Search

Ultrasonic guided wave is a promising technique for structural health monitoring and nondestructive testing. However, due to the anisotropy and complexity of composite materials, the imaging performance of numerous signal processing methods deteriorates with significant artifacts and unsatisfactory accuracy. To obtain a better damage imaging performance of Lamb waves in noisy and noise-free conditions, a weighted delay-and-sum (DAS) imaging method based on denoising autoencoder (DAE) learning is developed for complex composite structures. The traditional DAS formulation is modified to be more compatible with anisotropic materials. The DAE with feature learning capability is then employed to extract potentially efficient features and remove noise from ultrasonic signals. Several verification experiments conducted on flat or curved and stiffened composite structures have confirmed the ability of the DAE-DAS method to suppress artificial noise and to intensify singularities induced by the anomalies. By comparing with the unweighted DAS methods and the weighted DAS method without feature extraction, the proposed algorithm has satisfactory robustness to achieve higher localization accuracy and fewer artifacts. [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]