Shock & Vibration, Aircraft/Aerospace, Energy Harvesting, Acoustics & Optics, Volume 9

The Impedance-Matched Multi-Axis Test (IMMAT) technique has been shown to offer significant benefits when compared with the conventional single-shaker vibration test. Thus far, IMMAT has only been demonstrated on relatively small and lightweight structures and to moderate vibration severity. This paper describes the activities undertaken to scale-up the IMMAT technique to larger structures and to severe vibration levels.

Six degree of freedom (6-DOF) subsystem/component testing is becoming a desirable method, for field test data and the stress environment can be better replicated with this technology. Unfortunately, it is a rare occasion where a field test can be sufficiently instrumented such that the subsystem/component 6-DOF inputs can be directly derived. However, a recent field test of a Sandia National Laboratory system was instrumented sufficiently such that the input could be directly derived for a particular subsystem. This input is compared to methods for deriving 6-DOF test inputs from field data with limited instrumentation. There are four methods in this study used for deriving 6-DOF input with limited instrumentation. In addition to input comparisons, response measurements during the flight are compared to the predicted response of each input derivation method. All these methods with limited instrumentation suffer from the need to inverse the transmissibility function.

This research compares spatial damping identification methods, both theoretically and experimentally. In contrast to the commonly used damping methods (modal, proportional) the spatial damping information improves structural models with a known location of the damping sources. The real case robustness of full FRF matrix and local equation of motion methods were tested against: modal and spatial incompleteness, differences in viscous and hysteretic damping models and the effect of damping treatments. To obtain accurate results, a careful analysis of measurements in terms of reciprocity in the raw measurements, and in terms of how to preserve symmetry has to be done. It was found that full FRF matrix needs to be symmetrisized due to small deviations in reciprocity before the damping identification. Full frequency response function (FRF) matrix methods (e.g.: Lee-Kim) can identify the spatial damping if spatial and modal incompleteness are carefully evaluated, but the measurement effort increases with second order and, consequently, the size of the FRF matrix.

The objective of this testing activity was to examine and verify the interactions of a hydraulic mechanical load control system loading a thin skinned aerospace structure coupled with applied acoustic vibrations. The goal was to be able to actively control the load of four hydraulic cylinders loading two end plates while an electrodynamic shaker attached to the skin imposed simulated acoustic loading. The objective was to achieve mechanical load control within ±1% of desired static load for each load control channel. The mechanical loads applied ranged from the noise floor to Design Limit Load (DLL) (2000 lbf) using varying ramp rates in conjunction with the random dynamic vibration. The presence of dynamic load introduced an error of 0.4% DLL with traditional PID tuning methods. The testing also quantified the suitability of 2 channel master/slave control vs. 4 channel individual control. It was determined that using a dedicated servo control channel (non-slaved servo channel) per load cylinder drops the maximum difference to 2.2% of full scale applied load (25% DLL), as opposed to 7.8% of full scale applied load (25% DLL) when the top two load cylinders are slaved to one servo and the bottom two load cylinders are slaved to another. This amounted to a 72% reduction in error using non-slaved servos. The test activity’s technical approach and test data will be presented.

Real-world structures, such as civil and aerospace structures, are subjected to various dynamic loads which are spatially local and distributed. Assessment of operational performance, prediction of the dynamic responses, and prognosis of the remaining service life of the structure therefore requires accurate, high-resolution measurements, and modeling of the dynamic loads. This is extremely difficult, if not impossible, with the current state of the art. First, dynamic loads on structures usually come from a wide spectrum of sources, some of which are extremely challenging to accurately measure, such as the traffic loads on a bridge. Also, it is impractical to instrument a dense array of force measurement devices on the structure due to the high cost, the effect of mass-loading, and modification of the structure’s surface. On the other hand, digital video cameras are non-contact measurement device that are relatively low-cost, agile, and able to provide high spatial resolution, simultaneous, pixel measurements. This study develops a novel method for identification of the high-resolution, full-field loads on the structure from the video of the operational structures by leveraging advanced computer vision and unsupervised learning techniques. Impact and wind loads were applied on a cable structure to experimentally validate the method. The non-contact, remote, simultaneous sensing capability of the proposed technique should enable truly high-resolution, full-field force estimation that was previously not feasible.

In the field of vibration test including modal analysis, the completion of a traditional test requires many devices and takes several days due to its complex process. The development of digital image technology makes the application of the image sensor to complete vibration test and modal test possible, which is known for collecting a large amount of data within several hours. This paper developed a new testing method that targeting at operating deflecting shape vibration test based on stereo vision. Normally, it is required that the image sensor can be triggered at a very high frequency to oversample a high frequency vibration. With the application of undersampling technology, the method proposed in this paper breaks through this limitation and the camera can be triggered at less than 3 Hz to analyze structure that vibrates at more than 30 Hz. Furthermore, a timestamp-based optimization method is proposed and developed to address Jitter problem in the process of undersampling. More than two hundred test results validate the accuracy and reliability of this new method, which can satisfy the needs of operating deflecting shape modal analysis.

Many thin walled structures experience a combination of static thermal stress and large amplitude dynamic loads. From a dynamic perspective, the heating can change both the linear and nonlinear properties of the system so that completely different responses may be obtained (e.g. they may snap through rather than exhibiting a simple hardening nonlinearity). This work uses the “cold modes” approach to create various reduced order models of one panel from a concept hypersonic vehicle; heating is assumed to change only the linear natural frequencies of the panel, but not the mode shapes or geometrically nonlinear effects. The resulting reduced order models are studied by computing their nonlinear normal modes and their response subject to a random dynamic pressure. The comparisons reveal that much can be inferred about the response from the nonlinear modes.

One of the more common forms of passive vibration isolation in mechanical systems has been the use of elastomeric or foam pads. Cellular silicone foam is one such example which has been used for vibration isolation and mitigating the effects of mechanical shock. There are many desirable properties of cellular silicone, including its resilience and relative insensitivity to environmental extremes. However, there is very little test data that is useful for understanding its dynamic characteristics or for the development of a predictive finite element model. The problem becomes increasingly difficult since foam materials typically exhibit nonlinear damping and stiffness characteristics. In this paper we present a test fixture design and method for extraction of a few dynamic properties of one type of cellular silicone foam pad. The nonlinear damping characteristics derived from the experimental testing are then used to attempt to improve the predictive capability of a linear finite element model of the system. Difficulties and lessons learned are also presented.

Vibration and shock qualification testing of components can be an expensive and time-consuming process. If the component is small, often two or more units can be mounted on a fixture and tested simultaneously to reduce test time. There is an inherent danger in simultaneously testing two or more identical components as the fundamental natural frequencies and mode shapes of the individual components will be nearly identical with some slight variation due to manufacturing variability. Testing in this manner can create a situation where closely spaced vibration modes produce unwanted interference between the two units under test. This phenomenon could result in a case where one unit is over-tested while the other is under-tested. This paper presents some experimental results from simultaneously testing pairs of components which show distinct interference between the units. Some analysis will also be presented showing how variations in the components can alter the intended test response, potentially impacting component qualification.

Video cameras offer a versatile high resolution alternative to traditional sensor apparatuses for full field structural dynamics analysis. Previous work has extended phase based motion magnification techniques to achieve state of the art results in vibrational and structural analysis. One limitation on the current approach is that it does not work very well in situations where substantial rigid body motion is present in the scene. The large rigid motion drowns out the tiny vibrations that are relevant for a full-field structural analysis. This work extends upon previous phase based motion algorithms in order to allow them to work even in cases with large rigid body motion present. Through a keypoint tracking and video cropping scheme rigid body motion is subtracted from video streams, while preserving the smaller motions of interest. After this preprocessing step, the new video stream can be passed into the full-field analysis scheme as usual, in order to get a full-field modal decomposition.

As an alternative to traditional sensing methods, video camera measurements offer a non-contact, cost-efficient, and full-field platform for operational modal analysis. However, video cameras record large amounts of redundant background data causing video processing to be computationally inefficient. This work explores the use of a silicon retina imager to perform operational modal analysis. The silicon retina provides an efficient alternative to standard frame-based video cameras. Modeling the biological retina, each silicon retina pixel independently and asynchronously records changes in intensity. By only recording intensity change events, all motion information is captured without recording redundant background information. This asynchronous event-based data representation allows motion to be captured on the microsecond scale, equivalent to traditional cameras operating at thousands of frames per second. With minimal data storage and processing requirements, the silicon retina shows promise for real-time vibration measurement and structural control applications. This study takes the first step toward these applications by adapting existing video frame-based modal analysis techniques to operate on event-based silicon retina measurements. Specifically, blind source separation and video motion processing techniques are used to automatically output vibration parameters from silicon retina data. The developed method is demonstrated on a cantilever beam.

Aircraft braking systems may be subjected to friction-induced vibrations, during which the brake is unstable and behaves as a source of mechanical vibrations. This is an issue for aircraft brake manufacturers as it may jeopardize structural integrity due to accelerated fatigue life, or generate discomfort for the aircraft crew and passengers. The important cost associated to the occurrence of this phenomenon motivates the development of instability simulation and prediction methods that can be used as early as the design stage.Here, the self-excited vibrations are induced by a coupling of two structure modes by friction. It appears that modifications of the brake hydraulic command system configuration have important consequences on the vibration levels monitored during the tests. It is therefore necessary, when developing a vibration level simulation methods, to consider the hydro-mechanical coupling of the unstable brake structure with the brake hydraulic fluid.This paper presents a simulation methodology associating a structural reduced model with a unidimensional model of the hydraulic control system, including both passive and active components. Both submodels are validated against experimental data.The influence of several hydraulic system configurations on the coupled system vibration levels and on the braking performances is evaluated. It is shown that a compromise between vibration reduction and braking performance can be found through simulation. It is the first time that the interaction of self-excited brake structure with a heavy fluid is studied. The present method offers important industrial opportunities and gives an insight into the brake mode-coupling dynamics.

Materials subject to cyclic loading have been studied extensively and experimentally determined comparisons of stress to number of cycles are used to estimate fatigue life under various loading scenarios. Fatigue data are traditionally presented in the form of S-N curves. Normally, S-N data are derived from cyclic loading but the S-N results are also applicable to random vibration loading and, to some extent, shock. This paper presents an alternate presentation of fatigue data in terms of input energy and number of cycles to failure. In conjunction with this study, a series of shock tests was conducted on 3D printed cantilever beams using a 6-DOF shaker table. All of the beams were tested to failure at shock levels in the low-cycle fatigue regime. From these data, a nominal fatigue curve in terms of input energy and number of shocks to failure was generated and compared with the theoretical developments.

Recent advances in 6DOF testing has made 6DOF subsystem/component testing a preferred method because field environments are inherently multidimensional and can be better replicated with this technology. Unfortunately, it is rare that there is sufficient instrumentation in a field test to derive 6DOF inputs. One of the most challenging aspects of the test inputs to derive is the cross spectra. Unfortunately, if cross spectra are poorly defined, it makes executing the tests on a shaker difficult. In this study, tests were carried out using the inputs derived by four different inverse methods, as described in a companion paper. The tests were run with all 6DOF as well with just the three translational degrees of freedom. To evaluate the best way to handle the cross spectra, three different sets of tests were run: with no cross terms defined, with only the coherence defined, and with the coherence and phase defined. All of the different tests were compared using a variety of metrics to assess the efficacy of the specification methods. The drive requirements for the different methods are also compared to evaluate how the specifications affect the shaker performance. A number of the inverse methods show great promise for being able to derive inputs to a 6DOF shaker to replicate the flight environments.

Functionally Graded Material (FGM) is an advanced composite that finds increasing application in high-tech industries such as mechatronics, space technology, bio-materials etc. In the application of FGM, dynamics of structures such as beams, plates or shells made of the material is of a great importance. This report is devoted to develop general theory of vibration of cracked FGM beams based on the power law of material grading and Timoshenko beam theory. Crack is modeled by an equivalent spring of stiffness calculated from its depth. First, governing equations of motion of the beam are constructed in the frequency domain taking into account the actual position of beam neutral plane. This enables to obtain general solution of free vibration of the beam and condition for uncoupling of axial and flexural vibration modes. Using the solution natural frequencies and mode shapes of cracked FGM beam are examined in dependence upon material properties and crack parameters.

A novel procedure to eliminate blur in imagery captured by small unmanned aircraft is demonstrated. The whirling props and airflows often cause unwanted image blur due to the unstable platform from which they are captured. In multirotor aircraft these image artifacts often produce remote sensing data that is inferior to the imaging system capability. By eliminating the vibration of the host vehicle, the full potential of the instruments can be obtained, as demonstrated in imaging studies presented. To accomplish this an attachment system has been developed so that the props can be shut off in a perched configuration and the unwanted vibration will cease. This concept not only reduces the vibration levels, but also greatly prolongs the effective use of imaging or robotic systems. Typically, most of the power usage is for propulsion and not for payloads, therefore without the need for lift and maneuvering, the payload systems can operate for periods many times the flight operation duration. The dynamic performance of two aircraft is presented; first is a highly modified remote control helicopter clearly illustrating the concept and the second a custom built system in a small footprint enclosure with increased payload capacities for remote sensing and robotic equipment.

Hypersonic aircraft structures must operate in complex loading conditions and very high temperatures, making the design of a robust and reusable platform very challenging. An analytical and experimental test program was developed by the Air Force Research Laboratory (AFRL) and industry. The objective of the program is to review the design process of a thin skinned aircraft panel subjected to combined thermal-acoustic-mechanical loading, through a series of laboratory experiments at the AFRL’s Structural Dynamics Laboratory.This paper presents the results from a series of modal tests, performed to estimate the modal parameters of a hat-stiffened fuselage panel designed by industry. A comparison of the modal parameters of the panel, estimated from roving impact and a multiple input multiple output (MIMO) shaker test data of the panel is presented. Also, the variation in the natural frequencies and damping through various stages prior to the combined environment test is discussed.

In this paper, nonlinear vibration analysis of micro scale functionally graded material (FGM) beams with geometric nonlinearity due to large deflection is studied using modified couple stress theory (MCST). MCST is a nonlocal elasticity theory which includes a material length scale parameter since the size of an atomic microstructure becomes comparable to the length of the microbeam. Equations of motion of the micro scale FGM beam are obtained by using Hamilton’s principle. Nonlinear free vibrations of the FGM microbeam with simply supported boundary conditions is investigated where the effect of the length scale parameter on the nonlinear natural frequencies of the microbeam is studied. The nonlinear partial differential equations of motion are converted into nonlinear ordinary differential equations by using Galerkin’s Method. By using describing function method (DFM), a set of nonlinear algebraic equations are obtained which are solved by an iterative eigenvalue solver.

In an effort to understand the details of why a Shock Response Spectrum (SRS) has a particular shape, a method was designed to predict the shape of the SRS based on Frequency Response Functions (FRFs). Gaining a full understand of the relationship between the FRF of a structure and the SRS shape should prove to be very useful in reducing SRS test time and allow the general shape of an SRS response to be predicted more efficiently using finite element methods or experimentally obtained data. To allow comparisons of different shock response plates and fixtures through the use of FRFs a normalized Shock Response Spectrum (nSRS) was developed. The nSRS is derived directly from the FRF of a structure and when coupled with a library of characterized impactor input spectrums allows an SRS to be predicted without performing any testing. This approach allows modifications to the shock response plate/fixture to be evaluated efficiently and the effect of different impactors to be studied without performing a large number of experimental tests. It is hoped that this approach to understanding and predicting SRSs improves the understanding of how the structural dynamics effects an SRS and efficiency of testing.

There are many methods for vibration and modal testing. With the development of digital technology and 3-D imaging technology, the development and application of digital image correlation (DIC) technology becomes possible. Now we can use stereo digital image correlation and dynamic photography to obtain data on the structure mode. It should be noted that this is a noncontact measurement technique, meaning there will be a wide range of applications. In addition, the visualization of vibration modes provides the intuitive pictures. This paper focuses on how to build a test environment. The project uses BUMBLEBEE binocular stereo cameras to shoot the images of test object. The images then are transferred, stored, and processed to get the relative coordinates of each pixel. After that, a MATLAB program is developed to revert the coordinates of every pixel, reduce noise, and generate stereo images. The stereo images can be animated based on the timestamps they were acquired. As for data processing, this paper studies the effects of various parameters, such as shutter speed, exposure, gain, vibration frequency and amplitude. The relationship between these parameters and the accuracy of pixel coordinates is studied.

This paper considers a so-called “Mission Synthesis” procedure where a qualification test specification is derived for an electronic component (i.e., a VHF radio) mounted on a console inside a helicopter cockpit. The environmental vibrations, which are measured during various flight maneuvers, consist of deterministic sine tones superimposed on Gaussian random excitations, i.e. so-called “Sine-on-Random” (SoR) vibration. The Fatigue Damage Spectrum (FDS) model is utilized for analyzing the fatigue damage potential of these SoR excitations. A particular objective in this application example is to assess the induced fatigue damage for two design alternatives of the radio console, which is achieved by means of an FDS comparison. Furthermore, it is demonstrated that experimental modal analysis can provide valuable insights on the physical root causes for large induced fatigue damage, such that appropriate design modifications can be prescribed. Finally, the Mission Synthesis procedure enables the derivation of an accelerated but damage-equivalent shaker qualification test. For the component considered in this example it is demonstrated that the derived test specification is more severe than a MIL-STD standardized SoR test specification.

Modal analysis is used to extract dynamic characteristics of structures for correlation and validation purposes. These characteristics are usually extracted using conventional measurement tools. A known force is imparted to the structure using impact hammers or mechanical shakers and the response is measured using accelerometers. However, accelerometers may induce mass loading effects; thus, the results may not show the true dynamics of the structure. On the other hand, conventional measurement tools can only measure at few discrete locations. Digital Image Correlation and photogrammetry has provided test engineers with a new tool for measuring dynamics of structures. These techniques are non-contacting so they do not induce mass loading and they can provide full-field results. For a DIC measurement to extract mode shapes, operating data of a structure is measured while the cameras are in fixed positions. A stereo camera system has line of sight only on a section of the structure. If the entire dynamics of a structure is desirable, several stereo cameras needs to be used. Another approach is to move a single stereo camera measure dynamics of the structure from multiple field of views; however, the time domain results can only be stitched together if the entire structure is fixed or if the applied force is constant during the entire time of measurement. This assumption is not valid for many dynamic measurements. On the other hand if the DIC system is moved from a view to another view, because the response is measured at different instants of time, operating shapes from each camera view may have a different scaling factor. In the current work, we propose an approach to identify a uniform scaling factor that enables us to stitch these operating shapes extracted from different views of cameras. The new approach is proposed based on conventional modal analysis theory. In the proposed technique, a known force is applied to a structure using a mechanical shaker or an impact hammer. The measured response of the structure in the time domain is transferred to the frequency domain to extract the mode shapes of a section of the structure. A similar procedure is repeated for other sections of the structure to capture the entire area of interest. Mode shapes for all views of the structure are stitched together to extract the mode shape for the entire structure. The proposed technique suggests stitching the views in the frequency domain after being scaled rather than stitching them in the time domain. The proposed method was applied to measure mode shapes of a tire using measured time series from different sections of the tire.

Reduction/expansion approaches have been conventionally used in correlation and validation studies. Recently, these approaches have also been used to extract full-filed results on structures using limited set of data measured. The expansion techniques are used to expand real-time data measured on components of vehicle chassis, utility scale wind turbines, and helicopter rotors. The resulted full-field data is used to monitor structures and determine their durability. With the advances in Digital Image Correlation (DIC), researchers are able to readily extract strain mode shapes of structures. However, the conventional reduction/expansion techniques are usually limited to displacement, velocity, or acceleration data. This can hinder correlation studies that compare the strain data between two models. Furthermore, this limitation does not allow researchers to expand strain-gage measured data for full-field structural monitoring and durability analysis.In the current paper, a reduction/expansion technique has been developed to reduce/expand strain data. In this technique, measured strain at limited locations is expanded using strain mode shapes to extract full-field results on the entire surface and within the structure. This technique can also be used to reduce data for correlation studies. In order to demonstrate the merit of the approach, the proposed strain expansion approach was applied to the finite element model of a cantilever beam subjected to sinusoidal and impact excitations. The results show that the proposed approach can effectively expand the strain measured at limited locations. The approach could accurately predict the strain at locations where no sensors were placed.

In this paper, the vibration suppression capabilities of magnetorheological (MR) layer in smart beams is investigated. A three-layered beam including MR elastomer layer sandwiched between two elastic layers is considered. By assuming the properties of MR layer in the pre-yield region as viscoelastic materials behavior, the governing equations of motion as well as the corresponding boundary conditions are derived using Hamilton’s principle. Due to field-dependent shear modulus of MR layer, the stiffness and damping properties of the smart beam can be changed by the application of magnetic field. This feature is utilized to suppress the unwanted vibration of the system. The appropriate magnetic field applied over the beam is chosen through a fuzzy controller for improving the transient response. The designed fuzzy controller uses the modal displacement and modal velocity of the beam as its inputs. Free and forced vibration of smart sandwich beam is investigated using numerical simulations. The results show that the magnetorheological layer along with the designed fuzzy controller can be effectively used to suppress the unwanted vibration of the system. The qualitative and quantitative knowledge resulting from this research is expected to enable the analysis, design and synthesis of smart beams for improving the dynamic performance of smart engineering structures.

Many of energy harvesting devices use piezoelectric elements to convert mechanical vibrations into usable electrical energy. The input excitation is usually assumed to be a deterministic harmonic wave, while in practical situations; the mechanical excitation of the media is a random signal. The objective of this research is to study the energy harvesting in piezoelectric devices using the random vibration theory. At the first step a lumped parameter physical model of the device is presented. A mathematical model is then developed by obtaining the normalized differential equations governing the voltage induced in the energy harvesting circuit as well as the length of the piezoelectric material. The random vibration theory is then utilized to derive analytical expressions for the statistical properties of the voltage, power and the length of the piezoelectric material in terms of the statistical properties of the excitation which is assumed to be a band limited white noise. It is shown that with proper selection of the system parameters, the expected value of the harvested power can be effectively maximized. The qualitative and quantitative knowledge resulting from this effort is expected to enable the analysis, optimization, and synthesis of piezoelectric energy harvesting devices.

Improved testing and modeling capability has provided more accurate environments definition for environmental testing. When those efforts expand the shock testing performance envelope at a production test lab, legacy testing processes may need to be updated to ensure efficient testing and accurate environment reproduction. If new specifications contain reduced shock pulse durations or resonant fixture-type tests, legacy fixtures and legacy fixture designs may no longer be adequate. Failure to properly evaluate legacy fixture designs may lead to increased setup times and dramatically increased testing costs if those designs are carried over into shorter-duration shock tests. This work will examine approaches for fixture evaluation, attempt to evaluate different fixture designs, and investigate the effect of shock machine choice on fixture performance.

One step beyond the studies of transverse vibration of beams with a breathing edge crack is the verification of the theoretical crack beam models with an experimental test set up. Beams with a single breathing edge crack can be used as a specimen for the experimental test. However, there is no assurance against the unexpected additional cracks in the specimens, which may cause unexpected results. Therefore, in this paper nonlinear transverse vibration of a beam with multiple breathing edge cracks is considered as a preliminary study. The breathing effect of the cracks are modeled as piecewise linear stiffness and a harmonic force is applied to the cracked beam. Galerkin’s Method is applied with multiple trial functions and nonlinear differential equations obtained are solved by using harmonic balance method with multi harmonics. Utilizing the developed model, effects of crack parameters and crack locations are studied.

The Transiting Exoplanet Survey Satellite (TESS) program, led by the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology (MIT) will be the first-ever spaceborne all-sky transit survey. MIT Lincoln Laboratory is responsible for the cameras, including the lens assemblies, detector assemblies, lens hoods, and camera mounts. TESS is scheduled to be launched in August of 2017 with the primary goal to detect small planets with bright host starts in the solar neighborhood, so that detailed characterizations of the planets and their atmospheres can be performed.The TESS payload consists of four identical cameras and a data handling unit. Each camera consists of a lens assembly with seven optical elements and a detector assembly with four charge-coupled devices (CCDs) including their associated electronics. The optical prescription requires that several of the lenses are in close proximity to a neighboring element. A finite element model (FEM) was developed to estimate the relative deflections between each lens-bezel assembly under launch loads to predict that there are adequate clearances preventing the lenses from making contact. Modal tests using non-contact response measurements were conducted to experimentally estimate the modal parameters of the lens-bezel assembly, and used to validate the initial FEM assumptions.

Metastructures are known to provide considerable vibration attenuation for mechanical systems. With the optimization of the internal geometry of metastructures, the suppression performance of the host structure increases. While the zigzag inserts have been shown to be efficient for vibration attenuation, the geometric properties of the inserts affect the suppression performance in a complex manner when attached to the host structure. This paper presents a genetic algorithm based optimization study conducted to come up with the most efficient geometric properties of the zigzag inserts. The inserts studied in this paper are simply cantilever zigzag structures with a mass attached to the unsupported tips. Numerical simulations are run to show the efficiency of the optimization process.

This paper discusses the application of 3D vision-based techniques to recover the Operational Deflection Shapes of a reference specimen consisting in a rectangular section cantilever beam. The research work focuses on the analysis of the accuracy of this measuring approach and discusses the strong points and the limitation of the photogrammetric approaches to experimental modal analysis. Two different types of area-based matching techniques are considered here: the former is based on incremental displacement estimation of subsets of subsequent images. This technique works the approach of Particle Image Velocimetry (PIV), with an Eulerian approach. The second technique considered here is on the contrary based on the estimation of the absolute displacement, considering the first acquired image as the reference for all the subsequent ones. In this last case a Lagrangian approach is used and the applied algorithm is the Digital Image Correlation (DIC). The main advantage of Eulerian (i.e. PIV) approach is that it is possible to analyse also the vibration of a moving object (like in belt dynamic characterization), where the reference region imaged in the first acquired picture, comes out of the field of view much before the end of the test. On the other hand, the Lagrangian approach shows some accuracy problems in the applications where the estimation of the displacement is request instead of velocity. In these cases, the incremental displacement estimated at each new image has to be composed with all the previously estimated ones to obtain the overall motion. Experimental tests were done using a cantilever beam as a case study. The excitation of the beam is done by means of an electromagnetic shaker, while the reference transducer used to validate the output of vision-based techniques is a Scanning Laser Doppler Vibrometry. Results show that the two image-based methodologies have similar performances when estimating natural frequencies, loss factors and ODSs.

Independent Component Analysis (ICA) is used for recovering the various independent sources exciting a system and to separate their contributions. In this paper, ICA is applied to vibrational and acoustic data measured on undamaged and damaged rolling bearings, where those data contains information about the vibration frequencies related to the defect, if present, to the rotation of the bearing, to its dynamic behaviour (resonance frequencies) and to random noise. The aim of the method is to separate those different contributions and to enhance only the one related to the vibration frequency associated to the defect in order to diagnose its presence. To improve the power of the method to identify the contribution of the bearing fault characteristic frequency, a pre-processing step is introduced based on Empirical Mode Decomposition (EMD), allowing to reconstruct multiple sets of time series from a single sensor data, called multiple intrinsic mode functions, which are then given as input to the ICA.

Aircraft are more than ever pushed to their limits for performance reasons. Consequently, they become increasingly nonlinear and they are more prone to undergo aeroelastic limit cycle oscillations. Structural nonlinearities affect aircraft such as the F-16, which can undergo store-induced limit cycle oscillations (LCOs). Furthermore, transonic buzz can lead to LCOs because of moving shock waves in transonic flight conditions on many aircraft.This study presents a numerical investigation of passive LCO suppression on a typical aeroelastic system with pitch and plunge degrees of freedom and a hardening stiffness nonlinearity. The absorber used is made of a piezoelectric patch glued to the plunge springs and connected to a resistor and an inductance forming a RLC circuit. A mechanical tuned mass damper absorber of similar configuration is also considered. The piezoelectric absorber features significant advantages in terms of size, weight and tuning convenience.The results show that both types of absorber increase the linear flutter speed of the system in a similar fashion but, when optimal, they lead to a sub-critical bifurcation while a super-critical bifurcation was observed without absorber. Finally, it is shown that the addition of a properly tuned nonlinear spring (mechanical absorber) or capacitor (piezoelectric absorber) can restore the super-criticality of the bifurcation. The tuning of the nonlinearity is carried out using numerical continuation.

Parallel kinematic manipulator are for sure the best choice to fulfill the need of high accuracy and high stiffness. By the way due to the always increasing required performances, poor decisions during the early design stages might lead to the arising of vibration phenomena during the operating phase of the machine. Due to the intrinsic nonlinearities that characterize this kind of machines the modes of vibration of the structure strongly depend on the pose of the robot. In addition in many applications as the one presented in this paper the tasks the machine is required to perform are not known in advance. Usually the technical specification for a PKM in terms of workspace dimensions and maximum excitation frequencies are related to the end-effector. By the way the kinematics that links the motion of the mobile platform to the joints motion is highly nonlinear, and thus might produce the insurgence of higher frequencies excitations. This paper presents an approach to test the modes of vibration of a 6-DoF parallel kinematic simulator that relies on a statistical method to investigate different possible motions of the end-effector that involve a combination of translations and rotations on the basis of their probability density functions (PDF).

Understanding how damage progresses in engineering materials is of the utmost importance for ensuring safety and reliability. Mechanical components and structures must often perform safely and reliably for much longer than can be reasonably tested, or the components must operate in severe environments that are difficult to reproduce. The capability to perform real-time, real-scenario testing is not always present or attainable however, and failure to adequately test components can lead to catastrophic consequences or high preventative-maintenance costs resulting from the use of weaker presumptive models. As a solution, a criterion for ductile metals for equating high-stress single shock damage, to periodic, low-stress, multiple shock damage is presented. The correspondence in damage, exhibited as an equivalency in deformation, is derived and experimentally validated. To accomplish this process, we relate the plastic deformation a test coupon experiences under a shock input, to the shock parameters under both single shock and multiple shock regimes. We then compare the proposed theoretical damage model against the experimental data.

The lightness of the space and aerospace structures causes their vulnerability to vibrations. The cold temperatures do not allow using polymer materials. Active and semi-active control using piezos embedded on the structure can be used efficiently instead of polymers but they induce energy consumption. Friction damping is less efficient but it does not depend on temperature and it is energetically passive. Unfortunately its efficiency depends on the vibration amplitude as well as on the tightening force. The damping is very low for the lowest amplitudes and the largest tightening loads and increase up to an optimal value. This optimum damping is adjustable thanks to the tightening force. The purpose of our work is to evaluate the efficiency of the control of the tightening force in bolted joints to reduce the vibration of the assembled structure. In the first part, we present a original setup and a very detailed design of experiments that highlights the optimal sets of parameter in order to get a good control of the vibrations according to the frequency and the magnitude of the load. To conclude, we propose to share experimental data with the attendees for further discussions.

A closed-loop random vibration test is a common test used to qualify systems that will be submitted to similar vibration loads during their life. Numerical modeling of this type of test is an important tool to provide insight about the robustness and success of the tests. In this paper the response of an electro-dynamic shaker will be modeled and validated with experimental data.