Adhesive bonding is being increasingly employed in many applications as it offers possibility of light-weighting and efficient multi-material joining along with reduction in time and cost of manufacturing. However, failure initiation and progression in critical components like joints, specifically in fatigue loading is not well understood, which necessitates reliable NDE and SHM techniques to ensure structural integrity. In this work, concurrent guided wave (GW) and fiber Bragg grating (FBG) sensor measurements were used to monitor fatigue damage in adhesively bonded composite lap-joints. In the present set-up, one FBG sensor was strategically embedded in the adhesive bond-line of a lap-joint, while two other FBGs were bonded on the surface of the adherends. Full spectral responses of FBG sensors were collected and compared at specific intervals of fatigue loading. In parallel, guided waves were actuated and sensed using PZT wafers mounted on the composite adherends. Experimental results demonstrated that time-of-flight (ToF) of the fundamental modes transmitted through the bond-line and spectral response of FBG sensors were sensitive to fatigue loading and damage. Combination of guided wave and FBG measurements provided the desired redundancy and synergy in the data to evaluate the degradation in bond-line properties. Measurements taken in the presence of continuously applied load replicated the in-situ/service conditions. The approach shows promise in understanding the behavior of bonded joints subjected to complex loading.

A theoretical framework is established to study the interaction between circular-crested Lamb waves (CCLWs) and a fiber-optic ultrasound sensor (FOUS) bonded onto the surface of a plate structure. CCLW is a universal expression of guided Lamb waves, regardless of the source-sensor distance. In the theory developed here, strain transfer and transformation, birefringence of the fiber sensor, and modulation of refractive index by elasto-optic effects are incorporated. Explicit expression for calculating the ultrasound-induced wavelength shift of fiber Bragg gratings (FBGs) is derived. To showcase application of the established theory in a more general situation, a chirped FBG Fabry-Perot interferometer based FOUS is simulated, in combination with transfer matrix method. Results uncover quantitatively the interplay of the following factors governing the sensor response: 1) co-existence of both axial and lateral normal strains in the fiber, especially in the near field; 2) elasto-optic properties of the fiber material; 3) initial polarization state of the fiber sensor; 4) incident angle of the CCLWs; and 5) ratio between the equivalent ultrasonic wavelength and the effective sensor length. The calculated results uncover the underlying mechanisms responsible for several experimental observations reported but inadequately explained in the literature. The theoretical framework is also applicable to other non-FBG-based FOUSs that rely on phase modulation by ultrasounds.

Modern aerospace engineering places increasing emphasis on materials that combine low weight with high mechanical performance. Fiber metal laminates (FMLs), which merge metal layers with fiber-reinforced composites, meet this demand by delivering improved fatigue resistance, impact tolerance, and environmental durability, often surpassing the performance of their constituents in demanding applications. Despite these advantages, inspecting such thin, layered structures remains a significant challenge, particularly when they are difficult or impossible to access. As with any new invention, they always come with challenges. This study examines the effectiveness of the fundamental anti-symmetric Lamb wave mode (A0) in detecting weak interfacial defects within Carall laminates, a type of hybrid fiber metal laminate (FML). Delamination detectability is analyzed in terms of strong wave dispersion observed downstream of the delaminated sublayer, within a region characterized by acoustic distortion. A three-dimensional finite element (FE) model is developed to simulate mode trapping and full-wavefield local displacement. The approach is validated by reproducing experimental results reported in prior studies, including the author’s own work. Results demonstrate that the A0 mode is sensitive to delamination; however, its lateral resolution depends on local position, ply orientation, and dispersion characteristics. Accurately resolving the depth and extent of delamination remains challenging due to the redistribution of peak amplitude in the frequency domain, likely caused by interference effects in the acoustically sensitive delaminated zone. Additionally, angular scattering analysis reveals a complex wave behavior, with most of the energy concentrated along the centerline, despite transmission losses at the metal-composite interfaces in the Carall laminate. The wave interaction with the leading and trailing edges of the delaminations is strongly influenced by the complex wave interference phenomenon and acoustic mismatched regions, leading to an increase in dispersion at the sublayers. Analytical dispersion calculations clarify how wave behavior influences the detectability and resolution of delaminations, though this resolution is constrained, being most effective for weak interfaces located closer to the surface. This study offers critical insights into how the fundamental anti-symmetric Lamb wave mode (A0) interacts with delaminations in highly attenuative, multilayered environments. It also highlights the challenges in resolving the spatial extent of damage in the long-wavelength limit. The findings support the practical application of A0 Lamb waves for structural health assessment of hybrid composites, enabling defect detection at inaccessible depths.

This paper presents a dual-mode concentrical sensor for flaw detection in fiber-reinforced polymer (FRP) composites using eddy current testing (ECT) and capacitive imaging (CI). Combining ECT and CI techniques provides the necessary synergy to detect damage in both electrically conductive and insulative FRP materials. The proposed sensor, featuring rotational symmetry, comprises a central pancake coil and an outer ring coil, both printed on a multi-layered printed circuit board (PCB). When configured for ECT, the coils serve as a means for magnetic field excitation and pickup, while in CI mode, the central pancake coil and the outer ring coil function as charged electrodes of the disk-and-ring capacitor. A dual-mode array probe is created using staggered linear arrays of individual sensors. Experimental validation involves interfacing the array with the Fanuc robotic arm and the Ectane 2 multichannel data acquisition system. Initial results show the capability to detect volumetric flaws, fiber breakage, and interlaminar delaminations in carbon FRP.