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This paper investigates the scattering of the fundamental anti-symmetric Lamb wave mode by a delamination along the edge of a quasi-isotropic fiber-composite laminate. Delaminations pose a serious threat to the structural integrity of composite laminates as they often show no visible signs of damage on the surface. Due to the anisotropic nature of composites, analytical solutions are often complicated to derive for such problems. This investigation uses 3D Finite Element (FE) analysis and experimental tools to characterize the scatter by a mid-plane delamination in composites. A 5.5 cycle Hann windowed wave train was used as the input signal for both FE and experiments. A 2D Fast Fourier Transform was used to identify the mode and amplitude of the scattered field along various scattered directions. The experimental wave field image was captured using a 3D Laser Vibrometer. The results indicate that the scatter pattern is much more complex than previously obtained results for an isotropic plate. Even for the simple case of mid-plane delamination, the fiber steering effects in the outermost plies have a significant effect on the scattering pattern, relative to the isotropic case where the dominant part of the scattered field is an edge-guided wave. Computational and experimental results are presented for the scattering pattern and scattering amplitude as a function of damage size relative to wavelength. The implications of these results for the optimal placement of actuators and sensors for Structural Health Monitoring (SHM) purposes, and for devising effective approaches for the inverse problem of damage detection and sizing, are briefly discussed. [ABSTRACT FROM AUTHOR]

The complex design of composite structures makes it very difficult for conventional visual inspection techniques to detect defects in these materials. Therefore, numerous types of nondestructive testing and structural health monitoring techniques have been developed for damage identification in composite structures, among which ultrasonic guided waves, particularly Lamb waves, have gained much popularity. In recent years, apart from piezoelectric pointwise measurements, laser Doppler vibrometry has been used for full wavefield measurements of propagating Lamb waves. Damage imaging can be done with animations of Lamb waves interacting with defects. In this work, two end-to-end deep learning-based models of many-to-one sequence prediction were developed to perform pixel-wise image segmentation. A large dataset of full wavefield images resulting from the interaction with delamination of random size, shape, and location was utilised to train the proposed deep learning models. The main goal of this work is to investigate the feasibility of implementing deep learning methods for delamination identification in composite laminates by only utilising the animations of Lamb waves. The elaborated models worked well on the numerically generated test data and have also shown good performance on the real-world experimental data. These methods can enable the automation of delamination identification and to create damage maps without the intervention of the user. [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]

In this study, experimental and numerical approaches based on the nonlinear ultrasonic Lamb wave are used to detect the barely visible impact damages (BVID) in orthotropic CFRP laminates due to low velocity impact (LVI). A drop-weight impact test system and a RITEC nonlinear ultrasonic system are adopted for experimentally measuring the higher harmonics of lamb wave in the damaged CFRP laminates. An integrated finite element (FE) model is proposed to further identify the interaction between Lamb wave and impact damages, in which Hashin criterion and triangular cohesive zone model (TCZM) are used to predict the initiation and evolution of delamination and other damages under LVI. Then, the displacement–time signal and the relative acoustic nonlinear parameters (RANP) of the Lamb wave excited by sinusoidal displacement load are extracted to evaluate the damage in the same FE model. Finally, the relevance between LVI damage and RANP is investigated by numerical and experimental analyses under LVI with different energies, and shows that the amplitudes of harmonics and RANP increase with the raise of the impact energy, as well as the delamination areas. Therefore, the amplitude of harmonics and RANP can be used as indicators to detect BVID. [ABSTRACT FROM AUTHOR]