Papers by Author: Tomasz Wandowski

Abstract: The investigation was focused on Carbon Fibre Reinforced Polymers (CFRP). In the first part of research the aim was the characterization of CFRP surfaces. These surfaces were influenced by release agent, hydraulic fluid, moisture and overheating. In the second part of research adhesive bond quality was investigated. Three different cases of possible weak bonds were focused on. Weak bond caused by release agent contamination, moisture contamination and poor curing of adhesive. The characterization was conducted using laser vibrometry used as NDT tools. An active element in the form of piezoelectric transducer was used to excite the samples made out of CFRP material. Laser vibroemter was used to register the surface response. Combining the piezoelectric excitation with laser sensing a tool was obtained to measure precisely the propagating elastic waves. The excited waves were measured in defined points by the vibrometer obtaining the wavefield. In order to characterize the surface and bonding quality an indicator was proposed based on propagating wave parameters. The guided elastic wave velocity depends material properties (Young modulus, density, Poisson ratio) and thickness of the sample. It was assumed that comparison of the velocities can provide an information about the bond condition. All the investigated scenarios showed deviation from the reference case.

Abstract: The paper consists of two parts. First, Electromechanical Impedance (EMI) method is proven to be able to determine some vibrational characteristics of the investigated structure. In order to verify this statement, Scannig Laser Vibrometry (SLV) is used to correlate frequency peaks of velocity (or displacement) operational deflection shapes with corresponding ones obtained by EMI method. Finally, the influence of moisture content in CFRP sample on resonance peaks is investigated using EMI method. Damage detection indicator in this case is based on shifts of resonant peaks.

Abstract: The reported research concerns experimental investigation toward the monitoring of an aircraft panel. Guided wave propagation phenomena were used to obtain information about the state of the monitored structure. A curved aluminium panel with rivets was investigated. Piezoelectric transducer was used to excite guided waves in chosen structural element. The generated signal was amplified before applying it to the transducer in order to ensure measurable amplitude of excited guided waves. Measurement of the wave field was realized using laser scanning vibrometer that registered the velocity responses at a points belonging to a defined mesh. This contactless measurement technique allowed to investigate phenomena related to wave propagation in the aircraft panel. In the first stage, due to high complexity of the element, baseline measurements were taken. Next, a discontinuity (additional mass) was introduced on the panel surface and the measurements were repeated. Signal processing methods for features extraction from signals were proposed. These features were applied in order to detect and localize the presence anomalies in the investigated panel. The signal processing was conducted in MATLAB with the procedures developed by the authors. The used measurement technology (vibrometer) allowed to register whole wavefield of the propagating guided waves. This allowed to visualize the interaction of the waves with rivets. After introducing the discontinuity on the panel surface wave interaction with it was investigated. Two positions of the additional mass were considered. One just before the riveted stiffener and second after the stiffener. Because of this the influence of the stiffener on the damage detection abilities could be investigated. It can be concluded that the guided wave can be used for monitoring of such complex structures. The vibrometer measurements allowed learn about the guided wave propagation phenomena and perform successful damage localization.

Abstract: Damage detection techniques are becoming more and more important issues for prolonging the lifetime of the old structures. In the paper the strain measurements on offshore oil platform leg model as well as the preliminary measurement system installed on jack-up rig is presented. During experiments performed on frame model of the platform leg different types of damage scenarios were tested. The Fiber Bragg Grating (FBG) strain sensors were capable to detect damage. After laboratory experiments, preliminary FBG system was installed on the real construction (jack-up rig) excited by environmental and operation-induced forces. It is evident that strain measurement based on FBG technology is very promising technique for use in truss/frame structures.

Abstract: In this paper the investigation of a structural health monitoring method for thin-walled parts of structures is presented. The concept is based on the guided elastic wave propagation phenomena. This type of waves can be used in order to obtain information about structure condition and possibly damaged areas. Guided elastic waves can travel in the medium with relatively low attenuation, therefore they enable monitoring of extensive parts of structures. In this way it is possible to detect small defects in their early stage of growth. It is essential because undetected damage can endanger integrity of a structure. In reported investigation piezoelectric transducer was used to excite guided waves in chosen specimens. Dispersion of guided waves results in changes of velocity with the wave frequency, therefore a narrowband signal was used. Measurement of the wave field was realized using laser scanning vibrometer that registered the velocity responses at points belonging to a defined mesh. An artificial discontinuity was introduced to the specimen. The goals of the investigation was to detect it and find optimal sensor placement for this task. Determination of the optimal placement of sensors is a very challenging mission. In conducted investigation laser vibrometer was used to facilitate the task. The chosen mesh of measuring points was the basis for the investigation. The purpose was to consider various configuration of piezoelectric sensors. Instead of using vast amount of piezoelectric sensors the earlier mentioned laser vibrometer was used to gather the necessary data from wave propagation. The signals gather by this non-contact method for the considered network were input to the damage detection algorithm. Damage detection algorithm was based on a procedure that seeks in the signals the damage-reflected waves. Knowing the wave velocity in considered material the damage position can be estimated.

Abstract: In presented research a problem that belongs to the Structural Health Monitoring (SHM) topic was investigated. Special arrays of active sensors were used for damage detection. These sensors were piezoelectric transducers. They were attached to specimen under investigation and used to excite and sense guided elastic waves – Lamb waves. Each array comprised of uniquely placed transducers. The total number of transducers was the same for all considered arrays. This ensured that the same number of signals was used to obtain damage information. A numerical algorithm was proposed to process these signals. It was designed to be independent of sensor arrangement so it could be used for all considered arrays. The principal idea behind the algorithm is that obstacles on a wave path cause wave reflection. These reflections are represented in the time signals. The algorithm was used to associate energy of these reflections with a particular area of the investigated specimen. The value of the energy was extracted from all the signals and projected to coordinate system associated with the specimen edges. In order to test and compare proposed arrays artificial defects were introduced to the specimen to model damaged structure. Because the specimen with defect and signal processing algorithm were the same, the only variable that could influence damage detection was the type of the array. In the investigation damage detection results were obtained for considered arrays. Although the number of sensors were invariable, differences in damage indication exist. This suggest that the type of sensor array should be precisely chosen for a particular application. Even simplest linear array may be sufficient but it depends where we want to apply it.

Abstract: In this paper algorithm for damage localisation in thin panels made of aluminium alloy has been proposed. Mentioned algorithm uses Lamb wave propagation methods and geometrical approach for damage localisation. Elastic waves are generated and received using piezoelectric transducers. Excited elastic waves propagate and reflect from panel boundary and discontinuities existing in the panel. Wave reflection can be registered through the piezoelectric transducers and used in signal processing algorithm. Processing algorithm consists of two parts: signal filtering and extraction of damage location. The first part is used in order to remove noise from received signals. Second part allows to extract arrival time of waves reflected from discontinuity, very often called Time Of Flight (TOF). Localisation algorithm uses pairs of transducers from a concentrated transducers configuration. Using signals from pair of transducers two times of reflection can be extracted from received signals. Because coordinates of transducers are well known ellipse can be constructed based on extracted times of waves reflections. Damage lies one ellipse but it is not known exactly where. Therefore one ellipse is not enough to localise a discontinuity. In order of proper damage localisation more ellipses must be used. In this purpose signals received by larger number of transducers pairs are used in damage localisation algorithm. Points of ellipses intersections allow to indicate localisation of damage. Described signal processing algorithm has been coded in the MATLAB® environment. In this work experimental results has been presented.

Abstract: The aim of this work is the investigation and improvement of a Structural Health Monitoring method based on Lamb waves propagation. This research concentrates on ambiguity in damage localization using attached piezoelectric transducers as sources and sensors of the elastic waves. A linear phased array is chosen as a starting point of the investigation. It has a great advantage in damage localization, namely it enables to amplify the wave reflected from damage, increasing the signal to noise ratio, and precisely indicates not only the distance to damage from the array but also the direction on which the damage lies. However it has also a great disadvantage which needs to be handled – the localization results are symmetric in relation to the line on which the transducers of linear phased array are placed. This obviously does not facilitate Structural Health Monitoring process and precise indication of damage placement. Therefore this investigation aims to improve this localization method by removing the ambiguity in results. In this work the placement of transducers forming a linear phased array is modified to achieve this goal. Several array modification are investigated and compared in order to determine the best solution. Presented research is based on theoretical calculations as well as laboratory experiments on prepared specimens. The measurements are conducted with a compact 13–channel SHM system controlled by a MATLAB® script.

Abstract: This work is focused on two major applications of multi–functional materials. In the first one the use of piezoelectric transducers have been studied in order to monitor the health of composite plate–like structures. These transducers can act as signal sources and sensors for guided elastic waves in inspected structures. The excited waves propagating in the material can reflect from various discontinuities such like: boundaries, notches, cracks and delamination. In the next step the time responses registered by the sensors, as inputs for a signal processing algorithm, may be processed to correlate the measured arriving waves with the discontinuities in the structures enabling one to indicate the location of the discontinuities. In the second application the use of shape memory alloy (SMA) components integrated with composite structural elements are investigated. SMA elements in the forms of wires, strips, ribbons, beams, tubes, etc. can be bonded to, or integrated within, various structural elements in order to control their mechanical properties, static as well as dynamic behaviour. This can be obtained thanks to unique effects associated with thermal activation of SMAs leading to significant changes in SMA material properties, which next can also be applied for control purposes. The use of such controllable properties of SMA components in active control of static (deflection) and dynamic (natural frequencies, modes of vibrations, amplitudes of forced vibrations) characteristics of laminated composite beams–like structures have been demonstrated.

Abstract: A method for damage localisation has been developed, which is based on the phased array idea. Four arrays of transducers, instead of only one, are used to perform a beam-forming procedure. Each array consists of nine transducers placed along a line, which are able to excite and register elastic waves. The arrays are placed in such a way that the angular difference between them is 45º and the rotation point is the middle transducer, which is common for all the arrays. The idea has been tested on a square aluminium plate modelled by the Spectral Finite Element Method. Two types of damage were considered, namely distributed damage, which was modelled as stiffness reduction, and cracks, modelled as separation of nodes in selected finite elements. The plate is excited by a wave packet (5-cycle sine modulated by the Hanning window). The whole array system is placed in the middle of the plate. Each phase array in the system acts independently and produces maps of a scanned field based on the beam-forming procedure. These maps are made of signals that represent the difference between the damaged plate signals and those from the intact plate. An algorithm was developed to join all four maps. This procedure eliminates the necessity to analyse each map individually and also gives the possibility to extract common features only. It allows to remove ambiguity and helps to localise damage more precisely than in the case of a single map. The problem for damage localisation was investigated and exemplary maps confirming the effectiveness of the system proposed were obtained. The investigation is based exclusively on numerical data.