Showing papers by "Gian Marco Revel published in 2000"

Laser vibration measurements and data processing for structural diagnostic on composite material

Abstract: In recent years, the identification and characterization of defects in mechanical and civil structures have been widely investigated by processing of vibration measurement results. In this field laser Doppler vibrometry (LDV) offers large potentials in terms of spatial resolution and reduced intrusivity. In this work LDV technique has been applied to measure delamination extension, depth, and location in composite materials. A lumped parameter model has been developed to describe the dynamic behavior of a delaminated composite panel with the aim of determining which kind of measurement data must be extracted for damage monitoring and to design efficient postprocessing algorithms for experimental LDV data. It is shown that the root-mean-square (rms) values computed in different frequency bands are indicators of the delamination depth. In fact, the vibration exhibits a higher rms value in higher frequency bands, if the defect is deeper. Following the model results, an experimental investigation by LDV has been performed on panels with known detachments. Accuracy of results has been checked by comparison with thermal tomography, which at present is one of the most used measurement techniques for monitoring the state of composite materials. Experimental results show the effectiveness of the model and the real applicability of the proposed technique. The presented methodology has proved to be efficient to determine the delamination depth. Also issues connected with structural excitation and measurement uncertainty are addressed.

Measurement techniques for the acoustic analysis of synchronous belts

Abstract: At present, in the automotive field, the noise generated by belt drives is evaluated by using microphones in the proximity of the belt, crankshafts, idlers and so on. Such a method can be misleading, since it may easily include the contributions of other noise sources present during the measurement. Moreover, a large amount of data is needed in order to test various layouts and various running conditions. We present a method for the analysis and prediction of the noise generated by belt drives which consists of two distinct phases in this paper. For simplicity, a two-pulley belt drive has been considered and the results have been validated at the meshing frequency, at which, as has been shown in the existing literature, the phenomenon of noise generation is mainly concentrated. In the first stage of the work, the acoustic power generation of the belt drive being tested was measured by means of acoustic intensity techniques. Subsequently, an acoustic prediction was performed by using vibration data obtained with a scanning laser Doppler vibrometer (SLDV) as inputs for a boundary element code. The SLDV was used because of its capability of measuring in-operation data on the running belt, which would not have been possible using traditional contact sensors (accelerometers and so on). The results obtained in the two phases were finally compared in order to evaluate the relation between the vibratory behaviour and the total acoustic radiation determined experimentally. The experimental and numerical data agree fairly well, adding precious information on the noise generation mechanisms and showing the feasibility of modelling the vibro-acoustic behaviour of belt drives and the possibility of a totally numerical procedure. In particular the implementation of an entirely numerical procedure using, for example, data generated through the use of codes for the dynamic characterization of mechanical systems (multi-body mechanical models and so on) seems foreseeable. In the final section of the present work, the uncertainty arising from the measurement processes of the investigation method presented is also discussed.

An experimental technique for structural diagnostic based on laser vibrometry and neural networks

Abstract: In recent years damage detection techniques based on vibration data have been largely investigated with promising results for many applications. In particular, several attempts have been made to determine which kind of data should be extracted for damage monitoring.

Laser vibration measurement for damage detection on composite materials : Advances in signal processing

Abstract: In damage detection and characterisation non-contact techniques are continuously developing in order to obtain techniques more sensitive and able work not only in laboratory but also in operative conditions. The technique based on the vibration analysis by scanning laser Doppler vibrometer is one of the most promising, allowing to extract also small defect and to direct correlate it to local dynamic stiffness and structural integrity. In fact the measurement capabilities of vibrometers, such as sensitivity, accuracy and reduced intrusivity, allow to have a very powerful instrument in diagnostic. In this work the advances of the damage detection and characterisation technique based on laser Doppler vibrometer is presented. In particular the algorithms for data processing are analysed in detail, discussing the advantages and the limitations. Finite Element model was employed for the controlled characterisation of the procedure. With respect to the previous works, the aim of the present one is to further improve reliability, easiness and readability of the output of the diagnostic procedure and to reduce the amount of the relevant data to be managed. A new algorithm is presented and discussed in detail and numerical and experimental tests are presented.

Application of lasers for noncontact excitation and measurement of vibration

Abstract: This work is part of a research devoted to the development of a non-intrusive modal analysis procedure based on laser techniques both for excitation and for measurement. The attention is focused on the experimental evaluation of uncertainty sources in modal parameter measurement, when high energy laser pulses are used to excite the vibration of the structure. The tests were performed on a cantilever beam excited firstly with a hammer and then with laser pulses from a Nd-YAG source (532 nm, 100 mJ/pulse). The problem due to the lack of knowledge on laser-induced input force is here considered. This, in fact, makes the precise quantitative measurement of the mobility functions and of the damping ratio difficult. To this aim, an `equivalent' input force was estimated by solving an inverse problem: the `equivalent' force is useful to determine the features of the laser-induced impulsive excitation. The achieved results are in complete agreement with other characterizations presented in previous studies, in particular in terms of specific impulse, time duration and characteristic frequencies. Finally, several suggestions are given in order to minimize the influence of the problems connected with laser pulse excitation, in particular concerning the limited energy given to the structure under investigation.

Laser vibration measurements through combustive flows : application to an industrial burner in working conditions

Abstract: The paper presents the application of a laser Doppler vibrometer for the characterisation of the dynamic behaviour of a burner during normal working conditions. The burner is a 1:4 scale model of a real CH 4 industrial burner for gas turbines, with a power of 120 kW. A first series of tests has been performed in order to determine the resonance frequencies of burner components, in such a way as to correlate the results achieved in working conditions with the characteristics of the structure. In a second series of tests the burner has been tested in use, firstly with only a cold jet of air flowing from the nozzle, then in real working conditions with the flame. In each test both vibration and acoustic measurements have been performed, in order to find correlation between combustion noise and structural vibrations. The laser Doppler vibrometer has been chosen to carry out measurements on the burner because of its capability of 'remotely' and non-intrusively determining vibrations. In order to assess the accuracy of vibrometer measurements through the flame, a theoretical model previously developed by the authors has been employed. The model describes the interactions between laser interferometer and refractive index variations induced by the flame, in such a way as to estimate the interfering and modifying inputs of the measurement system.

Quality control of electronic devices by vibration analysis

Abstract: In this paper the problem of vibration measurement and testing for quality control of electronic components is approached. In general, many tests are performed on electronic devices (personal computers, power supply units, lamps, etc.), according to international standards (IEC), in order to verify their resistance to shock and vibrations, but these are mainly `go no-go' experiments, performed on few samples taken from the production batches. The idea here proposed is to improve the efficiency of these tests by using electro-optic techniques for the measurement of the vibration behavior of the components under known excitation. This would allow the on-line testing of a high percentage of the production and would be useful to give important feedback to the design process. Scanning laser Doppler vibrometry seems to be a valuable solution for the problem, thanks to its capabilities of measuring several spatially- defined points on a vibrating object with reduced testing time for on-line application, with high sensitivity and accuracy, non-intrusivity and with any kind of excitation signal. Experimental tests are performed on a power supply: the results show the effectiveness of the proposed approach.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Laser pulses in modal analysis : An experimental and numerical investigation

Abstract: In this paper it has been described part of the research devoted to the development of a complete non-intrusive experimental modal analysis procedure based on laser techniques both for excitation and for measurement. In particular, the attention has been focused to establish a mechanical equivalent system of forces and moments. An analytical model of the energy exchange between the light pulse and the target surface is proposed together with a finite element model of thermal and mechanical behaviour of the structure under excitation. Both the models (analytical and numerical) have been experimentally validated by measuring the thermal and the vibration responses induced by the laser pulses using an high-speed infrared camera and a scanning laser Doppler vibrometer.

Laser Pulses in Modal Analysis: An Experimental and Numerical Investigation #282

Abstract: In this paper it has been described part of the research devoted to the development of a complete non-intrusive experimental modal analysis procedure based on laser techniques both for excitation and for measurement. In particular, the attention has been focused to establish a mechanical equivalent system of forces and moments. An analytical model of the energy exchange between the light pulse and the target surface is proposed together with a finite element model of thermal and mechanical behaviour of the structure under excitation. Both the models (analytical and numerical) have been experimentally validated by measuring the thermal and the vibration responses induced by the laser pulses using an high-speed infrared camera and a scanning laser Doppler vibrometer.