Papers by Keyword: Guided Waves

Abstract: Characteristics of the symmetric coplanar strip line on multi-layer dielectric substrates expressed in analytic formulas have been obtained using conformal mapping. But in most previous presented papers, the metal strip is considered as infinitely thin or PEC while the influence of dielectric substrates are not considered. In this paper, the propagation characteristics of symmetric coplanar strip line with finite conductivity and finite metal strip thickness on multilayer substrates are analyzed by conformal mapping at THz regions. The influences of metal strips and dielectric substrates on the propagation characteristics of CPS are also given. The numerical results are very useful for the development of terahertz devices and terahertz material.

Abstract: Having a robust non-destructive evaluation (NDE) technique for friction stir welded (FSWed) joints is of interest to the processing community. Such a technique has to be sensitive to the different types and shapes of internal weld defects and has to be applicable for both similar and dissimilar material FSW joints. Investigated was the ability of ultrasonic guided waves to detect and assess the quality of FSW joints. The fundamental anti-symmetric (A₀) mode was selected to detect the flaws in FSW joints. Guided waves were excited (using PZT wafers) and received (using a laser Doppler vibrometer, LDV). Implemented was the frequency-wavenumber filtering technique to separate forward propagating wave from any back propagating reflected wave due to the welded joint. Identified was the reflection of the A₀ mode caused by the presence of the interface and/or defects within the joint. The findings indicate little sensitivity to the presence of material interface suggesting this technique to have a promising potential among guided-wave-based techniques in the qualitative and quantitative assessment of FSW joints.

Abstract: A Semi-Analytical Finite Element (SAFE) formulation is applied to determinethe dispersion curves in homogeneous and isotropic cylindrical waveguides subject touniaxial stress. Bulk waves are required for estimating the guided wave dispersion curvesand acoustoelasticity states a stress dependence of the ultrasound bulk velocities. Therefore,acoustoelasticity influences the wave field of the guided waves. Effective Elastic Constants(EEC) has emerged as a less complex alternative to deal with the acoustoelasticity; allowinga stressed material to be assumed as an unstressed material with EEC which considers thedisturbance linked to the presence of stress. In this approach the isotropic specimen subjectto load is studied by proposing an equivalent stress-free with a modified elasticity matrixwhich terms are the EEC. EEC provides an approximate stress-strain relation facilitating thedetermination of the dispersion curves using the well-studied numerical solution for the stressfreecases reducing the complexity of the numerical implementation. Therefore, a numericalmethod combining the SAFE and EEC is presented as a tool for the dispersion curve generationin stressed cylindrical specimens. The results of this methodology are verified by comparingthem with an approach previously reported in the literature based on SAFE including the fullstrain-displacement relation

Abstract: Guided-waves-based diagnostic imaging techniques have been attracting much attention due to their merits including easily interpretable image, high identification accuracy and suitable for online surveillance. In this study, to envisage the difficulty in detecting orientation-specific damage (crack, notch etc.), a novel guided-waves-based diagnostic imaging technique capable of inspecting complicated engineering structures was developed, in terms of the relationship between damage parameters (location, orientation and severity) and extracted guided waves signal features (time-of-flight, signal correlation and signal energy). Experimental studies were performed to verify the developed diagnostic imaging approach, where a through-thickness crack was successfully identified in a metallic plate and a part of real rail structure respectively.

Abstract: In this investigation, a flow rate estimation guided wave based scheme in pipes is proposed. The effect of the fluid over the propagation of longitudinal waves has been experimentally studied by using several laminar flows of water transported by a steel pipe. Results have shown a decrease of the guided wave pattern repeatability and the signal energy as the flow rate increase as a result of the energy leakage from the pipe to the fluid. A Matlab^® script is used to excite the PZT actuator via picoscope 2208 of Picotech^®, the captured signal is acquired also by the picoscope and the data is processed in Matlab. The test bench utilized is composed by a 1” sch 40 A-106 pipe, a needle valve and a centrifugal pump provides the flow energy. A couple of PZTs are used in a picth-catch configuration to produce and capture the longitudinal waves along the cross section of the pipe

Abstract: In this paper, a guided wave temperature robust PCA-based stress monitoring methodology is proposed. It is based on the analysis of the longitudinal guided wave propagating along the path under stress. Slight changes in the wave are detected by means of PCA via statistical T² and Q indices. Experimental and numerical simulations of the guided wave propagating in material under different temperatures have shown significant variations in the amplitude and the velocity of the wave. This condition can jeopardize the discrimination of the different stress scenarios detected by the PCA indices. Thus, it is proposed a methodology based on an extended knowledge base, composed by a PCA statistical model for different discrete temperatures to produce a robust classification of stress states under variable environmental conditions. Experimental results have shown a good agreement between the predicted scenarios and the real ones

Abstract: Detection of damage in rails can be achieved by using ultrasonic guided waves. It is practical that these waves can propagate as lengthy as possible along the rail structure. In this work, the generation of adequate guided waves is considered by performing pertinent laser irradiation of the rail head. Special laser spots were applied through out-of-phase masks collocated on both sides of the rail. Simulation of this excitation configuration was performed by means of the finite element method. It was found that the proposed method enables to trigger sufficiently high stress pulses and generate adequate pattern of waves from a lot of other modes that are likely to exist for a common rail. These waves were found to be sensitive to small localized transverse like defects in the rail system.

Abstract: The theme of the present work is the analysis of the guided waves propagation along a flat, narrow composite plate (beam). The investigated structure is made of the multi - layered composite material (carbon fabric with epoxy resin). The applied material posses the quasi - isotropic mechanical properties. The guided waves are generated with the use of the piezoelectric activator. The finite element method is used in order to carried out the computer simulations. The plain state of the strain is assumed. The computations are performed for the intact and the damaged structure. In the case of the damaged structure the received signal (voltage) is significantly different in comparison with the intact structure. The qualitatively similar results are obtained from the experimental investigations.

Abstract: Ultrasonic Non-destructive testing is a well known technique for inspecting fiber reinforced composite structures however; its capability is severely limited by the high attenuation in thick and multi layer structures. Guided wave ultrasonic inspection has been reported to be useful tool for quantitative identification of composite structures. It takes advantage of tailoring / generating desired ultrasonic wave modes (Symmetric and anti-symmetric) for improved transmission through the composite structure. For this, guided waves have to be generated selectively by precisely placing transducer at an angle to the test surface. Automation of two axis fixture for transmission and reception of transducers have to be used for avoiding manual errors. The captured signals have to be processed in order to extract useful information from the received ultrasonic signals. The proposed project aims at developing automated guided wave inspection methods along with digital signal processing for generating dispersion curves for thick composited. Using test laminates with implanted defects, methodology for thick composite inspection with guided wave ultrasonic’s will be established. For this data will be captured and analyzed using Labview software.

Abstract: Ultrasonic guided waves can propagate a long distance in pipeline with little attenuation. This means the damage in nuclear power plant can be detected from a remote single position. In the paper, the propagation of the guided waves are analyzed for the nuclear power plant pipes, and the axisymmetric torsional mode T(0,1) is chosen as the detection mode. An imaging method based on the synthetic focusing algorithm is used to obtain the damage information. The method is then verified by the finite element model. Results illustrate that the damage can be detected and located accurately by the damage imaging method. Not only the axial position, but also the circumferential position can be located simultaneously.