Dario Di Maio

Impact damage detection in composite chiral sandwich panels using nonlinear vibro-acoustic modulations

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.

Design of High Impedance Test Rig for Composite Structures Vibration Measurements

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.

Modeling and Validation of the Nonlinear Dynamic Behavior of Bolted Flange Joints

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.Copyright

Impact damage detection in composite chiral sandwich panels

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.

Experimental Non-linear Modal Testing of an Aircraft Engine Casing Assembly

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.

Development of testing methods for endurance trials of composites components

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.

Use of Continuous Scanning LDV for Diagnostics

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.

Identification of Dynamic Nonlinearities of Bolted Structures Using Strain Analysis

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.

Industrial-Graded Epoxy Nanocomposites with Mechanically Dispersed Multi-Walled Carbon Nanotubes: Static and Damping Properties

The majority of currently published dispersion protocols of carbon nanotubes rely on techniques that are not scalable to an industrial level. This work shows how to obtain polymer nanocomposites with good mechanical characteristics using multi-walled carbon nanotubes epoxy resins obtained by mechanical mixing only. The mechanical dispersion method illustrated in this work is easily scalable to industrial level. The high shearing force due to the complex field of motion produces a good and reproducible carbon nanotube dispersion. We have tested an industrial epoxy matrix with good baseline mechanical characteristics at different carbon nanotube weight loads. ASTM-derived tensile and compressive tests show an increment in both Young’s modulus and compressive strength compared with the pristine resin from a starting low wt %. Comparative vibration tests show improvement in the damping capacity. The new carbon nanotube enhanced epoxy resin has superior mechanical proprieties compared to the market average competitor, and is among the top products in the bi-components epoxy resins market. The new dispersion method shows significant potential for the industrial use of CNTs in epoxy matrices.

Sensor Location Analysis in Nonlinear Acoustics Used for Damage Detection in Composite Chiral Sandwich Panels

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.