5,456 research outputs found

    A General Bayesian Framework for Ellipse-based and Hyperbola-based Damage Localisation in Anisotropic Composite Plates

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    This paper focuses on Bayesian Lamb wave-based damage localization in structural health monitoring of anisotropic composite materials. A Bayesian framework is applied to take account for uncertainties from experimental time-of-flight measurements and angular dependent group velocity within the composite material. An original parametric analytical expression of the direction dependence of group velocity is proposed and validated numerically and experimentally for anisotropic composite and sandwich plates. This expression is incorporated into time-of-arrival (ToA: ellipse-based) and time-difference-of-arrival (TDoA: hyperbola-based) Bayesian damage localization algorithms. This way, the damage location as well as the group velocity profile are estimated jointly and a priori information taken into consideration. The proposed algorithm is general as it allows to take into account for uncertainties within a Bayesian framework, and to model effects of anisotropy on group velocity. Numerical and experimental results obtained with different damage sizes or locations and for different degrees of anisotropy validate the ability of the proposed algorithm to estimate both the damage location and the group velocity profile as well as the associated confidence intervals. Results highlight the need to consider for anisotropy in order to increase localization accuracy, and to use Bayesian analysis to quantify uncertainties in damage localization.Projet CORALI

    Guided wave propagation and skew effects in anisotropic carbon fiber reinforced laminates

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    Guided ultrasonic waves provide a promising structural health monitoring (SHM) solution for composite structures as they are able to propagate relatively long distances with low attenuation. However, the material anisotropy results in directionally dependent phase and group velocities, in addition to energy focusing, wave skewing, and beam spreading phenomena. These effects could lead to inaccurate damage localization if not accounted for. In this contribution, the guided wave propagation behavior (A0 mode) for a highly anisotropic, unidirectional carbon fiber reinforced polymer laminate is systematically investigated through both finite element analysis and non-contact laser measurements and compared to theoretical predictions. The directional dependency of phase and group velocity measured for a point and line source shows good agreement with theoretical predictions, once a correction for wave skew effects is applied. Wave skew angles were evaluated from the experimental and numerical wave propagation in multiple directions and matched theoretical predictions based on the phase slowness curve. Significant guided wave beam spreading from a line source was observed and quantified from both experiments and simulations and compared with theoretical predictions using the anisotropy factor. The impact of anisotropic guided wave propagation behavior on SHM is discussed

    Numerical prediction of temperature effect on propagation of rubbing acoustic emission waves in a thin-walled cylinder structure

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    Temperature field has serious effects on the accuracy of rubbing acoustic emission (AE) source localization in a thin-walled cylinder structure, but it is difficult to explore the functioning mechanism through experiments. This paper aims to propose a thermos-elastic coupling simulation procedure to reveal the effect of the uniform temperature and non-uniform temperature field on the propagation characteristics of AE waves. To obtain the behaviors of guiding wave in the thin-walled cylinder, an efficient numerical simulation tool for AE wave propagation modeling is explored. The numerical results of AE propagation in a plate are compared with the experimental data. Then the semi-analytical finite element method is introduced to calculate the characteristics of multi-modal and dispersion. To remove the unwanted reflections from boundaries generated by the numerical simulation, a methodology combined with the infinite element and Rayleigh damping is presented. Consequently, several AE wave propagation simulations are carried out respectively, including the model with the uniform temperature in a range of 20-700 °C, and the non-uniform temperature field with the temperature of the central region, 649 °C. On the basis of the modeling and evaluation results, both the peak-to-peak amplitude and arrival time versus temperatures are summarized and analyzed. The validation results demonstrate that the proposed approach could be used efficiently to research rubbing AE source localization applications with a high degree of accuracy

    Damage identification in FRP-retrofitted concrete structures using linear and nonlinear guided waves

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    Structural health monitoring (SHM) involves the implementation of damage identification methods in engineering structures to ensure structural safety and integrity. The paramount importance of SHM has been recognised in the literature. Among different damage identification methods, guided wave approach has emerged as a revolutionary technique. Guided wave-based damage identification has been the subject of intensive research in the past two decades. Meanwhile, applications of fibre reinforced polymer (FRP) composites for strengthening and retrofitting concrete structures have been growing dramatically. FRP composites offer high specific stiffness and high specific strength, good resistance to corrosion and tailorable mechanical properties. On the other hand, there are grave concerns about longterm performance and durability of FRP applications in concrete structures. Therefore, reliable damage identification techniques need to be implemented to inspect and monitor FRPretrofitted concrete structures. This thesis aims to explore applications of Rayleigh wave for SHM in FRP-retrofitted concrete structures. A three-dimensional (3D) finite element (FE) model has been developed to simulate Rayleigh wave propagation and scattering. Numerical simulation results of Rayleigh wave propagation in the intact model (without debonding at FRP/concrete interface) are verified with analytical solutions. Propagation of Rayleigh wave in the FRP-retrofitted concrete structures and scattering of Rayleigh waves at debonding between FRP and concrete are validated with experimental measurements. Very good agreement is observed between the FE results and experimental measurements. The experimentally and analytically validated FE model is then used in numerical case studies to investigate the scattering characteristic. The scattering directivity pattern (SDP) of Rayleigh wave is studied for different debonding size to wavelength ratios and in both backward and forward scattering directions. The suitability of using bonded mass to simulate debonding in the FRP-retrofitted concrete structures is also investigated. Besides, a damage localisation method is introduced based on the time-of-flight (ToF) of the scattered Rayleigh wave. Numerical case studies, involving different locations and sizes of debonding, are presented to validate the proposed debonding localisation method. Nonlinear ultrasonics is a novel and attractive concept with the potential of baseline-free damage detection. In this thesis, nonlinear Rayleigh wave induced at debondings in FRPretrofitted concrete structures, is studied in detail. Numerical results of nonlinear Rayleigh wave are validated with experimental measurements. The study considers both second and third harmonics of Rayleigh wave. A very good agreement is observed between numerical and experimental results of nonlinear Rayleigh wave. Directivity patterns of second and third harmonics for different debonding size to the wavelength ratios, and in both backward and forward scattering directions, are presented. Moreover, a damage image reconstruction algorithm is developed based on the second harmonic of Rayleigh wave. This method provides a graphical representation for debonding detection and localisation in FRP-retrofitted concrete structures. Experimental case studies are used to demonstrate the performance of the proposed technique. It is shown that the proposed imaging method is capable of detecting the debonding in the FRP-retrofitted concrete structures. Overall, this PhD study proves that Rayleigh wave is a powerful and reliable means of damage detection and localisation in FRP-retrofitted concrete structures.Thesis (Ph.D.) -- University of Adelaide, School of Civil, Environmental and Mining Engineering, 201

    Triple correlation for detection of damage-related nonlinearities in composite structures

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    Nonlinear effects in vibration responses are investigated for the undamaged composite plate and the composite plate with a delamination. The analysis is focused on higher harmonic generation in vibration responses for various excitation amplitude levels. This effect is investigated using the triple correlation technique. The dynamics of composite plate was modelled using two-dimensional finite elements and the classical lamination theory. The doubled-node approach was used to model delamination area. Mode shapes and natural frequencies were estimated based on numerical models. Next, the delamination divergence analysis was used to obtain relative displacements for delaminated plies. Experimental modal analysis test was carried out to verify the numerical models. The two strongest vibration modes as well as two vibration modes with the smallest and largest motion level of delaminated plies were selected for nonlinear vibration test. The Fisher criterion was employed to verify the effectiveness and confidence level of the proposed technique. The results show that the method can be used not only to reveal nonlinearities, but also to reliably detect impact damage in composites. These results are confirmed using the statistical analysis

    Guided Wave Energy Focusing and Steering in Composite Laminates

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    Lightweight, carbon fiber reinforced composites are often selected for aerospace components but are prone to barely visible impact damage, caused by low velocity impacts, during service. Guided-wave-based structural health monitoring (SHM) techniques can efficiently detect impact damage impact in composite structures. However, wave propagation is influenced by material anisotropy resulting in a number of effects. The phase and group velocity of propagating wave modes depend on the wave launching direction, with increased wave speeds in the high stiffness (fiber) directions. Wave energy tends to be focused along the fiber directions, resulting in beam steering or skewing away from the initial wave launching direction. These anisotropic effects, if unaccounted for, could lead to inaccurate localization of damage, and potential regions of the structure where guided waves cannot propagate with sufficient amplitude, reducing damage sensitivity. Wave propagation in an undamaged unidirectional carbon fiber reinforced polymer (CFRP) panel was investigated for the A0 mode for multiple wave launching directions. Finite Element (FE) modelling was carried out using homogenized anisotropic material properties to investigate the directional dependency of velocity. Point and line sources were modelled to investigate the influence of the excitation source on the guided wave evaluation and signal processing. Wave skewing behavior was visualized for the line source, and wave skew angles and beam spread angles were calculated for a range of propagation angles. Experimental non-contact guided wave measurements were obtained using a laser vibrometer. A PZT strip transducer was developed in order to measure wave skew angles. Experimental and numerical velocities and skew angles were compared with theoretical predictions and good agreement was observed

    Large plate monitoring using guided ultrasonic waves

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    Areas of stress concentration around welded structures are likely to lead to fatigue cracks and corrosion pitting during the life time of technical machinery. Performing periodical non-destructive testing of the critical area is crucial for the maintenance of structural integrity and the prevention of unforeseen shutdowns of the system. Low frequency guided ultrasonic waves can propagate along thin structures and allow for the efficient testing of large components. Structural damage can be localized using a distributed array of guided ultrasonic wave sensors. Guided waves might be employed to overcome the accessibility problem for stiffened plate structures where access to some parts of the inspected structure is not possible. The transmission and reflection of the A0 Lamb wave mode for a variation of the stiffener geometry and excitation frequency was investigated numerically and verified experimentally. The dispersive behaviour of the guided waves has been studied to ascertain a frequency thickness product that provides limited pulse distortion. The limitations of the plate geometry as well as the excitation and monitoring locations were discussed. The radial spreading of the incident, transmitted and reflected waves from a stiffener has been investigated. The efficient quantification of the transmitted and reflected waves from the stiffener for a wide range of angles has been obtained from a single Finite Element model containing two parallel lines of nodes in front of and past the stiffener. The research outcomes have shown the dependency of the scattered wave on the incident angle and stiffener dimensions. Reasonably good A0 wave mode transmission was obtained from the oblique wave propagation (up to an angle of 45o) across realistic stiffener geometries. The choice of an optimum excitation frequency, which can ensure maximum transmission across the stiffener for specific plate geometry, was recommended. The ability for defect detection in inaccessible areas has been investigated numerically and validated experimentally. The possibility of detecting and characterizing the reflection of a guided wave pulse (A0 mode) from a through-thickness notch located behind the stiffener has been discussed. Two different approaches, based on the access to the sides of the stiffener on the plate, were employed. The limitations of the detectable defect size and location behind the stiffener have been investigated. The energy of the transmitted wave across the stiffener was adequate to detect simulated damage behind the stiffener. The evaluation has shown that defect detection in inaccessible areas behind stiffeners is achievable if the signal-to-noise ratio is high enough. In experimental measurements the noise level was of similar magnitude to the observed reflections at the defect. Thus, there is necessity to enhance the signal-to-noise ratio in experimental measurements
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