the sixth volume of six from the Conference, brings together 27 contributions to this important area of research and engineering. The collection presents early findings and case studies on fundamental and applied aspects of Structural Dynamics, including papers on Structural Health Monitoring, High Intensity Noise Generation and other Special Topics.

Inhaltsverzeichnis

Frontmatter

Experimental and Numerical Investigation of Forming Limit Diagram for AA3105

Abstract
In this work, forming limit diagram for aluminum alloy 3105 is performed experimentally using out of plane test. In addition, using ductile fracture criteria and finite element simulation, forming limit diagram of aluminum alloy 3105 is performed numerically. Finally, it is shown that the results obtained from numerical prediction are in good agreement with experimental results.
Mehdi. Safari, S. J. Hoseinipour, H. D. Azodi

Multi-field Microphone – when the Sound Field is unknown

Abstract
Only a small percentage of all acoustical measurements are performed in the welldefined and well-controlled environment of a calibration laboratory – on the contrary most acoustical measurements are done under non-controlled conditions which in many cases are not even known in beforehand. This is the reason that some acoustical standards such as the IEC 61672 series (the “Sound Level Meter standard”) specify the performance of the measuring microphone over a wide range of environmental conditions. Modern quality measuring condenser microphones often meet or exceed the requirements even under very varying conditions. However one important – and unfortunately in many cases major – source of error is often neglected: The response of the actual microphone type in the actual sound field. The influence of different sound fields on the measurement error is discussed in some detail with practical examples and it is shown how a worstcase error exceeding 10 dB @ 20 kHz is a real risk. After a brief discussion of a condenser microphone which drastically reduces the error caused by influence of an unknown sound field or varying angle of incidence. Finally, test results from production samples of the new microphone are shown.
Svend Gade, Niels V. Bøgholm

Separating Three Sources of Changes in Structural Health Monitoring

Abstract
Discrimination between three sources of variability in a structural health monitoring system are investigated: environmental or operational effects, sensor faults, and structural damage. Different environmental or operational effects are included in the training data and can be accounted for by the model. Distinguishing between sensor fault and structural damage utilizes the fact that the sensor faults are local, while structural damage is global. A time domain approach is used to model the sensor network and the generalized likelihood ratio test (GLRT) is then used to detect and localize a change in the system. An experimental study is performed to validate the proposed method.
Jyrki Kullaa

Structural Damage Identification with Various Uncertainties

Abstract
This paper presents an investigation of structural damage identification with various uncertainty conditions. The model-based damage identification techniques compare the change of the identified model and the change of the analytical model, such as a finite element model, due to damage. The analytical model is different from the identified model because of errors from boundary conditions, material properties, and other variability. The uncertainty associated with identification errors and other random factors may also result in false indications of damage. In this study, we examine the damage identification with various uncertainties, such as boundary condition variation, temperature change, and environmental variation. Experiments of a beam structure under various uncertainties will be used to demonstrate the study.
Jiann-Shiun Lew, Michael Williamson

Excitation techniques for testing bike vibration transmission in the laboratory

Abstract
Vibrations generated by road surface defects are a significant source of discomfort for cyclists. This paper presents two very different laboratory techniques for studying road bike vibration. The first technique uses a treadmill with a modified belt surface. The second technique is based on the use of a road simulator that was developed specifically to generate displacement excitation under the wheels of the bike. Broadband excitation generated by coarse pavement surface is also evaluated in this study. The objective of this paper is to evaluate and compare the relative merits of these two approaches. For the purposes of evaluation, we have described a technique to obtain a realistic measurement of input in real road conditions. Our results demonstrate that the road simulator succeeds in producing adequate displacement profiles in the vertical axis resulting in a vibration frequency spectrum that closely resembles the measurements in real road conditions. Limitations in current actuator capacity prevent to reproduce very coarse road conditions. Finally, more work is needed to develop an appropriate belt surface that can generate sufficient energy excitation above the 25 Hz range.
Julien Lépine, Yvan Champoux, Jean-Marc Drouet

Damage Detection Using Blind Source Separation Techniques

Abstract
Blind source separation (BSS) techniques are applied in many domains since they allow separating a set of signals from their observed mixture without the knowledge (or with very little knowledge) of the source signals or the mixing process. Two particular BSS techniques called Second-Order Blind Identification (SOBI) and Blind Modal Identification (BMID) are considered in this paper for the purpose of structural damage detection or fault diagnosis in mechanical systems. As shown on experimental examples, the BMID method reveals significant advantages. In addition, it is demonstrated that damage detection results may be improved significantly with the help of the block Hankel matrix. The main advantage in this case is that damage detection still remains possible when the number of available sensors is small or even reduced to one. Damage detection is achieved by comparing the subspaces between the reference (healthy) state and a current state through the concept of subspace angle. The efficiency of the methods is illustrated using experimental data.
Nguyen Viet Ha, Golinval Jean-Claude

Application of Image Sensing to Motion Study of High-voltage Switchgear

Abstract
Video sensing method is applied to the motion study of high voltage switchgears, in which electric contacts are driven in high speed by hydraulic or spring powered mechanism in order to interrupt large current. The presented system combining high speed video and image processing algorithm realized precise measurement of linear or rotational motions of links, levers and cams that are too small to install conventional displacement sensors. Furthermore experimental properties as damping ratio and friction coefficient can be estimated by this method. By implementing those values in the multi-body motion analysis of operating mechanism, numerical simulation of motion shows excellent agreement with the prototype test.
Satoshi Marushima, Yoshiaki Ohda, Masaharu Shimizu, Hirokazu Takagi

CAE Based Approach to Understand a Behavior of Vibro-Acoustic System

Abstract
In this paper, a CAE based approach to understand a behavior of vibro-acoustic system is introduced. The sound pressure at an evaluation point in an enclosure can be described by inner product of (i)an acoustic transfer function from coupling surface to the evaluation point, and (ii)a structural vibration on the coupling surface. By addition of (iii)the correlation angle between above (i) and (ii), the sound pressure can be exactly estimated. In this paper, the correlation (iii) is called Vibro- Acousitc Coupling Factor (VAC). Using VAC, it is possible to understand not only the radiation efficiency, but also the interference of sound at the evaluation point. The proposing method explains the behavior of vibro-acoustic system using the frequency response of (i), (ii) and (iii). As an example, the proposing method is applied to FE model for improving the vibroacoustic characteristic.
Kohei Furuya, Takuya Yoshimura, Nobuyuki Okubo, Takeshi Toi

Verification of coverage of inverse-numerical acoustic analysis

Abstract
This paper describes the influence of both air-borne noise and structure-borne noise (input) by using inverse-numerical acoustic analysis. Inverse-numerical acoustic analysis is a technique for calculating the vibration of the surface of the sound source by measuring sound pressure around the sound source. Therefore, vibrations with a high contribution to sound radiation can be identified by this technique. Moreover this technique can calculate the vibration of the engine with a complex shape. Therefore, this technique is a powerful method in acoustic problems. However, accuracy of the identification result of the engine vibration might changes because of various factors. Therefore, Noise source’s form must be considered. Noise source’s form is whether the sound source contained in the structure-borne noise or the air-borne noise. Then, this paper focused on the oil pan that is penetrated by the combustion sound and the engine vibration easily. In addition, the accuracy using inverse-numerical acoustic analysis for oil pan was verified for the structure-borne noise or the air-borne noise respectively.
Takayuki Koizumi, Nobutaka Tsujiuchi, Akihiro Kobayashi, Hiroshi Uehara

Prototype WSN Platform for Performing Dynamic Monitoring of Civil Engineering Structures

Abstract
Structural Health Monitoring represents the present and future of the civil engineering since, until few years ago, structural diagnosis works had been performed with few resources regarding to experimental techniques. Precisely in the field of monitoring sensors, the progress of new technologies based on wireless communications and Micro-Electro-Mechanical- Systems (MEMS) are of high interest for replacing the handle difficult wired sensors. However, three major limitations of the commercial off-the-shelf technology on WSN (combination of MEMS and wireless technology) for performing dynamic monitoring were identified by means of: (1) not enough sensitivity of the accelerometers; (2) low resolution of the ADC embedded; and (3) lack of synchronization algorithms implemented. This paper presents a new prototype system conceived for performing dynamic monitoring civil engineering structures. This system was jointly conceived by a team of civil, electrical and communication engineers and is a combination of the last technology on high resolution MEMS accelerometers and the state of the art of communication technologies. Despite the fact that the prototype system needs more improvements; the results of several rounds of validation experiences confirm the feasibility for its consideration as an alternative to the conventional wired based sensors.
Rafael Aguilar, Luis F. Ramos, Paulo B. Lourenço, Ricardo Severino, Ricardo Gomes, Paulo Gandra, Mario Alves, Eduardo Tovar

Damage Identification in CRFP Laminates using a Statistical Method

Abstract
The purpose of this research is the detection and classification of impact damage in structures made from composite materials. This paper discusses classification of damage extent and type based on the use of X-ray analysis and a Scanning Electron Microscopy (SEM) technique. The composite material chosen for this research is a Carbon Fibre Reinforced Plastics (CFRP) laminate. The material was laid-up to produce plate specimens of 250 mm × 150 mm with 11, 12 or 13 layers. Piezoelectric sensors were placed on three different points of each test specimen in order to record the responses from impact events. An experimental modal analysis system was used to collect responses from both damaging and undamaging impacts. To perform the damaging impact tests, an instrumented drop test machine was used and the impact energy was set to range from 0.37 J to 41.72 J. The signals captured from each specimen were recorded for evaluation and the impacted specimens were X-rayed and SEM techniques were used to evaluate the extent and type of the damages. Statistical methods were used to classify the impact events into damaging and non-damaging and also to correlate the damage type and extent with the impact energy.
M. T. H. Sultan, K. Worden, W. J. Staszewski

High Intensity Noise Generation for Extremely Large Reverberant Room Test Applications

Abstract
A recent operational need for the development of a large (101,000 ft3) reverberant acoustic chamber at the Space Power Facility of NASA Glenn Research Center’s Plum Brook Station with the requirement of generating sound pressure levels (SPL) as high as 163 dB has resulted in the need to re-examine the generation of noise in reverberant rooms. Early in the design stage, it was realized that the acoustic power level capability (10-30 kW) of conventional electrodynamic air modulators, such as those supplied by the Wyle Corporation, would be required in unprecedented numbers to meet the test spectra requirements. The design team then turned to a lesser known modulator, the hydraulically driven air modulator supplied by the Team Corporation, which has 150-200 kW acoustic power capability. The advantage to the project was a significant reduction in the number of modulators required to meet the requirements. However, since only limited characterization of Team modulator’s performance has been reported, a test program was required in order to mitigate the risk of the design of the RATF. Aiolos Corporation, which is responsible for the acoustic design of the RATF, and the Institute of Aerospace Research (IAR) of the National Research Council of Canada (NRC), entered into a collaborative agreement with the objective of characterizing, optimizing and investigating the controllability of the Team modulators. The test program was performed at the NRC-IAR reverberant chamber, a 19,000 ft3 facility located in Ottawa, Ontario, Canada. The current paper provides details of the principle of operation of the Team modulators, including their servo control loops and provides of a summary of the characterization and controllability test program.
Anant Grewal, Ramani Ramakrishnan, William O. Hughes, Bill Woyski, Gary Elfstrom, Chon Mech, Yong Chen

Human Induced Dynamic Loads Estimation Based on Body Motion

Abstract
Crowd motion on civil structures that host musical and sport events is one of the main causes of excessive vibrations. In scientific literature many works, related to the measurement of forces induced by a single volunteer, or few of them, can be found; but the extension of these results to a crowd is a non trivial operation because of crowd synchronisation effect; moreover obvious practical problems arise in measuring the forces due to many people. This work presents an innovative algorithm to estimate such an excitation without needing a direct force measurement but relying only on the knowledge of the motion of each subject in the crowd. As shown in a previous research, the motion of people in a crowd can be estimated through digital image correlation, starting from a movie of the stadia stand. The method proposed in this work was tested both for jumping and bobbing, within all the typical range characterizing these kinds of movements (1.5-3.5 Hz) and the obtained results are then validated with the signal acquired by a dynamometric platform. The results show an excellent correlation between expected and measured loads in the whole range, both for jumping and bobbing.
Paolo Mazzoleni, Emanuele Zappa

In the Field Validation of Human Induced Loads Measuring Technique

Abstract
Human induced vibrations on civil structures is a key problem in comfort and security design. The estimation of loads due to human motion is a critical and still unsolved problem, but it is fundamental for a correct design of new buildings and proper monitoring of existing. This work presents a validation of an imaged based measuring technique of these loads. The test-bed is the Meazza Stadium in Milan. The validation of the proposed technique is done as follows: some volunteers jump on a stand and the induced vibration is acquired; a video recording of the jumping crowd is acquired simultaneously. The video is analyzed using a digital image correlation (DIC) code in order to obtain a motion estimation and, starting from the DIC results, induced loads are predicted. Using the frequency response function of the stand, previously estimated through a dynamic characterisation of the structure, it is now possible to forecast the induced vibration and compare it with the acquired one. The results show a very high correspondence between the measured and estimated accelerations and, above all, the performances do not degrade increasing the number of volunteers.
Paolo Mazzoleni, Emanuele Zappa

Optimal Selection of Artificial Boundary Conditions for Model Update and Damage Detection

Abstract
Sensitivity-based model error localization and damage detection is hindered by the relative differences in modal sensitivity magnitude among updating parameters. The method of artificial boundary conditions is shown to directly address this limitation, resulting in the increase of the number of updating parameters at which errors can be accurately localized. Using a single set of FRF data collected from a modal test, the artificial boundary conditions (ABC) method identifies experimentally the natural frequencies of a structure under test for a variety of different boundary conditions, without having to physically apply the boundary conditions, hence the term "artificial." The parameter-specific optimal ABC sets applied to the finite element model will produce increased sensitivities in the updating parameter, yielding accurate error localization and damage detection solutions. A method is developed for identifying the parameter-specific optimal ABC sets for updating or damage detection, and is based on the QR decomposition with column pivoting. Updating solution residuals, such as magnitude error and false error location, are shown to be minimized when the updating parameter set is limited to those corresponding to the QR pivot columns. The existence of an optimal ABC set for a given updating parameter is shown to be dependent on the number of modes used, and hence the method developed provides a systematic determination of the number of modes required for localization in a given updating parameter. These various concepts are demonstrated on a simple model with simulated test data.
Joshua H. Gordis

Use of Damping Solution for Acceleration Measurements in High Vibration Environments

Abstract
In many accelerometer applications, one can be faced with the challenge of making low amplitude, low frequency measurements in the presence of spurious high vibrations. Wideband, undamped accelerometers are commonly used for acceleration measurements in these applications but dynamic range must be sacrificed so as not to risk distortion and amplifier overload, or even damage to the device. A more suitable approach incorporates damping such that high frequency content above the passband of interest is directly mitigated. This paper contrasts and compares the effect of damping on the performance characteristics of piezoresistive MEMS and bonded strain gage accelerometers. It addresses the compromises that have to be made in terms of temperature response, phase delay, and noise for both fluid and gas damping. Also discussed are the inherent limitations of electronic filtering as an alternate approach for rolling off high frequency noise when using wideband accelerometers. A discussion of specific applications and test results are presented.
Tom Connolly

Efficiency comparison of CMS vibroacoustic formulations for uncertain damped problems

Abstract
When dealing with internal vibroacoustics, many formulations are available. The classical (u,p) approach uses physical variables (displacement of the structure, pressure in acoustic cavity), and alternatives exist based on a displacement or velocity potential variable instead of the acoustic pressure in the fluid. Each formulation has its own advantages and drawbacks which are addressed in this paper in the context of Component Mode Synthesis for model reduction, but globally all formulations exhibit the same behavior. Thus, the decoupled mode bases which are classically considered for CMS are identical in all formulations providing that the static pressure is not included in the formulation. So, the question addressed in this work deals with the strategies to take the static response of the fluid domain into account in the projection and on the ability of each formulation to adapt to these strategies. Many options are presented for each of the formulations and are applied for two study cases: a shoe box, that is a parallepipedic cavity and a curved box with a more complex geometry.
Morvan Ouisse, Emeline Sadoulet-Reboul

Nonlinear Parameteric Health Monitoring for Vibrating Structures Under Non-Stationary Excitation

Abstract
Nonlinear system identification has been used to predict and monitor cracks as they form, propagate, and eventually cause the catastrophic failure of a vibrating cantilevered beam. The Continuous Time based system identification technique allows for estimation of model parameters based on collected stimulus and response data. For this study the estimated cubic stiffness term in the model is mapped as a function of time. The purpose of this investigation is to strengthen results from a previous study [1] through repetition, and to expand the scope of this system identification technique. This study mainly explores the effectiveness of using nonstationary excitation in the identification process, with an understanding that in implementation on real systems the selection input amplitude and frequency may not be readily controlled. Additionally, the robustness of this method is demonstrated in direct comparison to a wellaccepted linear-based approach. The results show this method to be successful with little prior knowledge of the accurate model form or parametric values for the systems being studied.
Timothy A. Doughty, Natalie S. Higgins

Displacement Based Approach for a Robust Operational Modal Analysis

Abstract
Robust estimation of the dynamic modal parameters of structures during shaking table experiments is done by means of efficient time domain data-driven Crystal Clear Stochastic Subspace Identification (CC-SSI) of vibration data recorded by a new, innovative, high resolution 3-D optical movement detection and analysis tool tracking the dynamic displacement of several selected points of the structures during the dynamic tests of natural (earthquake) and artificial (mechanical) induced vibrations. The measure of the displacements is a crucial task for the numerical and experimental studies in structural dynamics, especially within the displacement based approach in seismic design and calculations. The innovative monitoring technique measures 3 axial absolute displacements with easy and fast test setup, high accuracy and the possibility to link the 3D-motion time histories of the tracked markers with CAD drawings of the structure and validate the FE models in real time experimental data assimilation.
Gerardo De Canio, Palle Andersen, Ivan Roselli, Marialuisa Mongelli, Enrico Esposito

Effect of systematic FRF errors on matrix inversion based vibro-acoustic analysis methods

Abstract
The establishment of a non-parametric system description in terms of a Frequency Response Function matrix is an essential step in many structural and vibro-acoustic analysis approaches such as Modal Analysis, Load Identification, Transfer Path Analysis and Substructuring. A crucial step in many of these applications is the inversion of a matrix of this FRF matrix. Several studies have been reported on the influence of stochastic FRF errors on this matrix inversion, leading to the use of a pseudo-inverse approach, SVD truncation, regularization etc. but little is known about the effect of systematic errors, such as amplitude or frequency shifts and inter-FRF inconsistencies on the matrix inversion result. The present paper investigates the effect of selected systematic FRF measurement errors on applications involving matrix inversion, with emphasis on the Transfer Path Analysis method. The subject is reviewed through analytical considerations and through simulations on a reference numerical model (the Garteur model). In particular, the sensitivity of two Transfer Path Analysis methods with respect to the studied systematic errors is analyzed and considerations on increasing the robustness of these methods are derived.
Peter Gajdatsy, Paul Sas, Wim Desmet, Karl Janssens, Herman Van der Auweraer

Peri-Prosthetic Fracture Vibration Testing

Abstract
The purpose of this study was to establish a test setup and vibration analysis method to predict femoral stem seating and prevent bone fracture using accelerometer and force response data from an instrumented stem and impactor. This study builds upon earlier studies to identify a means to supplement a surgeon’s tactile and auditory senses by using damage identification techniques normally used for civil and mechanical structures. Testing was conducted using foam cortical shell sawbones prepared for stems of different geometries. Each stem was instrumented with an accelerometer. Two impactor designs were compared: a monolithic impactor and a twopiece impactor, each with an integrated load cell and accelerometer. Acceleration and force measurements were taken in the direction of impaction. Comparisons between different methods of applying an impacting force were made, including a drop tower and a surgical hammer. The effect of varying compliance on the data was also investigated. The ultimate goal of this study was to assist in the design of an integrated portable data acquisition system capable of being used in future cadaveric testing. This paper will discuss the experimental setup and the subsequent results of the comparisons made between impactors, prosthetic geometries, compliances, and impact methods. The results of this study can be used for both future replicate testing as well as in a cadaveric environment.
Jesse Cruce, Jenny Erwin, Kevin Remick, Phillip Cornwell, R. Michael Meneghini, Joe Racanelli

Performance Comparison of Fiber Optic Tips in Interferometric Displacement Measurements

Abstract
Fiber optic displacement sensors have many potential advantages over traditional displacement measurement techniques, including small size, immunity to electromagnetic interference, electrical isolation, and high resolution. In this report, we focus on an interferometric fiber optic sensor, where the gap between the fiber tip and the device under test forms a Fabry-Perot resonant cavity. An optical interrogator measures the reflected intensity at wavelengths ranging from 1510 to 1590 nm. The spacing between resonant frequencies allows us to determine the distance from the tip to the device under test. We consider ferrule connector angled physical contact (FC/APC), ferrule connector ultra physical contact (FC/UPC) and unpolished cleaved tips and compare their influence on sensor performance. A plane wave propagation model is proposed for predicting tip effects. Comparisons are made on the basis of sensor measurement range, resolution, and sensitivity to changes in test conditions. In this paper, we discuss the experimental setup, detail our analysis, and present test results with recommendations for the applications of each tip.
Rick Grahn, Hussain Karimi, Kyle Wilson, Erik Moro, Anthony Puckett

Development of Polymer ‘Chips’ used in Medical Diagnostics

Abstract
In recent years, there has been growing interest in creating bio-inspired devices that feature artificial bilayer lipid membranes (BLM), or lipid bilayers. These membranes can be tailored to mimic the structure and transport properties of cellular walls and can be used to selectively transport ions and other species between aqueous volumes. One application of this research is the formation of a standardized BLM contained within a portable and disposable housing for use in medical diagnostics. This concept utilizes a flexible polymer ‘chip’ that has internal compartments for housing both an organic solvent and an aqueous solution, which contains phospholipid molecules, proteins, and specific analyte molecules. The formation of a BLM within the chip enables integration of the chip into an electronic reader to perform diagnostic measurements of the sample. A key element of the bilayer formation process requires a single aqueous volume to first be separated into multiple volumes such that it can then be reattached to form a bilayer at the interface. This process, called the regulated attachment method, relies on the geometry of the deformable ‘chip’ to separate and reattach the aqueous contents held inside by opening and closing an aperture that divides adjacent compartments through the application of mechanical force. The purpose of this research is to develop an optimized chip that provides a controllable method for initially separating the aqueous phase via dynamic excitation. This study focuses on two specific aspects: designing an efficient excitation method for separating the aqueous volume, and optimizing the geometry of the chip to decrease the required input energy and better target the location and duration of the separation. Finite Element (FE) models are used to optimize the chip geometry and to identify suitable excitation signals. A series of experimental studies are also presented to validate the FE models.
Zachary G. Brush, Laura M. Schultz, Justin W. Vanness, Kevin M. Farinholt, Stephen A. Sarles, Donald J. Leo

Acoustic Fluid-Structure Interaction of Cars and Ships (Tutorial)

Abstract
Acoustic fluid-structure interaction is a common issue in automotive applications. An example is the pressure-induced structure-borne sound of piping and exhaust systems. Efficient model order reduction and substructuring techniques accelerate the finite element analysis and enable the vibro-acoustic optimization of such complex systems with acoustic fluid-structure interaction. This tutorial reviews the application of the Craig-Bampton and Rubin method to fluid-structure coupled systems and presents two automotive applications. First, a fluid-filled brakepipe system is assembled by substructures according to the Craig-Bampton method. Fluid and structural partitions are fully coupled in order to capture the interaction between the pipe shell and the heavy fluid inside the pipe. Second, a rear muffler with an air-borne excitation is analyzed. Here, the Rubin and the Craig-Bampton method are used to separately compute the uncoupled component modes of both the acoustic and structural domain. These modes are then used to compute a reduced model which incorporates full acoustic-structure coupling. For both applications, transfer functions are computed and compared to the results of dynamic measurements. The vibro-acoustic behavior of ship-like structures is noticeably influenced by the surrounding water and thus represents a multi-field problem. In this tutorial, fast boundary element methods are applied for the semi-infinite fluid domain. As an advantage, the Sommerfeld radiation condition is satisfied in an exact way and only the boundary, i.e. the ship hull, has to be discretized. To overcome the draw-back of fully populated matrices, fast boundary element methods are applied. The focus is on the comparison of the multipole method with hierarchical matrices, which are set up by adaptive cross approximation. In both cases, a half-space fundamental solution is used to incorporate the water surface, which is treated as pressure-free. The structural domain is discretized with the finite element method. A binary interface to the commercial finite element package ANSYS is used to import the mass and stiffness matrices. The coupled problems are formulated as Schur complements, which are solved by a combination of iterative and direct solvers. Depending on the applied fast boundary element method, different strategies arise for the preconditioning and the overall solution. The applicability of these approaches is demonstrated using a realistic model problem.
Lothar Gaul

The Testing Behind the Test Facility: The Acoustic Design of the Nasa Glenn Research Center’s World-Class Reverberant Acoustic Test Facility

Abstract
The National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC) is leading the design and build of the new world-class vibroacoustic test capabilities at the NASA GRC’s Plum Brook Station in Sandusky, Ohio, USA. Benham Companies, LLC is currently constructing modal, base-shake sine and reverberant acoustic test facilities to support the future testing needs of NASA’s space exploration program.
William O. Hughes, Mark E. McNelis, Aron D. Hozman, Anne M. McNelis

Detection of Damage in Space Frame Structures with L-shaped Beams and Bolted Joints Using Changes in Natural Frequencies

Abstract
It is difficult to use conventional non-destructive testing methods to detect damage, such as loosening of bolted connections, in a space frame structure due to the complexity of the structure and the nature of the possible damage. A vibration-based method that uses changes in the natural frequencies of a structure to detect the locations and extent of damage in it has the advantage of being able to detect various types of damage in the structure. Since the vibration-based method is model-based, applying it to a space frame structure with L-shaped beams and bolted joints will face challenges ranging from the development of accurate dynamic models for the structures to that of a robust damage detection algorithm for severely under-determined, nonlinear least-square problems under the effects of relatively large modeling error and measurement noise.With the development of the modeling techniques for fillets in thin-walled beams (He and Zhu, 2009, “Modeling of Fillets in Thin-Walled Beams Using Shell/Plate and Beam Finite Elements,” ASME J. Vibr. Acoust., 131(5), p. 051002) and bolted joints (He and Zhu, “Finite Element Modeling of Structures with L-shaped Beams and Bolted Joints,” ASME J. Vibr. Acoust., in press) by the authors, accurate physics-based models of space frame structures can be developed with a reasonable model size. A new damage detection algorithm that uses a trust-region search strategy combined with a logistic function transformation is developed to improve the robustness of the vibration-based damage detection method. The new algorithm can ensure global convergence of the iterations and minimize the effects of modeling error and measurement noise. The damage detection method developed is experimentally validated on an aluminum three-bay space frame structure with L-shaped beams and bolted joints. Three types of introduced damage, including joint damage, member damage, and boundary damage, were successfully detected. In the numerical simulation where there is no modeling error and measurement noise, the almost exact locations and extent of damage can be detected.
K. He, W. D. Zhu

Steels for Caribbean Steel Drums

Abstract
The Caribbean steel drum also known as the pan or steelpan is a percussion instrument which has its beginnings in the twin island Republic of Trinidad and Tobago. The instrument surfaced around WWII and for more than seven decades a majority of pan makers have continued to fabricate these instruments from used 55 gallon oil barrels. Barrel material which is predominantly low-carbon steel has long been the de-facto material for the production of these instruments in part due to its low cost and high availability in addition to having tuning methods centred on its usage. The manufacture of oil barrels is a regulated industry with standards that stipulate the grade of low-carbon steel that is to be used for the construction of oil containers. These grades of steels may not be entirely suitable for pan making and at times grades of steels which do not conform to the specifications for drum making may be deliberately or ignorantly used. Currently, there is no material standard for Caribbean steel drums. In this work, three low carbon steels were examined and compared on the basis of vibration damping properties. The results indicate that a pan standard must incorporate specifications on chemical composition and vibration properties for the recommended steels.
Soren E. Maloney
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