9TH INTERNATIONAL CONFERENCE ON VIBRATION MEASUREMENTS BY LASER AND NON‐CONTACT TECHNIQUES AND SHORT COURSE
1253(2010); http://dx.doi.org/10.1063/1.3455482View Description Hide Description
Continuous Scan Laser Doppler Vibrometry (CSLDV) modifies the traditional mode of operation of a vibrometer by sweeping the laser measurement point continuously over the structure while measuring, enabling one to measure spatially detailed mode shapes quickly and minimizing the inconsistencies that can arise if the structure or test conditions change with time. When a periodic scan path is employed, one can decompose the measurement into the response that would have been measured at each point traversed by the laser and obtain the structure’s mode shapes and natural frequencies using conventional modal analysis software. In this paper, continuous‐scan vibrometry is performed on a rotating fan, using computer controlled mirrors to track the rotating fan blades while simultaneously sweeping the measurement point over the blades. This has the potential to circumvent the difficulty of attaching contact sensors such as strain gauges, which might modify the structure and invalidate the results. In this work, impact excitation was used to excite a 3‐blade fan rotating at various speeds, and the blades were scanned with a cloverleaf pattern that captured the bending of all three blades simultaneously. Some specialized signal processing is helpful in minimizing the effect of rotation frequency harmonics in the measurements, and specific scan strategies are needed to avoid those frequencies, both of these issues are discussed in the paper. While noise in the laser vibrometer does pose some difficulty, the results show that several modes could be extracted and that the tracking‐CSLDV results agree with measurements obtained from the parked fan.
1253(2010); http://dx.doi.org/10.1063/1.3455457View Description Hide Description
Helicopters are aircraft machines which are subjected to high level of vibrations, mainly due to spinning rotors. These are made of two or more blades attached by hinges to a central hub, which can make the dynamic behaviour difficult to study. However, they share some common dynamic properties with the ones expected in bladed discs, thereby the analytical modelling of rotors can be performed using some assumptions as the ones adopted for the bladed discs. This paper presents results of a vibrations study performed on a scaled helicopter rotor model which was rotating at a fix rotational speed and excited by an air jet. A simplified analytical model of that rotor was also produced to help the identifications of the vibration patterns measured using a single point tracking‐SLDV measurement method.
Development of a Comprehensive Mathematical Model for Simulating the Effects of Misalignments in Vibration Measurements using Scanning LDV Measurement Systems1253(2010); http://dx.doi.org/10.1063/1.3455470View Description Hide Description
Scanning mirrors are very important contributors of a SLDV system. These mirrors are often installed inside the laser head thereby the optical access of the targeted structure must be always available during testing. However, some applications could benefit of having a laser head decoupled from the scanner so to positioning the twos in an optimum way, although this can introduce possible sources of misalignments.
This paper presents the development of a mathematical model for simulating the effects of misalignment in vibration measurements using scanning LDV measurement systems. The misalignments between the laser source and the scanning unit, and between the latter and the target structure is a source of uncertainty, producing pseudo‐vibrations and causing relative motion of the laser beam on the target surface. Using such a mathematical model, the effects of each parameter of misalignment can be simulated and studied. This work aims to presents simulations of all sources of misalignments when the scanners are decoupled from the laser head.
1253(2010); http://dx.doi.org/10.1063/1.3455487View Description Hide Description
Spectrally encoded endoscopy is a recently demonstrated imaging technique in which a diffractive element is used to encode locations with wavelengths, enabling simultaneous imaging of multiple image points on the sample. Using spectral domain interferometry, the technique has been demonstrated useful for vibration measurements through a sub‐millimeter endoscopic probe. We demonstrate a full‐field bench‐top imaging system, which is capable of simultaneous two‐dimensional imaging and vibration measurement at each resolvable point on the sample. Multitone signal recovery from the spectral interference signal is experimentally demonstrated and the limitations of the technique are discussed.
1253(2010); http://dx.doi.org/10.1063/1.3455488View Description Hide Description
A laser speckle contrast imaging (LSI) setup has been designed and used to estimate heartbeat rate and microvascular perfusion non‐invasively. LSI measurements were performed on the human index finger and thumb during various finger perfusion conditions, such as before and after gently rubbing of a finger or the healing of a small inflammation of the eponychium on the finger. Heartbeat was retrieved with 0.5 to 10 ms exposure time using LASCA (Laser Speckle Contrast Analysis) and dLASCA (dynamic Laser Speckle Contrast Analysis) processing methods. Additionally, a noise analysis model for laser speckle contrast imaging was established to evaluate Signal to Noise Ratio (SNR) in speckle imaging and provide a guideline for camera selection and imaging system design.
1253(2010); http://dx.doi.org/10.1063/1.3455489View Description Hide Description
This paper deals with the generation, characterization and analysis of ultrasonic waves generated in a thick stepped sample of inconel super alloy using Laser Based Ultrasonic Technique. Nd‐YAG pulsed laser is used for ultrasonic generation while He‐Ne laser is used for heterodyne detection. Ultrasonic signals are analyzed using Fourier and wavelet transforms. Here the identification and estimation of velocity of pressure waves is presented. The mechanism of pressure wave generation is discussed in brief. Laser ultrasonics studies of inconel are being reported for the first time.
Laser‐vibrometric measurement of oscillating piezoelectric actuators and of Lamb waves in CFRP plates for structural health monitoring1253(2010); http://dx.doi.org/10.1063/1.3455490View Description Hide Description
The use of Lamb waves is attractive for structural health monitoring of plate and shell structures since their propagation in thin‐walled structures is disturbed at damage locations especially at high frequencies. Lamb waves can easily be generated by thin piezoelectric plates which are attached to the surface of the structure. For the observation of the oscillating actuator as well as the propagating Lamb waves laser vibrometry is a powerful tool. Examples of vibrations of free and bonded piezoelectric actuators are given with special regard to the influences of contacts and other parameters, affecting the effectiveness of the wave generation. The determination of important features of Lamb wave propagation in carbon fibre reinforced plastics includes the measurement of dispersion curves and the estimation of attenuation and anisotropy. The interactions of Lamb waves with defects represented by reflections, transmissions and mode conversions are visualised and are easily to interpret.
Lamb Waves Amplitude Losses in Air‐Coupled Ultrasonic NDT System due to the Beam Deviation Produced by Inclination of the Test Structure1253(2010); http://dx.doi.org/10.1063/1.3455491View Description Hide Description
In Lamb waves inspection, an air‐coupled transmitter transducer is oriented at a specific angle such that it generates a pure Lamb mode which propagates along the structure and interacts with any existing defects. For this inspection system as shown in Fig. 1, an amplitude loss appears when small inclinations of the tested structure occur. An important factor which affects directly these losses has been observed, it consists on the Lamb waves beam (LWB) deviation due to this bad alignment. A simple expression of LWB deviation has been deduced. This expression includes the test structure angle, phase velocity of generated Lamb mode, and the phase velocity of waves propagating in the coupled medium. In this work, the bad alignment between the ultrasonic transducers and the plate test structure is modelled for any symmetric and asymmetric Lamb modes. Interesting results has been obtained using this model. Experimental LWB deviation angles and amplitude losses of both and modes have been measured for different inclination angles of the aluminium plate sample. A comparative study is released with theoretical results. It is shown that, for both theoretical and experimental studies, the LWB deviation and its measured amplitude are very sensitive to the alignment of the tested structure with respect to the transmitter‐receiver transducers plane. The effects of a change in the plate orientation is greater for a Lamb mode which propagates with higher velocity. Therefore, it is important to take into account this problem when inspecting large structures.
1253(2010); http://dx.doi.org/10.1063/1.3455492View Description Hide Description
In this paper, experimental measurements of the vibration pattern that results when an air‐coupled A0 Lamb wave mode propagates in a plate are presented. Lamb waves are generated using an air‐coupled piezoelectric transducer array. The measurements were performed in 304 stainless steel using laser interferometry. Air coupled piezoelectric ultrasonic array transducers are a novel tool that could lead to interesting advances in the area of non‐contact laminar material testing using Lamb wave’s propagation techniques, and it is interesting to know the vibration pattern in the plate. Using this system, some snapshots of the interaction of the A0 Lamb wave mode with a crack‐defect in the plate are also presented.
Ultrasonic characterization of materials by means of under water Laser Doppler Vibrometer measurements of continuous waves1253(2010); http://dx.doi.org/10.1063/1.3455493View Description Hide Description
Pulse signals are widely use for several ultrasonic testing. They indeed allow an easy estimation of the delays occurring in echo and transmission measurements and give the possibility to filter the noise (i.e undesired reflections occurring in the surface of the transducers) applying a window in the time domain. However their high crest factor makes these signals unsuitable to test attenuating materials. For this reason this paper proposes a new method, based on continuous waves, for ultrasonic characterization of materials. A a wave propagation model in the frequency domain is presented, to determine simultaneously acoustic velocity, mass density, and thickness of two Plexiglas plates, during transmission experiments. The Ultrasonic waves are captured by a Scanning Laser Doppler Vibrometer (SLDV) in order to guarantee a large number of spatial points, acquired with a high resolution.
1253(2010); http://dx.doi.org/10.1063/1.3455446View Description Hide Description
This paper deals with the characterization and analysis of ultrasonic Lamb waves generated in a long aluminium plates using Laser Based Ultrasonic Technique. Nd‐YAG pulsed laser is used for ultrasonic generation while He‐Ne laser is used for heterodyne detection. The Lamb wave signals are analyzed using Fourier and wavelet transforms. The Lamb wave signals recorded at three diferent source detector distances are analysed and the resuls are presented.
1253(2010); http://dx.doi.org/10.1063/1.3455447View Description Hide Description
This study is aimed at developing a technique for the characterisation of air‐coupled ultrasound probes, starting from the analysis of the mechanical behaviour of the probe membrane. The vibratory behaviour of the emission membrane is studied using laser‐Doppler vibrometry techniques with high frequency demodulation system (20 MHz). The determination of the vibration provides information which are useful for the assessment of the performance of the probe, in particular concerning the Quality factor and the portion of the membrane which really contributes to the emission. During the second step the results of the vibration measurements are used to calculate, by means of numerical boundary element method, the ultrasound beam emitted in terms of intensity in space. The obtained field is compared with the direct measurements carried out by scanning with the receiver probe and a pinhole plate. This comparison allows the potential and the problems of the two different characterisation techniques to be determined, even if the pinhole technique (which is currently considered the state of the art) cannot be used as an absolute reference. This study appears to be useful for paving the way for a new methodology for the calibration of air‐coupled ultrasound probes, which potentially could be used not only to improve the probe manufacturing process, but also to control conformity to specifications.
1253(2010); http://dx.doi.org/10.1063/1.3455448View Description Hide Description
This paper describes a high voltage pulser developed for an air‐coupled ultrasonic array NDE system based on Lamb waves. Unlike the classical single pulse excitation circuits, this pulser is able to generate a bipolar return‐to‐zero square‐wave burst signal up to of amplitude. The system is composed of two 32 elements concave array with a resonant frequency of 800 kHz. To excite all the transducers an array of pulsers with small volume and weight to be able to place it close to the concave array has been designed.
1253(2010); http://dx.doi.org/10.1063/1.3455449View Description Hide Description
The laser Doppler vibrometer (LDV) is an instrument based on a heterodyne interferometer that can be used to measure vibrations at a location on a structure. By moving the laser beam using scanning mirrors a full field vibration measurement can be performed. While the main purpose of the scanning LDV is to measure vibrations, recently several authors proposed to use the LDV for flow and sound measurements. The idea is to aim the laser vibrometer at a rigid block. When a pressure field is present between the LDV head and the rigid block this will result in changes of the refraction index that are related to the measured Doppler shift. Because no seeding particles are needed, the method opens new opportunities to measure phenomena that were previously difficult to measure. Unfortunately, the current LDV measurement techniques are only able to measure dynamic flow fields (both turbulent and coherent).
In this paper we present a novel method that is able to measure a steady‐state velocity flow field. The technique is based on the use of an ultrasound field that is generated in addition to the flow field. At locations where a non‐zero flow velocity is present, the ultrasonic wave fronts will be phase shifted. The ultrasound fields can be measured using a scanning LDV. By calculating the phase shift from the measurements the flow velocity can be obtained. The proposed method will be validated on measurements of a jet.
1253(2010); http://dx.doi.org/10.1063/1.3455450View Description Hide Description
Exploitation of deep mine shafts is connected with vitally important aspects of work and public safety. The latter becomes clear keeping in mind the fact that in many locations of the European coal and ore mining countries, the shafts can be found in close proximity of free access areas such as, for instance, public roads. On the other hand, proper operational capabilities of mine shaft installation determine the level of safety of hundreds underground miners, in a single underground mine, who use the shaft at least twice a day. Reliable operation of shaft installations is a derivative of many factors influencing the shaft frame as such, and its surroundings, e.g. changing of water conditions in the ground. Consequently, systematic control (including continuous monitoring if need be) of the vibration level of both the rotating machinery installed within the shaft structure, and frame supporting elements (including continuous monitoring) is compulsory. Currently, only in the Upper Silesia coal mining region, approximately 35 such structures are under extensive vibrational control. The paper presents sets of vibration parameters measurements obtained using conventional, and laser‐based instrumentation, performed on some of those shaft structures. In relation to these results safety aspects have been discussed.
1253(2010); http://dx.doi.org/10.1063/1.3455451View Description Hide Description
This paper presents research on the damage localization method. The method is based on guided wave propagation phenomena. The investigation was focused on application of this method to monitor the condition of structural elements such as aluminium or composite panels. These elements are commonly used in aerospace industry and it is crucial to provide a methodology to determine their condition, in order to prevent from unexpected and dangerous collapse of a structure. Propagating waves interact with cracks, notches, rivets, thickness changes, stiffeners and other discontinuities present in structural elements. It means that registering these waves one can obtain information about the structure condition—whether it is damaged or not. Furthermore these methods can be applied not only to aerospace structures but also to wind turbine blades and pipelines. In reported investigation piezoelectric transducer was used to excite guided waves in considered panel. Measurement of the wave field was realized using laser scanning vibrometer that registered the velocity responses at a defined points belonging to a defined mesh. Mesh spacing was investigated in order to ensure fine wave propagation visualisation. Firstly, wave propagation in pristine specimen was investigated. Secondly, artificial damage was introduced to the specimen. Finally, wave interaction with damage was visualised and conclusions regarding potentials of application of laser vibrometer for damage detection were drawn. All the processing was made with the developed MATLAB procedures.
1253(2010); http://dx.doi.org/10.1063/1.3455452View Description Hide Description
The power flow and energy distribution of a vibration mode of a damaged plate is studied experimentally. Variation of the modal reactive power distribution of a damaged plate is experimentally evaluated with a scanning LDV and compared to the theoretical predictions. Large variation of local reactive power flow in or around the damage region of a plate under resonant vibration is found to be related to the change of strain and kinetic energies in the damage region. Feasibility of damage identification based on the detection of this local variation of modal reactive power flow in a structure is studied.
1253(2010); http://dx.doi.org/10.1063/1.3455453View Description Hide Description
Non‐Destructive Evaluation (NDE) techniques have achieved a great development during the last decades as a valuable tool for material characterization, manufacturing control and structural integrity tests. Among these tools, the guided wave technology has been rapidly extended because it reduces inspection time and costs compared to the ordinary point by point testing in large structures, as well as because of the possibility of inspecting under insulation and coating conditions. This fast development has motivated the creation of several inspection and material characterization systems including different technologies which can be combined with this technique.
Different measurements systems based on laser techniques have been presented in order to inspect pipes, plates and diverse structures. Many of them are experimental systems of high cost and complexity which combine the employment of a laser for generation of waves in the structure and an interferometer for detection. Some of them employ air‐coupled ultrasound generation transducers, with high losses in air and which demand high energy for exciting waves in materials of high stiffness. The combined employment of a commercial vibrometer system for Lamb wave sensing in plates has been successfully shown in the literature.
In this paper we present a measurement system based on the combined employment of a piezoelectric wedge transducer and a laser vibrometer to sense guided acoustic waves in carbon steel pipes. The measurement system here presented is mainly compounded of an angular wedge transducer, employed to generate the guided wave and a commercial laser vibrometer used in the detection process. The wedge transducer is excited by means of a signal function generator whose output signal has been amplified with a power signal amplifier. A high precision positioning system is employed to place the laser beam at different points through the pipe surface. The signal detected by the laser vibrometer system is amplified with a signal amplifier and then it is displayed in a digital storage oscilloscope. This set‐up offers the possibility of analyzing in a simpler way the wave propagation and the material evaluation in pipes of certain wall thickness. The material characterization considering distinct wave propagation modes can be easily achieved, changing the different incident angles of the wedge piezoelectric probe and their combined employment with several driving signals. Moreover, this experimental sensing system offers other possibilities of inspecting and analyzing the wave propagation in some features (bends, flange joints, welds,…) of the pipe surface which cause very large reflections and mode conversions and which in practice limits the inspection range when are inspected with conventional receiving transducer arrangements.
1253(2010); http://dx.doi.org/10.1063/1.3455454View Description Hide Description
Piezoelectrically excited langasite transducers are studied by laser doppler vibrometry (LDV) at temperatures up to 860 °C. The scope of our research is to design and realize different resonator concepts based on this material. The properties of langasite tuning forks and cantilevers are subject of our investigations. Room temperature investigations of the displacement for at least three resonance frequencies result in values up to 5 μm for a tuning fork and 27 nm for a cantilever. Vibration nodes are identified using the spatial distribution of displacement. Further, the resonances are assigned to different vibration modes. The electrical impedance of the tuning forks is determined simultaneously in the temperature range from 50 °C to 425 °C. Trends in the temperature dependence of the displacement are consistent with that of the resonator quality factor and indicate increasing losses with increasing temperature. The observation corresponds to earlier studies at elevated temperatures where increasing mechanical and electrical losses are attributed to mobile ions.