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Most nonlinear identification problems often require prior knowledge or an initial assumption of the mathematical law (model structure) and data processing to estimate the nonlinear parameters present in a system, i.e. they require the functional form or depend on a proposition that the measured data obey a certain nonlinear function. However, obtaining prior knowledge or performing nonlinear characterisation can be difficult or impossible for certain identification problems due to the individualistic nature of practical nonlinearities. For example, joints between substructures of large aerospace design frequently feature complex physics at local regions of the structure, making a physically motivated identification in terms of nonlinear stiffness and damping impossible. As a result, black-box models which use no prior knowledge can be regarded as an effective method. This paper explores the pragmatism of a black-box approach based on Polynomial Nonlinear State Space (PNLSS) models to identify the nonlinear dynamics observed in a large aerospace component. As a first step, the Best Linear Approximation (BLA), noise and nonlinear distortion levels are estimated over different amplitudes of excitation using the Local Polynomial Method (LPM). Next, a linear state space model is estimated on the non-parametric BLA using the frequency domain subspace identification method. Nonlinear model terms are then constructed in the form of multivariate polynomials in the state variables while the parameters are estimated through a nonlinear optimisation routine. Further analyses were also conducted to determine the most suitable monomial degree and type required for the nonlinear identification procedure. Practical application is carried out on an Aero-Engine casing assembly with multiple joints, while model estimation and validation is achieved using measured sine-sweep and broadband data obtained from the experimental campaign.

This research article presents a novel application of travelling wave excitation method applied to a composite bladed disc. The objective of this work is to develop a non-contact excitation method for research applications where (i) blades are non-ferromagnetic and (ii) damping is nominally high. This goal was achieved by spinning a disc, on which 14 powerful DC magnets were installed, in front the composite bladed disc. Small DC magnets were attached near each blade root to provide repellent forces. Twenty blades were manufactured with pre-pregs IM7–8552, using unidirectional stacking sequence and were installed on a rigid metallic mounting hub. The paper will present the design and make of the bladed disc, the theoretical study of normal and tangential forces in a magnet-to-magnets configuration and, finally, the experimental validation of a 14-DC magnetic exciter. The forced responses were measured in one test case by a 3D single point LDV system and in another test case by a Scanning LDV system. This work will also present an attempt to develop a DC electromagnetic exciter with its limitation and potential.

The rise of output-only modal analysis has offered an economical and efficient way to identify modal parameters of civil engineering structures, namely natural frequencies, damping ratios and mode shapes. However, since the forcing is unknown, it is not possible to directly estimate modal masses, and estimates of damping ratios may be inaccurate. With the advancement of wireless sensor networks both vehicle and bridge responses can be simultaneously measured. This offers the possibility of estimating true Frequency Response Functions (FRFs), since the vehicle acceleration gives an estimate of the force input to the bridge. Hence in principle it is possible to estimate modal masses and more accurate damping ratios. However, the spatial and temporal variation of the moving load from a passing vehicle gives challenges to this idea and precludes the direct use of existing single-input-multiple-output (SIMO) system identification methods. Even if the system is treated as a multiple-input-multiple-output (MIMO) one, the inputs are highly correlated so existing methods for these systems are not applicable either. For this reason, a two-stage strategy is proposed to modify an existing method to solve this moving load problem.

In 1964, just a few years after the invention of the laser, a fluid velocity measurement based on the frequency shift of scattered light was made and the laser Doppler technique was born. This comprehensive review paper charts advances in the development and applications of laser Doppler vibrometry (LDV) since those first pioneering experiments. Consideration is first given to the challenges that continue to be posed by laser speckle. Scanning LDV is introduced and its significant influence in the field of experimental modal analysis described. Applications in structural health monitoring and MEMS serve to demonstrate LDV's applicability on structures of all sizes. Rotor vibrations and hearing are explored as examples of the classic applications. Applications in acoustics recognise the versatility of LDV as demonstrated by visualisation of sound fields. The paper concludes with thoughts on future developments, using examples of new multi-component and multi-channel instruments.

This work presents a strategy for testing and validating structures connected together with bolted joints, which are the most common components in mechanical structures. Considering the great number of coupled mechanical structures and research studies on this subject, the authors focused this research work on bolted flanges of aircraft engine casings. In fact, the coupling of engine casings is generally obtained by a large number of joints which assure the correct sealing at the flanges’ interfaces. From a finite element (FE) modelling perspective, joints are often modelled by either rigid connections or springs, otherwise incurring a very expensive computational time. This modelling approach is not a problem when dealing with low amplitude levels of vibrations. For higher levels of vibrations, joints and flanges cannot be considered rigidly connected and that exerted flexibility at the joints’ area can determine nonlinear dynamic behaviour. This work aims to study the dynamic behaviour of bolted flanges by using modal testing performed under controlled response amplitude. Two test structures, (1) a simple bolted flange test case and (2) a sector of a Rolls-Royce aero-engine casing, are tested under high level of vibrations. Both test structures are modelled by FE method, and nonlinear elements are used for modelling the flanges’ interfaces so as to perform prediction of nonlinear responses. These predictions are eventually correlated with the measured data.

This study focuses on the variability of the dynamic response of jointed structures. It is well known that the dynamic response of joints is prone to variability issues when they are open and closed again. This paper will present one numerical example and two experimental ones on different type of structures used for studying this issue of variability. The numerical example will focus on parameters such as torque magnitude and contact area. The analysis, which is linear, will present the changes of natural frequency caused by the variation of these two parameters. The experimental examples will focus on parameters like torque magnitude, torque sequence and number of assembling. Both natural frequency and damping changes will be measured from a number of repeated sequences and analysed using statistical methods.

Complex shaped aerodynamic structures such missiles are prone to exhibit some level of nonlinear phenomena due to their aerodynamically tailored design and application. Aside from the aerodynamic and aeroelastic challenges experienced by a missile, an important but fundamental challenge encountered by a deployable missile is the inevitable concentrated structural nonlinearities which are observed around the hinge of its fins. Due to the current design and manufacturing process, the hinge of the fin of a missile often consist of complex configurations, joints and other nonlinear features that leads to concentrated structural nonlinearities. Some of the nonlinearities encountered includes off sets, piecewise linear, bilinear nonlinearity, hysteresis, coulomb friction and damping nonlinearities. These nonlinearities are frequently triggered at large vibration amplitudes caused by high pressure loads during operational flight. Activation of these nonlinearities often affect the dynamic response of the missile and in some cases lead to structural failures in the air vehicle. In this context, identifying and predicting the vibration response of aerodynamic structures with nonlinearities will be of great advantage to the present aerospace industries. In this paper the nonlinear dynamic behaviour of a prototype missile is examined using established nonlinear identification methods applied to measured data obtained from experimental test. The nonlinear identification is achieved using the acceleration surface method and the Hilbert transform FORCEVIB method, these methods are applied to sine-sweep excitation and stepped sine excitation measurements to obtain nonlinear parametric coefficients. The nonlinear experimental model was developed using the white box identification process (Detection, Characterisation and Parameter Estimation). In addition, Force controlled stepped sine experiments at several excitation levels were conducted to gain useful insight into the amplitude dependant behaviour of the missile in the existence of structural nonlinearities.

Endurance of composites materials is a critical and important factor that enables engineers to design structural components more efficiently. In fact, what it matters for many industries is to define the life of their products and, within that life, their safe operational conditions. Endurance of composites materials goes by the definition of failure criteria that can be obtained by cooperation between research groups acting both in a modelling and in an experimental framework. This research work is focussed on the development of an experimental test planning directed at testing composites components. Endurance trials for full reverse loading conditions, such as R = −1, can be executed by exploiting the first resonant mode of a beam, or a narrow plate. The major obstacles for this type of testing are (a) the fixture of the sample and (b) the amplitude of vibrations required for initiating fatigue behaviours. This paper will present two experimental methods based on (i) pulsed air-jet contactless excitation method and (ii) a novel fixture for electromagnetic shaker type testing. These methods will show how to overcome these obstacles in order to perform endurance trials on Carbon Fiber Reinforced Polymer (CFRP) components. The experimental results obtained from two sets of components are presented here and an interpretation of the fatigue behaviour of the trialled specimens will be also attempted.

The dynamic response of a structure can consist of up to six degrees of freedom (DOF). Not all of them may be present at all times in a structure, but for a detailed finite element model validation it is important to provide as much information as possible. Current contact transducers can pick up a combination of some of the mentioned DOFs, but are not able to provide all six DOFs from a single transducer. Several contact-less transducers, such as a Scanning Laser Doppler Vibrometer (SLDV), can be combined to a single measurement system that allows the determination of up to three translational degrees of freedom at a point, but they are still not providing rotational information as the laser beam is fixed to a point. Continuously Scanning Laser Doppler Vibrometer (CSLDV) methods can partly overcome this issue by travelling in small circles around the measurement location and thus also sensing the rotation around the three axes at a point. An introduction of CSLDV methods for measuring DOFs at a point is given, focussing on a method for capturing up to 5 DOFs using a lens. However, the lens can make the measurement of the in-plane rotation very difficult and impractical. This paper presents a new type of scanning device which allows measuring all six degrees of freedom at a measurement location with a single setup and a single point LDV, by combining variable small circle scans. Experimental validation of the scanning head is presented and the measurement of the last DOF, the in-plane rotation, is demonstrated.

The definition of failure is fundamental to the characterisation of fatigue strength of components and structures and is often expressed as a percentage of stiffness degradation. This article proposes a new method to capture damage initiation and structural degradation during a fatigue test, by exploiting resonance vibrations. The method entails monitoring dynamic parameters, including the phase angle between excitation and response, which suddenly changes as soon as the overall stiffness changes as a consequence of damage. In this paper, a theoretical approach based on equations of motion is presented in order to describe the possible scenarios in which a component undergoes structural degradation. In the proposed approach, an abrupt change in the time history of the stiffness of the component is assumed and analysed in both the quasi-static case at low excitation frequencies and the dynamic case, when the excitation frequency is close to the resonance frequency. The resulting analytical solution shows that any small stiffness variation is amplified by the dynamic response of the component as a large phase change, even when it is too small to be captured by the quasi-static response. The idea is successfully applied to capture the initiation of delamination in CFRP laminates, and the relative S-N curve to initiation is drawn. The experimental evidence of a critical event in the fatigue life of a component is found both in the dynamic phase response and in the base acceleration. The critical event is captured from a sudden change in the dynamic parameters of the structure as soon as a delamination is initiated. Thermography, C-scan and micrograph confirmed that the onset of delamination occurs at the moment of the critical event, after an initial stage of microcracking.

This paper presents a novel approach for testing structural component to nonlinear vibrations. Nowadays, nonlinear testing is mainly carried out by using electromagnetic shakers. These are efficient and powerful excitation systems which transmit the force by a rigid stinger and can be driven by different excitation signals. The rigid connection contributes to create mechanical impedance mismatch between the shaker and the test structure thus reducing the efficiency of the driving force. An alternative solution to shakers is represented by use of a pulsed air jet excitation method, which drives the force by a pulsed air-jets and therefore contactless. This condition eliminates the mechanical impedance mismatch with the test structure and the excitation can be more efficient than the one created by shakers. The pulsed air-jet excitation system is used to study nonlinear vibrations of composites components. These were designed to be mock-ups of fan blades the layup of which was varied for the three types of components used in this work. Tests were carried out by performing forced response and free decay measurements. The free decay type of test revealed interesting results and the novelty of using such an exciter for nonlinear testing. The major novelty consists of interrupting the air flow from a steady state condition and let happen the free decay, all these without experiencing undesired dynamics as experienced by contact excitation.

Identification of nonlinear dynamical systems have enjoyed significant progression over the past few years with the outcome of various developed identification methods, however there is still no generalised method applicable to structures with arbitrary nonlinearity. In the analysis of nonlinear dynamical system, it is essential to establish accurate and reliable tools that are capable of estimating the parameters from measured data for both the linear and nonlinear system. This paper presents a modular framework approach for upgrading a valid linear finite element structural model to accommodate any nonlinearities present in a system. To validate the efficiency of the proposed method, numerical and experimental studies are conducted on a “Multiple Beam Test Structure”, the method uses an iterative process to upgrade the nonlinear terms in the system. The results are verified by comparing predicted new response with measured data.

AIP Publishing; International audience; This study aims to present a novel work aimed at locating discrete nonlinearities in mechanical assemblies. The long term objective is to develop a new metric for detecting and locating nonlinearities using Scanning LDV systems (SLDV). This new metric will help to improve the modal updating, or validation, of mechanical assemblies presenting discrete and sparse nonlinearities. It is well established that SLDV systems can scan vibrating structures with high density of measurement points and produc e highly defined Operational Deflection Shapes (ODSs). This paper will present some insights on how to use response phase mapping for locating nonlinearities of a bolted flange. This type of structure presents two types of nonlinearities, which are geometr ical and frictional joints. The interest is focussed on the frictional joints and, therefore, the ability to locate which joint s are responsible for nonlinearity is seen highly valuable for the model validation activities.

Vibration fatigue testing is a verification method for structural components. It is rapid and cost efficient, and it is often performed for aero-engine components because it replicates closely stresses under normal operation conditions. This testing methodology is well known for metallic components but it can also be applied to composites for studying failure caused by high cycle fatigue. Resonant frequency decay is usually the main parameter used to assess fatigue behaviour of metallic components at a given excitation level. However for composites this alone is not very useful and several measurement parameters need to be monitored in order to understand the behaviour and develop a set of failure criteria. Scanning LDV is an excellent measurement system which enables a variety of options thanks to its non-contact nature. This manuscript will show how this system can be used for monitoring several parameters during vibration fatigue testing. A custom made control panel called MONTEVERDI and the independent use of the scanning mirrors allow the SLDV to perform several tasks: (1) Phase Lock Loop (PLL) to excite the component always at resonance, (2) measuring of Operational Deflection Shape (ODS) using either step or continuous scanning method, (3) custom calibrated strain measurement and (4) phase portrait for nonlinear vibration analysis. This paper will present the great potential of using SLDV for performing time consuming vibration fatigue testing via an automated control panel.

This work investigates the identification of the nonlinear behaviour of bolted structures through experimental and numerical analysis. Friction joints (especially bolted joints) generate nonlinear dynamic behaviours in a bolted assembled structure subject to dynamic loadings (especially high level vibrations) due to energy dissipation. The causes of nonlinearities are multiple and the ones related to joints will be researched in this piece of work. Although numerical simulation of nonlinear dynamic behaviour is complex, Finite Element models of a bolted flange will be used for strain analysis in order to develop a strategy for test planning of nonlinear vibration testing. Experimental tests and parameters that can be used to identify the joint nonlinearities will be used for developing correlation methods.

Nowadays, diagnostic is an important assessment of integrity of structural components. Amongst the technologies capable of monitoring structural integrity the continuous scanning (CS) measurement method can be a valid alternative in quasi real-time acquisition methods. Its major feature is the capacity of continuously recording vibration response by sweeping a laser beam over a surface. The LDV output signal will be amplitude modulated because of the periodicity of the scanning for a given vibration oscillation. The spectral content of the LDV output signal is made of sidebands, which can be used for representing an Operational Deflection Shape of the structure. This paper proposes a numerical study focused on the changes of the spectral signature of the sidebands when a damage occurs in a structure. The major objective is to map these changes against an undamaged spectral signature and evaluate if this approach can be used for diagnostic purposes.

This paper demonstrates impact damage detection in a composite sandwich panel. The panel is built from a chiral honeycomb and two composite skins. Chiral structures are a subset of auxetic solids exhibiting counterintuitive deformation mechanism and rotative but not reflective symmetry. Damage detection is performed using nonlinear acoustics,involves combined vibro-acoustic interaction of high-frequency ultrasonic wave and low-frequency vibration excitation. High-and low-frequency excitations are introduced to the panel using a low-profile piezoceramic transducer and an electromagnetic shaker, respectively. Vibro-acoustic modulated responses are measured using laser vibrometry. The methods used for impact damage detection clearly reveal de-bonding in the composite panel. The high-frequency weak ultrasonic wave is also modulated by the low-frequency strong vibration wave when nonlinear acoustics is used for damage detection. As a result frequency sidebands can be observed around the main acoustic harmonic in the spectrum of the ultrasonic signal.

The study of dynamic properties of rotating structures, such as bladed discs, can be conveniently done using simple bladed discs where the blades do not have staggering angles. Simplified design, although not truly representative of real structures, can be easy and economic to manufacture and, still, very helpful for studying specific dynamic properties. An example of this can be called as mass mistune blisk study. Experimental measurements of vibrations of bladed discs under rotating conditions can be performed using Scanning Laser Doppler Vibrometer (SLDV) systems. However, in the aerospace industry, the vibrations of complex bladed discs must be measured under operating conditions which are more hostile than laboratory simulations. The Blade Tip Timing (BTT) measurement method is a measurement technique, which can be used to measure vibrations of bladed discs of an engine aircraft under operating conditions. However, the BTT technique is ineffective when used with a flat bladed disc whose blade vibrations cannot be measured. This can be detrimental when the use of controlled dynamic parameters, such as those obtained from a simple bladed disc design, can improve the confidence for the validation of post-processing software. This paper presents a work about experimental measurements of a simple bladed disc design whose vibrations were measured synchronously by Scanning LDV and BTT measurement systems. A rotating test rig and its mechanical modifications for the installation of the BTT probes are introduced. Implications of rotating a specimen inconsistently are presented so as solutions to obtained constant revolving speed. The experimental comparisons of forced response vibrations measured synchronously at one blade are presented and explained.

This paper presents the first part of a work about modal testing using meaningful measurement parameters. Scanning Laser Doppler Vibrometer (SLDV) systems are becoming largely used both in industry and university for performing vibration measurements. A reason for the success of SLDV systems can be found in their capability of measuring vibration remotely and under different environmental conditions which, when hostile, can inhibit other transducers to work correctly. Hence, SLDV system can be very practical and useful in many engineering applications. SLDV systems are being used as a contactless transducer measuring vibrations from a discrete number of measurement positions marked on the specimen whenever an optical access to it is available. Hence, the advantage of a modal test carried out using accelerometers and one carried out using a SLDV system can be: (i) the automation of the measurements and (ii) the increase of the spatial resolution of the measured modes. This suggests that SLDV systems can be used as a practical replacement of accelerometers operating the same measurement method. Continuous Scanning method is a novel approach of using contactless transducers for measuring vibrations. The most important difference between a discrete and a continuous approach is the method of measuring a vibration pattern. A discrete method measures the level of vibrations at discrete positions on a structure whereas a continuous method captures the modulation of the vibrations produced by the excited modes. This is possible when a transducer can travel across a vibrating surface. This first part of the work presents a new approach of continuous scanning measurement method using a multi-tonal excitation waveform. The paper starts from a comparison between a step and continuous scan mode to introduce a novel approach of continuous scan and multi-tonal excitation waveform. The objective of this first part of work is to present and understand that measurement parameters, such as measurement positions, and can be carefully chosen to improve the measurement technique. A laboratory test piece and a helicopter tail cone are used as examples for the application of this new measurement method approach.

This paper presents the latest developments of the continuous-scanning laser Doppler vibrometry (C-SLDV) measurement methods applied to rotating axially symmetric structures such as bladed discs. Measurement of vibrations of rotating structures are still difficult to perform. The main reason is due to the limitations of conventional transducers used for capturing the vibrations, these are often unable to measure and/or transfer a good quality signal remotely. The LDV became an ideal candidate to replace/aid such transducers, but only the introduction of two scanning mirrors in front of a laser head produced a step forward in measurements of vibrations of rotating targets; SLDV systems are now widely used both in industry and academic communities. This work shows how a commercial SLDV system can be used for measurement of vibrations of bladed discs under rotating conditions. The features of the scanning head, which was not modified for this study, were exploited up to the limit to achieve the synchronization between the scanner and a rotating target. The simplest tracking technique is performed when a circle-line scan pattern is synchronized with the rotating bladed disc so as to produce the point tracking measurement method. The extension of such a discrete measurement method to a full-field one is made when the laser beam is capable of tracing continuously either a line (LineScan tracking) or an area (AreaScan tracking) over the surface of the rotating structure. The development of tracking C-SLDV measurement techniques is achieved by the use of the traditional excitation methods and the development of a new excitation system, which will bring the excitation and the measurement to be in the same rotating frame of reference. Several experimental results are provided to illustrate the use of such techniques in turbomachinery industries.

Continuous Scanning LDV measurement methods allow estimates to be made of high spatial density vibration deflection shapes (ODSs) in very short testing times. The correct reconstruction of the ODS depends on the quality of the acquired data. Speckle and other types of noise, usually resulting from surface treatment of the specimen can be detrimental in this recovery process.

Composite components are being studied by many researchers due to complex nature of their nonlinear dynamic behaviour. This behaviour can be a function of several factors, such as lay-up configuration and/or visco-elastic material properties etc. Recent studies on failure criteria of composite structures under endurance testing have shown that vibration forced responses can be nonlinear because of structural modification occurring during a test. Typically, endurance testing can cause initiation and propagation of a delamination(s) of the test structure, shifting dynamics of composites from quasi linear to non-linear regime. This transition was observed during laboratory experiments but very little effort was made to generate an understanding of the phenomenon. The aim of this paper is to focus on the study of nonlinear behaviour of composites before and after endurance trials. Recently, tools capable of addressing detection, characterisation, localisation and quantification aspects of nonlinearity have been developed. Some of these tools will be used in this exercise with an overall objective of quantifying the level of nonlinearity.

Linear dynamic finite element analysis can be considered very reliable today for the design of aircraft engine components. Unfortunately, when theses individual components are built into assemblies, the level of confidence in the results is reduced, since the joints in the real structure introduce nonlinearity that cannot be reproduced with a linear model. Certain types of nonlinear joints in an aircraft engine, such as underplatform dampers and blade roots, have been investigated in great detail in the past, and their design and impact on the dynamic response of the engine is now well understood. With this increased confidence in the nonlinear analysis, the focus of research now moves towards other joint types of the engine which must be included in an analysis to allow an accurate prediction of the engine behaviour. One such joint is the bolted flange, which is present in many forms on an aircraft engine. Its main use is the connection of different casing components to provide the structural support and gas tightness to the engine. This flange type is known to have a strong influence on the dynamics of the engine carcase. A detailed understanding of the nonlinear mechanisms at the contact is required to generate reliable models and this has been achieved through a combination of an existing non-linear analysis capability and an experimental technique to accurately measure the nonlinear damping behaviour of the flange. Initial results showed that the model could reproduce the correct characteristics of flange behaviour, but the quantitative comparison was poor. From further experimental and analytical investigations it was identified that the quality of the flange model is critically dependent on two aspects: the steady stress/load distribution across the joint and the number and distribution of non-linear elements. An improved modelling approach was developed which led to a good correlation with the experimental results and a good understanding of the underlying nonlinear mechanisms at the flange interface.

This paper presents a novel measurement technique for measuring in-plane vibrations of a clamped circular ultra lightweight membrane. This work is divided in two parts a (i) modelling and an (ii) experimental part. The modelling part focuses both on construction of an FE model of an ultra-thin membrane and the simulation of an LDV output time signal using conical spiral scan method. The experimental part focuses on the design and manufacturing of a mechanism for performing spiral conical scan and, eventually, concludes with in-plane measurements of the membrane using a Scanning LDV and 6DOF Scanning LDV measurement methods. This experimental work follows the development of a novel laser scanning head which was designed for measuring up to 6 degrees of freedom using continuous scanning measurement methods. For this specific test case a spiral conical scan method had to be developed. Test results are presented and discussed here. keywords: in-plane vibrations, Scanning 6DOF LDV system, continuous laser Doppler vibrometry, modal testing.

A new scanning laser head is designed to use single Laser Doppler Vibrometer (LDV) for performing measurements up to 6 degrees of freedom (DOF) at a target. The scanning head is supported by a rotating hollow shaft, which allows the laser beam to travel up to the scanning head from an opposite direction where an LDV is set up. The scanning head is made of a set of two mirrors, which deflects the laser beam with an angle so that the rotation of the scanning head produces a conical scan. When measurements are performed at the focal point of the conical scan then three translational vibration components can be measured, otherwise the very small circle scan, before and after the focal point, can measure up to 6 degrees of freedom, including three translations and three rotations. This paper presents the 6DOF scanning head and the measurements of 3D operational deflection shapes of a test structure.

This paper aims to present experimental work on an aircraft engine casing assembly. Nowadays single components of casings can be modeled with such high accuracy that they can be validated by carrying out the model validation process using measured data from a sector of the entire assembly. This smart validation process can be achieved by carrying out the modal analysis with a Scanning LDV (Laser Doppler Vibrometer) system which allows good spatial resolution of the measured mode shapes. The validation process can be assumed valid under linear response conditions obtainable for low vibration amplitudes. Casings are typically connected together by joints which may or may not respond non-linearly under high levels of vibration. Therefore, prior to conducting any non-linear validation, the mode(s) responding non-linearly must be identified beforehand in order to correctly specify the non-linear modal testing required. The work presented here will use a large civil engine casing assembly comprising a Combustion Chamber Outer Casing (CCOC), High Intermediate Pressure Turbine Casing (HIPTC) and Low Pressure Turbine Casing (LPTC.) The Fine Mesh Finite Element Model (FMFEM) was successfully validated using linear modal analysis test data. One of the objectives of this work is to define the key points for conducting non-linear modal testing of such large casing assemblies and sub-assemblies. One outcome of the experimental work was a set of recommendations for performing measurements, which should be carried out within the frequency bandwidth selected during the model validation process. Experimentally derived non-linear response curves are presented in this paper.

This paper demonstrates damage detection in a smart sandwich panel with integrated piezoceramic transducers. The panel is built from a chiral honeycomb and two composite skins. A low-profile, surface-bonded piezoceramic transducer is used for high-frequency ultrasonic excitation. Low-frequency excitation is performed using a piezoceramic stack actuator. Ultrasonic sensing is performed using laser vibrometry. Nonlinear acoustics is applied for damage detection. The study is focused on sensor location analysis with respect to vibro-acoustic wave modulations. The paper demonstrates that when structure is damaged, the high-frequency “weak” ultrasonic wave is modulated by the low-frequency “strong” vibration wave. As a result frequency sidebands can be observed around the main acoustic harmonic in the spectrum of the ultrasonic signal. However, intensity of modulation strongly depends on sensor location.

In recent years, a number of studies have addressed the possibility of replacing the conventional rigid mirrors that are used in space-based telescopes with optical membranes. Weight reduction, reduced cost of transportation and ability to provide a continuous surface for the attenuation of wave front aberrations are some of the benefits given by optical membranes. Given the harsh environmental loading conditions represented by thermal radiation, debris impact and slewing maneuvers, the ability to characterise fully the dynamics of such low-density thin-film membranes is essential. Nevertheless, the testing of membrane-like structures has proven to be a non-trivial process, and requires a considerable amount of work to achieve accurate results, as well as validating experiments against numerical models. Despite typical testing, the inherent low-density feature of membranes requires the use of non-invasive, non-contact sensors and excitation capability. The work presented here, investigates the possibility of using Operational Modal Analysis (OMA) techniques to extract modal parameters from an acoustically-exited membrane, where responses are collected by using a Scanning Laser Doppler Vibrometer (SLDV) as a non-contact velocity transducer. Results from this experiment are validated against an impedance-based numerical model.

Experimental vibration measurement of mechanical components are very important for studying the dynamic properties. Electromagnetic (EM) shakers are the most widely used exciters in mechanical testing because of both the broadband dynamic range of excitation and the excitation power. However, there are applications where these exciters can work inefficiently, and so underperform. This can be caused by an impedance mismatch between the shaker armature and test rig, which causes dissipation of the generated power into heating the armature rather than moving the test structure. Clearly, mechanical components presenting a high level of structural damping will require higher level of power to obtain high levels of vibration. Hence, it is important to minimize as much as possible any unwanted power dissipation due to both the test rig design and the connection between the shaker and/or the test rig. This paper demonstrates that a bladed disc type of structure can be used as a high impedance connector for a test rig in order to increase the excitation force level. This is possible thanks to otherwise an undesirably dynamic characteristic of bladed discs, which is represented by mistuning of the blades. When this mistuning characteristic is enhanced, it is possible to produce several resonances each with a high impedance match between the shaker and the test rig and this can increase the force applied to the specimen and thus its displacement amplitude. Also, the test rig proposed here can be used of several resonance frequencies depending on the number of blades. Hence the proposed test rig can improve both the performance of a shaker and increase the amplitude of vibration of the test structure. Further to this the application of the amplification process can be used for fatigue trials of composite material component. This has been an application which has caused some considerable difficulty: few cases have been successful and the results in this paper show evidence of how to proceed for future trials.

A new scanning laser hear has been designed for measuring up to 6 degrees of freedom at a point. The scanning head is supported by a rotating hallowed shaft, which allows the laser beam to travel up to the scanning head. A set of two mirrors deflects the laser beam with an angle and the rotation of the scanning head produces a conical scan. This work shows that this device can perform a conical scan whose the very small circle scan, before and after the focus point, can measure up to 6 degrees of freedom; the paper focuses on the in-plane rotation. A validation of the novel scanning head is performed. This paper concludes with the presentation of a more compact design for the 6DOF scanning head, which is aimed for complex engineering applications.

Underplatform friction dampers are commonly used to control the vibration level of turbine blades in order to prevent high-cycle fatigue failures. Experimental validation of highly non-linear response predictions obtained from FEM bladed disk models incorporating underplatform dampers models has proved to be very difficult so as the assessment of the performance of a chosen design. In this paper, the effect of wedge-shaped underplatform dampers on the dynamics of a simple bladed disk under rotating conditions is measured and the effect of the excitation level on the UPDs performances is investigated at different number of the engine order excitation nearby resonance frequencies of the 1st blade bending modes of the system. The measurements are performed with an improved configuration of a rotating test rig, designed with a non-contact magnetic excitation and a non-contact rotating SLDV measurement system.

Aircraft engine components, such as bladed discs, are subjected to high levels of vibration due to their service conditions and vibratory stresses can reduce the expected operational life. High Cycle Fatigue (HCF) is the most common cause of component failure in gas turbine engines. Laboratory testing is important for understanding the fatigue properties of materials and for producing a database eventually used during the design of new components. HCF test can be performed in the laboratory using a test rig on which is installed a test structure, connected to an electromagnetic shaker supplying the excitation. A simple test rig can be made comprising a holding block connected by push rod to the armature of a shaker so as to produce a base excitation. Such a test rig can be specifically designed to increase the test piece vibration levels by tuning it to one resonance of the structure. However, in doing so, a test rig of this type can present an impedance which is mismatched with the shaker, thereby causing dissipation of the excitation force. Any power loss can be a problem, exacerbated by components presenting high levels of structural damping, because of the higher force levels required to achieve high levels of vibration. Hence, any HCF test can be ineffective because of the diversion of shaker power from the test component to other parts of the test setup. The aim of this paper is to study the impedance mismatch between test rig and shaker by modelling a simple test rig, using a lumped-parameter model, for designing and measuring vibrations of the test rig to identify its weakness for HCF.

Continuous-Scanning LDV (CSLDV) methods are now available as alternative approaches to the conventional Stepping mode for Operational Deflection Shape (ODS) measurements. CSLDV techniques were initially used with sinusoidal excitation where the LDV time history could be also used to extract modal parameters by post-processing the data using modal analysis software. The latest extension of CSLDV is the use of a pseudo random signal to excite the tested structure for this work, a cantilever steel beam was used. Analytical models of the structure, the LDV output signal and excitation waveform were produced to simulate several responses which could be studied to identify the relationship between three fundamental parameters: (i) LDV scan rate, (ii) sampling frequency and (iii) frequency resolution of the pseudo random signal. This paper produces a theoretical background of the study presenting simulations and validations of both LDV output time signals and ODSs, respectively.

In the past decade, Continuous Scanning Laser Doppler Velocimetry (CSLDV) has been successfully used inexperimental modal analysis and vibration tests. Essentially, this technique consists in the use of a LaserScanning Velocimeter (LDV) moving continuously across a vibrating surface rather than stepping in a point-by-point fashion. If the structure is undergoing harmonic vibration, its Operating De”ection Shape (ODS) canbe recovered in a very short time and without the limitations in spatial resolution typical of discrete pointmeasurements. Recently, this technique has been extended to transient excitation -i.e. the object is swept by thelaser transducer during a period of transient free response caused by an impact. The combination of a broadbandexcitation with a full-“eld measurement results in a highly complex signal, modulated both by the mode shapesof the structure and by the exponential decays due to damping eects. Thus, the signal contains in its spectruma mixture of temporal information (the eigenvalues of the structure) and spatial information (the eigenvectors),all in a single time history. This paper presents a mathematical model of this new concept, “rst applied to asimple one-dimensional scan and then to a rectangular 2-D area scan. The numerically-simulated signals of the1-D case are compared with actual measurements, both in time and frequency domains. Finally, an analysis ofthe measurement parameters involved in this method is provided.Keywords: Impact Testing, CSLDV, Continuous Scanning, LDV

One major problem facing finite element model updating is the spatial incompatibility between the measurement of mode shapes through a limited set of physical sensors and the corresponding analytical predictions at a large number of finite element DOFs. Expansion of the measured mode shape data or reduction of the size of the initial FE model to the measured degrees of freedom is normally used to match the requirement. However, both techniques bring some erroneous information into the process. With the Continuous Scanning Laser Doppler Vibrometry (CSLDV) technique, a large number of degrees of freedom (DOFs) can be measured and accurate mode shapes can be achieved in a relatively short testing time. Such features provide great potential for dealing with the problem of the spatial incompatibility in FE model updating. In this paper, an application of finite element model updating using the continuous scanning laser Doppler vibrometry measurement is discussed. A real test piece of a square steel plate and its FE model were built. The first 20 natural frequencies and the corresponding mode shapes expressed by a large number of DOFs were measured using the CSLDV technique and further used for FE model updating. The initial FE model was assumed that random errors up to 20% of the theoretical values of Young's modulus were distributed among 25 areas of the plate due to uncertainties on the physical parameters. Model updating was undertaken to minimise the discrepancy between the FE model predicted and the experiment measured modal parameters and satisfactory results were obtained.

In recent years, the CSLDV (Continuing Scanning LDV) technique has been developed to obtain the ODS (Operational Deflection Shape) of a structure within a very short period of time. The ability to predict and to measure the ODS of a vibrating structure suggests its use to increase the potential for structural damage detection, localization and severity assessment. Previous research based on simulation of some simple test cases showed the effect of excitation locations on structural damage detection. Four steel plates were acquired and a FEM model of that structure was produced. Modal analysis, theoretical and experimental, was performed on the plate to obtain eigenvalues and eigenvectors and to update the FEM model. This time, the damage was simulated either using two permanent magnets which could be attached and de-attached easily without compromise the integrity of the structure or reducing the thickness of some elements. Hence, 4 damaged plates were modelled in FE software upon the position of the damage. The simulation and testing of the ODSs and the MSE (Mean Square Error) of the 4 plate structures in the damaged and the undamaged cases were performed based on CSLDV measurement method and compared respectively. Results show effect of vibration excitation location in the damage detection.

The aim of this paper is to present an overview of the CAISER MYMESIS software platform that has been developed to simulate and to perform the vibration measurement procedures using Tracking/Continuous Scanning LDV (T/CSLDV) methods, such as area scan on structures under both stationary and rotating conditions. Mathematical models to simulate the LDV output for all CSLDV methods are also included in the software platform as well as the traditional Step-Scanning approach. The software platform can be used in one of two modes: (i) first, to conduct a 'virtual test' to explore various proposed test configurations in order to define an optimum test specification, and (ii) second, to carry out an actual measurement, including driving the scanning mirrors, acquiring data and quickly post-processing data for an immediate inspection of the ODS of the vibrating structures.

This paper reports an application of nonlinear acoustics to impact damage detection in a composite chiral sandwich panel. The panel is built from a chiral honeycomb and two composite skins. High-frequency ultrasonic excitation and low-frequency modal excitation were used to observe nonlinear modulations in ultrasonic waves due to structural damage. Low-profile, surface-bonded piezoceramic transducers were used for ultrasonic excitation. Non-contact laser vibrometry was applied for ultrasonic sensing. The work presented focuses on the analysis of the modulation intensities and damage-related nonlinearities. The paper demonstrates that the method can be used for impact damage detection in composite chiral sandwich panels.

We describe the production development and experimental tests related to an hybrid honeycomb-truss made of shape memory alloy (Ni48Ti46Cu6), and used as a demonstrator for a deployable antenna in deep-space missions. Specific emphasis is placed on the modal analysis techniques used to test the lightweight SMA structure.

Typically, the identification of resonant frequencies in railway bridges is carried out from free-decay stationary signals as a train leaves the bridge. The same identification proves very challenging when nonstationary vibrations are measured as a train traverses the bridge. Despite the numerous attempts, nonstationary signals with low modulating frequencies are still difficult to be processed. This paper attempts to evaluate the bridge-vehicle first bending resonance by a method known as Wavelet Synchro-Squeezed Transform (WSST). The significant advantage of this signal processing method is to deal with low-frequency modulations, which are typical of long bridges. This research focusses on a Finite Element Model (FEM) of a bridge simulating the nonstationary vibration responses exerted by a spring-mass model traversing the bridge. The paper sets two objectives, and the first one is to investigate how the WSST analyses nonstationary signals generated by the FE model. The instantaneous frequency trace of the bridge-vehicle system will be compared to a similar frequency trace, that is created by performing several modal analyses at different locations of the bridge. The second objective of the paper is to investigate if the instantaneous frequency obtained from WSST is suitable for damage detection, as the FE model is fitted with damages. Both objectives are met, and the results will be presented. The trace of the first natural frequency matches well the one calculated by the WSST, and the instantaneous frequency shows to be capable of detecting damages included in the model.

The structural dynamic behavior of 3D-printed and conventional aluminum alloys are studied experimentally. Heat treated Scalmalloy and Al 7075 T-6 beam-like structures are fabricated using additive and extrusion manufacturing, respectively. The beams are exposed to vibration fatigue. Linear and nonlinear dynamic characterizations are performed to extract the stiffness and damping properties for pristine and fatigued structures. Additionally, the study includes optical imaging, Electron Backscatter Diffraction (EBSD), and Atomic Force Microscopy (AFM) to obtain the alloys’ materials properties. While the preliminary results show that Scalmalloy fatigue life is approximately 3-4 times lower than Al 7075 T-6, failure in Scalmalloy appears to be more graceful and predictable than that for aluminum.