Search

• The aim of the present work is to develop a multi-mode imaging technique that will allow for identification of damage size and location using ultrasonic guided waves. • The proposed technique combines a reverse-time migration (RTM) imaging algorithm with a 3D wave propagation simulator using different wave modes. • This combination enables the separation of multiple modes using the wavefield filtering techniques, potentially providing more information to damage types. • Without the limitation of generating single dominant mode waves, wide frequency ranges are accessible, enabling optimal frequencies for a variety of ultrasonic data acquisition systems. The sensitivity of Lamb wave modes to a particular defect or instance of damage is dependent on various factors (e.g., the local strain energy density due to that wave mode). As a result, different modes will be more useful than others for damage detection and quantification, dependent on damage type and location. For example, prior work in the field has shown that out-of-plane modes may have a higher sensitivity than in-plane modes to surface defects in plates. The excitability of a certain data acquisition system and the corresponding resolution for damage imaging also varies with frequency. The aim of the present work was to develop a multi-mode damage imaging technique that enables characterization of damage type and size, general sensitivity to unknown damage types, higher resolution imaging, and detectability regardless of the data acquisition system used. A reverse-time migration (RTM) imaging algorithm was combined with a numerical simulator—the three-dimensional (3D) elastodynamic finite integration technique (EFIT)—to provide multi-mode damage imaging. The approach was applied to two simulated case studies featuring damaged isotropic plates. Sensitivities of damage type to wave mode were investigated by separating the A 0 and S 0 Lamb wave modes obtained from the resultant RTM wavefields. [ABSTRACT FROM AUTHOR]

• Study of varying temperature effect on guided wave (GW) propagation in a CFRP panel. • Calculation of a temperature-independent compensation factor for numerical data. • Validation of different FE modelling approaches for GW through dispersion curves. • Numerical and experimental GW S 0 and A 0 modes dispersion curves under thermal loads. • Assessment of S 0 and A 0 modes group velocity sensitivity to temperature variations. Guided wave (GW) structural health monitoring (SHM) systems offer an attractive solution as an in - situ quasi real-time assessment of structural damage, but their sensitivity and efficiency may be impaired under varied environmental and operational conditions. Thus, virtual tests, such as that based on the Finite Element (FE) method, represent a valid approach for simulating and investigating SHM systems, enabling a substantial reduction in experimental campaigns. In this work, GW propagation characteristics in a carbon fibre-reinforced composite plate are studied under a varying temperature condition, representative of the aeronautics application. At first, GW SHM system was physically tested at room temperature (20 ° C), and the results were used to calibrate and assess the proposed FE modelling approaches, characterised by different element types and mesh sizes. A temperature independent averaged time compensation factor is proposed to mitigate the numerical data dependency on excitation frequency and propagation angle. Two temperature variations (from 20 ° C to - 50 ° C , and 20 ° C to 65 ° C) were experimentally and numerically considered to investigate the effect of varying temperature on the GW. For all test cases, the compensated numerical data was compared to the experimental results, and discussed in terms of dispersion curves, focusing on the zero-order symmetric, S 0 , and antisymmetric, A 0 , modes. Results show that both 2D and 3D FE approaches can accurately predict the changes in GW due to varying temperature, with the group velocity of the A 0 mode being less sensitive to temperature variations. [ABSTRACT FROM AUTHOR]

Interaction of ultrasonic guided waves with kissing bonds (closed delaminations and incipient surface breaking cracks) gives rise to nonlinear features at the defect location. This causes higher harmonic frequency ultrasonic radiation into the ambient air, often referred to as Nonlinear Air-Coupled Emission (NACE), which may serve as a nonlinear tag to detect the defects. This paper summarizes the results of a numerical implementation and simulation study of NACE. The developed model combines a 3D time domain model for the nonlinear Lamb wave propagation in delaminated samples with a spectral solution for the nonlinear air-coupled emission. A parametric study is conducted to illustrate the potential of detecting defect location, size and shape by studying the NACE acoustic radiation patterns in different orientation planes. The simulation results prove that there is a good determination potential for the defect parameters, especially when the radiated frequency matches one of the resonance frequencies of the delaminated layer, leading to a Local Defect Resonance (LDR). [ABSTRACT FROM AUTHOR]

[Display omitted] • Instantaneous phase characteristics of guided waves contain a rich source of information for defect characterisation. • Gaussian process regression technique is capable of predicting the real defect depth with a large-scale simulated dataset. • Gaussian process regression can successfully estimate the depth of wide defects (≥100% of wavelength) in practice. Ultrasonic Guided Waves (UGW) offer the possibility of inspecting a strip across a structure rather than just the point under a traditional bulk wave transducer. This can increase the rate of inspection and enable inspection under obstructions. This paper investigates the instantaneous phase characteristics of the shear horizontal guided waves for various defect depths and widths. The Gaussian process regression is then evaluated for estimating the minimum remaining wall thickness between a pair of transducers. A Gaussian process regression model is built using the fusion of large-scale simulated and low-scale real experimental data. For this purpose, a more precise model of an electromagnetic acoustic transducer is initially built by integrating both electromagnetic and elastic wave fields. Then the simulated data set is built after having been calibrated using a genetic algorithm. The examination of an unseen simulated evaluation data set shows that 96 % of data has an error during thickness gauging of less than 10 per cent of wall thickness. Finally, an experimental testing data set containing three different defects with depths of 3.7, 5.7 and 9.2 mm was examined, resulting in a good depth prediction of large defects with less than 1 mm error for defects wider than one wavelength. [ABSTRACT FROM AUTHOR]

In this study, the effect of friction on the generation of dry-coupled Lamb waves is experimentally investigated. Recurrence analysis is performed to analyze the complex behavior of friction based dry-coupled Lamb waves. In particular, the effect of the normal force, which is necessary for a stronger dry-coupled Lamb wave generation and the friction, on the transmission of mechanical energy and determinism characteristics of Lamb waves are investigated. The results verify that larger friction coefficient and friction force are crucial for generation and propagation of strong Lamb waves supporting the fact that the main mechanism to transfer mechanical energy using dry-couplant is friction. The sensitivity of Lamb waves to the friction coefficient, highlights the importance of designing specific pads with respect to condition of the surface. Besides, the results show that the normal force and friction coefficient can change the determinism characteristics behavior of multimode Lamb waves. Furthermore, it is shown that the determinism value is sensitive to the friction coefficient and normal force. A similar trend is observed in the determinism values and friction coefficient. In general, a smaller friction coefficient indicates smaller determinism value. Additionally, it is shown that a normal load can change the behavior of a system, as observed from recurrence plots, owing to changes in the Lamb waves trajectories in the phase-space domain. In addition, it is shown that recurrence plots enable the detection of mode transitions in multimode Lamb waves. Recurrence analysis is a complementary tool to frequency domain methods for accurate analysis of multimode Lamb waves behavior. • The effect of friction and normal force in exciting dry-coupled Lamb waves is studied. • To increase the SNR the friction between pads and surface has to be maximized. • Recurrence analysis is applied to analyze dry-coupled ultrasonic Lamb waves. • It is shown that the determinism of ultrasonic Lamb waves and friction are correlated. • Recurrence plot is successfully used for Lamb wave modes arrival detection. [ABSTRACT FROM AUTHOR]

In ultrasonic guided wave inspections, incident waves propagate as a finite-bandwidth wave packet. During propagation, the shape of the wave packet is distorted because of dispersion even if there are no viscous or scattering effects. Although the presence of distortion is well known, how the wave packet is distorted has not been sufficiently investigated. We use the method of multiple scales (MMS), a perturbative scheme, to investigate the dispersion of the Lamb waves comprising the finite-bandwidth wave packet. Solutions obtained by the MMS to order O (ε 2) show that the shape of an input wave-packet remains invariant during propagation along the waveguide. The wave packet propagates with a group velocity obtained from a classical analysis. The O (ε 3) -order MMS solutions exhibit a symmetric distortion of the shape, whereas the O (ε 4) -order MMS solutions feature symmetric and anti-symmetric distortion. For the derivation of the O (ε 3) - and O (ε 4) -order MMS solutions, the Fourier transform of the wave-packet shape is introduced. The proposed MMS solutions were verified by comparing them with the numerical time-domain solutions constructed from the classical frequency-domain solutions. The strength of the distortion depends on propagation mode, input frequency, and shape of the input wave packet. Because the wave-packet distortion induces a shift and reduction of the peak, the propagation velocity is different from the group velocity. For long-distance inspections, the proposed method provides the precise propagation velocity and details of the propagation-mode selection. • The method of multiple scales is applied to analysis of the dispersion. • The strength of the distortion depends on propagation mode, input frequency, and shape of the input wave packet. • Using the MMS, we obtained insights into distorting the shape of the wave packet. [ABSTRACT FROM AUTHOR]

• Guided ultrasonic wavefield measurements performed on quasi-isotropic CFRP laminate. • Scattering at artificial, ellipse delamination, located at an asymmetric depth. • Wave trapping on top of delamination and forward scattered wave verified. • Numerical, Finite Element simulations validated from experiments. • Parameter study of guided wave scattering at different delamination shape and depth. Carbon fibre composite laminates are increasingly being used for aerospace structures due to their low weight and improved mechanical performance. Impact damage can cause delaminations below the visible surface of the structure due to limited interlaminar strength. Guided ultrasonic waves can detect and characterize delaminations in composite laminates. The scattering of the A 0 Lamb wave mode at an artificial delamination, located at an asymmetric depth in a quasi-isotropic laminate, was investigated. Full field non-contact laser measurements were used to visualise wave trapping and scattered waves. A three-dimensional finite element model was developed and validated against the experiments. The influence of delamination shape and depth on guided wave scattering were studied. Small variations in delamination shape significantly affected the interference pattern on top of the delamination, but had limited effect on the scattered wave outside the delamination. Delamination depth was found to strongly influence the angular direction and amplitude of scattered waves. Implications for structural health monitoring were discussed. [ABSTRACT FROM AUTHOR]