Abilasha Ramdhas

Abstract: Recent research by the authors shows that bends in plates can act as features that can concentrate and guide ultrasonic energy along their axis. At low frequencies, two feature-guided modes are identified when the bent plate is subjected to ‘in-plane’ or axial excitation applied uniformly along a through-thickness line bisecting the bent edge. Of these, the slower mode has properties similar to the A0 (fundamental antisymmetric) Lamb mode in flat plates. This paper focuses on the faster bend-guided mode that is similar to the S0 (fundamental symmetric) Lamb mode in flat plates. Using 3D finite element (FE) simulation validated with experiments, this mode is shown to be more strongly generated in smaller angle bends. Features of the mode including velocity, attenuation and modal structure are considered in detailed studies. Results are discussed in light of simple modal studies using the Semi Analytical Finite Element (SAFE) method, suggesting a relationship of bend-guided waves to modes of curved bars.

Abstract: The influence of bends constituting annular polygonal structures on ultrasonic guided waves propagating along their axis is investigated. Considering a single bend as a bent plate connects this problem to the better-understood physics of guided waves in straight plates. Using a three-dimensional finite element simulation validated with experiments, bends in plates are shown to act as features that can concentrate and guide ultrasonic energy along their length. Two interesting feature-guided modes are identified when the bent plate is subjected to “in-plane” or axial excitation applied uniformly along a through-thickness line bisecting the bent edge. Of these, the faster traveling mode has properties similar to, but travels at group velocities lower than, the S0 (fundamental symmetric) Lamb mode in flat plates. This paper however focuses on the slower bend-guided mode that is similar to the A0 (fundamental anti-symmetric) Lamb mode in flat plates. This mode is shown to be more strongly generated in smaller...

Abstract: This paper proposes a new method for detecting and locating delaminations in laminated composite beams using nonlinear guided wave. It is shown that when incident wave interacts at the delamination, the nonlinear effect of wave interaction with contact interfaces at the delamination generates higher harmonic guided waves due to contact acoustic nonlinearity (CAN). The proposed method employs a transducer network to detect and locate the delamination using the higher harmonic guided waves. A sequential scan is used to inspect the laminated composite beams by actuating the fundamental anti-symmetric mode (A0) of guided wave at one of the transducers while the rest of the transducers are used for measuring the impinging waves. A series of numerical case studies are performed using three-dimensional explicit finite element simulations, which consider different delamination locations, lengths and through-thickness locations. Experimental case studies are carried out to further validate and demonstrate the capability of the proposed method. The results show that the proposed method is able to accurately detect and locate the delamination in the laminated composite beams using the higher harmonic guided waves. One of the advantages of the proposed method is that it does not rely on baseline data to detect and locate the delamination, and hence, it has less influence by varying operational and environmental conditions.

Abstract: Complex-shaped composite components have been extensively incorporated as reinforcing structures in the aerospace industry. Various types of damages can be initiated in these structures due to the stress concentration and out-of-plane impacts during the in-service use, which have to be detected timely in case they propagate at subsurface laminae and ultimately lead to catastrophic failure. This paper explores the feasibility of using ultrasonic feature guided waves (FGW) for rapid screening of typical 90° bends made of quasi-isotropic composite laminates. Such FGWs are capable of focusing the propagation energy along the feature, with limited leakage to the adjacent plate. Modal studies of the composite bent plate are carried out by applying the Semi-Analytical Finite Element (SAFE) method, revealing properties of the FGWs that exist in the structure. A shear horizontal type bend-guided mode has been identified as a promising candidate. The mode is almost non-dispersive and non-leaky with strong energy confinement in the bend region, which is attractive to be applied as a screening tool for composite bends. Both 3D Finite Element (FE) simulations and experiments are performed to study the interaction of the identified FGW mode with different defects occurred in the bend region such as the interlaminar delamination and the transverse crack, showing good agreement. The wave-defect resonance phenomenon and the reflection behavior are investigated for localizing these two types of defects, and the potential of the FGW for efficient damage detection in composite bends is well demonstrated.

Abstract: This paper presents the implementation of holey structured metamaterial lens for ultrasonic characterization of subwavelength subsurface defects in metallic components. Experimental results are presented, demonstrating ultrasound-based resolution of side drilled through-holes spaced (λ/5) in an aluminum block. Numerical simulation is then used to investigate the parameters that can help improve the resolution performance of the metamaterial lens, particularly, the addition of end-conditions. This work has important implications for higher resolution ultrasonic imaging in the context of practical non-destructive imaging and non-invasive material diagnostics.

Abstract: Adhesive bonding is widely used in aerospace composite structures. A continuous well-cured bond can offer good joint strength and improved fatigue and impact resistance, and is therefore crucial to the performance of the entire structure. This paper explores the feasibility of using feature guided waves (FGW) for rapid screening of the bond line between a stiffener and a carbon fiber reinforced polymer (CFRP) composite panel. Such FGWs are capable of focusing the wave energy along the stiffener and the bond layer, with limited radiation to the adjacent plate. The Semi-Analytical Finite Element (SAFE) approach is employed to understand the modal properties of FGWs that exist in the structure, and criteria are suggested to choose proper mode-frequency combination that is sensitive to adhesive defects. A shear horizontal type FGW mode is identified to be well suited, as it is easy to excite, and propagates with little dispersion and relatively low attenuation, while it retains sufficient energy around the bond layer. Both 3D Finite Element (FE) simulations and experiments are performed to study the interaction of the selected FGW mode with defects in the adhesive bond, and the results show excellent agreement. The reflection behavior and the wave-defect resonance phenomenon are investigated, which demonstrate the capability of the FGW for the bond line inspection.

Abstract: This article reports on the characterization of delamination damages in composite laminates using wave visualization method. A combination of plate-guided ultrasound and air-coupled ultrasonics is used to locate and visualize delaminations. The study focuses on the physics of Lamb wave propagation and interaction with delaminations at various through-thickness locations and positions. Three-dimensional finite element simulations are used to study, in detail, the changes in wave features such as mode velocity, wavelength and wave refraction in the delamination region. These wave features provide information on the location, position and orientation of the delamination. These studies are validated by experimental measurements. The influence of position of source and delamination on wave refraction in the delamination region is examined. This method also correlates the results obtained from experiments and finite element simulations to theoretical dispersion curves in order to distinctly determine the delami...