Milena Martarelli

Laser Doppler vibrometry on rotating structures in coast-down: resonance frequencies and operational deflection shape characterization

In rotating machinery, variations of modal parameters with rotation speed may be extremely important in particular for very light and undamped structures, such as helicopter rotors or wind turbines. The natural frequency dependence on rotation speed is conventionally measured by varying the rotor velocity and plotting natural frequencies versus speed in the so-called Campbell diagram. However, this kind of analysis does not give any information about the vibration spatial distribution i.e. the mode shape variation with the rotation speed must be investigated with dedicated procedures. In several cases it is not possible to fully control the rotating speed of the machine and only coast-down tests can be performed. Due to the reduced inertia of rotors, the coast-down process is usually an abrupt transient and therefore an experimental technique, able to determine operational deflection shapes (ODSs) in short time, with high spatial density and accuracy, appears very promising. Moreover coast-down processes are very difficult to control, causing unsteady vibrations. Hence, a very efficient approach for the rotation control and synchronous acquisition must be developed. In this paper a continuous scanning system able to measure ODSs and natural frequencies excited during rotor coast-down is shown. The method is based on a laser Doppler vibrometer (LDV) whose laser beam is driven to scan continuously over the rotor surface, in order to measure the ODS, and to follow the rotation of the rotor itself even in coast-down. With a single measurement the ODSs can be recovered from the LDV output time history in short time and with huge data saving. This technique has been tested on a laboratory test bench, i.e. a rotating two-blade fan, and compared with a series of non-contact approaches based on LDV: traditional experimental modal analysis (EMA) results obtained under non-rotating conditions by measuring on a sequence of points on the blade surface excited by an impact hammer, continuous scanning LDV measuring the ODS of the structure excited by an impact hammer modulating the laser output, tracking laser Doppler vibrometry (TLDV) operating at different rotation speeds under stationary conditions, tracking continuous scanning laser Doppler vibrometry (TCSLDV) operating at different rotation speeds under stationary conditions. EMA and TLDV have been performed over the same grid of points sufficiently dense to have ODSs with adequate spatial resolution, it requiring long measurement time. The application of different techniques allowed us to completely characterize the tested bladed rotor and to validate the continuous scanning application to transient rotator processes.

A new laser vibrometry-based 2D selective intensity method for source identification in reverberant fields: part I. Development of the technique and preliminary validation

The selective intensity technique is a powerful tool for the localization of acoustic sources and for the identification of the structural contribution to the acoustic emission. In practice, the selective intensity method is based on simultaneous measurements of acoustic intensity, by means of a couple of matched microphones, and structural vibration of the emitting object. In this paper high spatial density multi-point vibration data, acquired by using a scanning laser Doppler vibrometer, have been used for the first time. Therefore, by applying the selective intensity algorithm, the contribution of a large number of structural sources to the acoustic field radiated by the vibrating object can be estimated. The selective intensity represents the distribution of the acoustic monopole sources on the emitting surface, as if each monopole acted separately from the others. This innovative selective intensity approach can be very helpful when the measurement is performed on large panels in highly reverberating environments, such as aircraft cabins. In this case the separation of the direct acoustic field (radiated by the vibrating panels of the fuselage) and the reverberant one is difficult by traditional techniques. The first aim of this work is to develop and validate the technique in reverberating environments where the location and the quantification of each source are difficult by traditional techniques. The reverberant field is clearly challenging also for the proposed technique, affecting the achievable accuracy, mainly due to the fact that coherence between radiated and reverberated fields is often unknown and may be relevant. Secondly, the applicability of the method to real cases is demonstrated. A laboratory test case has been developed using a large wooden panel. The measurement is performed both in anechoic environment and under simulated reverberating conditions, for testing the ability of the selective intensity method to remove the reverberation.

Experimental investigation by laser ultrasonics for high speed train axle diagnostics

The present paper demonstrates the applicability of a laser-ultrasonic procedure to improve the performances of train axle ultrasonic inspection. The method exploits an air-coupled ultrasonic probe that detects the ultrasonic waves generated by a high-power pulsed laser. As a result, the measurement chain is completely non-contact, from generation to detection, this making it possible to considerably speed up inspection time and make the set-up more flexible. The main advantage of the technique developed is that it works in thermo-elastic regime and it therefore can be considered as a non-destructive method. The laser-ultrasonic procedure investigated has been applied for the inspection of a real high speed train axle provided by the Italian railway company (Trenitalia), on which typical fatigue defects have been expressly created according to standard specifications. A dedicated test bench has been developed so as to rotate the axle with the angle control and to speed up the inspection of the axle surface. The laser-ultrasonic procedure proposed can be automated and is potentially suitable for regular inspection of train axles. The main achievements of the activity described in this paper are: – the study of the effective applicability of laser-ultrasonics for the diagnostic of train hollow axles with variable sections by means of a numerical FE model, – the carrying out of an automated experiment on a real train axle, – the analysis of the sensitivity to experimental parameters, like laser source – receiving probe distance and receiving probe angular position, – the demonstration that the technique is suitable for the detection of surface defects purposely created on the train axle.

Exploiting continuous scanning laser Doppler vibrometry (CSLDV) in time domain correlation methods for noise source identification

This paper proposes the use of continuous scanning laser Doppler vibrometry (CSLDV) in time domain correlation techniques that aim at characterizing the structure-borne contributions of the noise emission of a mechanical system. The time domain correlation technique presented in this paper is based on the use of FIR (finite impulse response) filters obtained from the vibro-acoustic transfer matrix when vibration data are collected by laser Doppler vibrometry (LDV) exploited in continuous scan mode (CSLDV). The advantages, especially in terms of source decorrelation capabilities, related to the use of CSLDV for such purpose, with respect to standard discrete scan (SLDV), are discussed throughout the paper. To validate this approach, vibro-acoustic measurements were performed on a planetary gear motor for home appliances. The analysis of results is also supported by a simulation.

A new laser vibrometry-based 2D selective intensity method for source identification in reverberant fields: part II. Application to an aircraft cabin

The selective intensity technique is a powerful tool for the localization of acoustic sources and for the identification of the structural contribution to the acoustic emission. In practice, the selective intensity method is based on simultaneous measurements of acoustic intensity, by means of a couple of matched microphones, and structural vibration of the emitting object. In this paper high spatial density multi-point vibration data, acquired by using a scanning laser Doppler vibrometer, have been used for the first time. Therefore, by applying the selective intensity algorithm, the contribution of a large number of structural sources to the acoustic field radiated by the vibrating object can be estimated. The selective intensity represents the distribution of the acoustic monopole sources on the emitting surface, as if each monopole acted separately from the others. This innovative selective intensity approach can be very helpful when the measurement is performed on large panels in highly reverberating environments, such as aircraft cabins. In this case the separation of the direct acoustic field (radiated by the vibrating panels of the fuselage) and the reverberant one is difficult by traditional techniques. The work shown in this paper is the application of part of the results of the European project CREDO (Cabin Noise Reduction by Experimental and Numerical Design Optimization) carried out within the framework of the EU. Therefore the aim of this paper is to illustrate a real application of the method to the interior acoustic characterization of an Alenia Aeronautica ATR42 ground test facility, Alenia Aeronautica being a partner of the CREDO project.

V-belt transverse vibration measurement by means of laser Doppler vibrometry

The dynamic behaviour of a power transmission V-belt system with two fixed pulleys has been analysed by applying the theory of the forced non-linear response of a moving string driven harmonically by eccentrically mounted pulleys. The model has been validated experimentally with reference data obtained by measuring out-of plane components of the belt vibration. The experimental data have been acquired by means of a single-point laser Doppler vibrometer (LDV), measuring the transverse vibration of the belt. Another experimentally technique applied here, for transverse vibration acquisition, was the continuous scanning LDV (CSLDV) that has been used for the first time in translating objects. From the model and the measurements, it has been found that the frequency crossing diagrams, analogous to the so-called Campbell plot used in rotating machinery, perfectly agree. Essentially, this plot demonstrates that the natural frequencies are strongly dependent on the belts transport speed. Consequently, the model can be employed as useful tool for identifying the transport speeds at which resonances are expected.

Performance analysis of continuous tracking laser Doppler vibrometry applied to rotating structures in coast-down

In this paper a performance analysis of the so-called tracking continuous scanning laser Doppler vibrometry (TCSLDV) exploited in coast-down has been performed. This non-contact measurement system is able to scan continuously over a rotating surface during coast-down and to determine vibration operational deflection shapes (ODSs) and natural frequencies in short time, i.e. the temporal extent of the coast-down. The method is based on a laser Doppler vibrometer (LDV) whose laser beam is driven to scan continuously over the whole rotor surface synchronously with its rotation, so that the LDV output is modulated by the structures ODSs. This technique has a full-field nature that enables it to measure simultaneously the time and spatial dependence of the vibration in a unique measurement. However, the TCSLDV presents some criticalities in practical applications, especially when applied to rotary transient and fast processes. In fact, if the vibration is transient and decays very fast, then the laser beam could not have had the time to scan the complete structure surface and the modulation of the ODS could be partial. An analytical model reproducing a representative experiment has been developed in order to evaluate the sensitivity of results to testing conditions. The laser beam trajectory in both the fixed and rotating reference systems has been synthesized showing its dependence on experimental parameters as the rotation speed variation during coast-down. It has been demonstrated the decrease in speed induces the deformation of the laser trajectory influencing the LDV output time history, spectrum and consequently the recovered ODS.

Aeroacoustic characterization of turbulent free jets using scanning laser Doppler vibrometry

In this work a procedure for the aero-acoustic characterization of air jets was presented. The ability of a laser vibrometer to assess the changes in the optical path induced by local fluctuation of air refraction index was used to calculate the pressure oscillations produced by turbulence. The signal from the laser must be treated in order to extract the required information. From the pseudo velocity, which represents the integral of the refraction index distribution along the laser beam, it is possible to calculate, by a tomographic algorithm, the volumetric distribution of the refraction index at each frequency, and then the pressure oscillation. Each pressure voxel represents a sound source, i.e. monopole, that gives contribution to the Sound Pressure Level in each point of the surrounding. The uncertainty of the procedure was also evaluated.

Diagnostic procedure on brake pad assembly based on Young's modulus estimation

Quality control of brake pads is an important issue, since the pad is a key component of the braking system. Typical damage of a brake pad assembly is the pad?backing plate detachment that affects and modifies the mechanical properties of the whole system. The most sensitive parameter to the damage is the effective Youngs modulus, since the damage induces a decrease of the pad assembly stiffness and therefore of its effective Youngs modulus: indeed its variation could be used for diagnostic purposes. The effective Youngs modulus can be estimated from the first bending resonance frequency identified from the frequency response function measured on the pad assembly. Two kinds of excitation methods, i.e. conventional impulse excitation and magnetic actuation, will be presented and two different measurement sensors, e.g. laser Doppler vibrometer and microphone, analyzed. The robustness of the effective Youngs modulus as a diagnostic feature will be demonstrated in comparison to the first bending resonance frequency, which is more sensitive to geometrical dimensions. Variability in the sample dimension, in fact, will induce a variation of the resonance frequency which could be mistaken for damage. The diagnostic approach has been applied to a set of undamaged and damaged pad assemblies showing good performance in terms of damage identification. The environmental temperature can be an important interfering input for the diagnostic procedure, since it influences the effective Youngs modulus of the assembly. For that reason, a test at different temperatures in the range between 15??C and 30??C has been performed, evidencing that damage identification technique is efficient at any temperature. The robustness of the Youngs modulus as a diagnostic feature with respect to damping is also presented.

Measuring strain response mode shapes with a continuous-scan LDV

A continuous-scan LDV can be used to give the response mode shape (of a vibrating surface as a spatial polynomial series. Second spatial derivative(s) of the deflection equation are then easily derived, and these should, in principle, give a curvature equation from which, for a beam or plate of known cross-section, stresses and strains can be obtained. Unfortunately, the stress and strain distributions depend critically on higher terms in the series, which are not accurately measured. This problem is avoided by a method described in this paper, which enables accurate stress and strain distributions to be derived for uniform beams, from a straight-line LDV scan, using only five terms in the mode- shape polynomial series. An application to uniform plates is being developed; the analysis in this latter case is rather more complicated.