Longitudinal guided waves confined in radius filler regions of composite joints.

Damage detection in quasi-isotropic composite bends using ultrasonic feature guided waves

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.

Feature guided wave inspection of bond line defects between a stiffener and a composite plate

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.

Deep subwavelength ultrasonic imaging using optimized holey structured metamaterials

Abstract: This paper reports the experimental demonstration of deep subwavelength ultrasonic imaging of defects in metallic samples with a feature size of λ/25 using holey-structured metamaterial lenses. Optimal dimensions of the metamaterial’s geometric parameters are determined using numerical simulation and the physics of wave propagation through holey lenses. The paper also shows how the extraordinary transmission capacity of holey structured metamaterials comes about by the coupling of higher frequencies in the incident ultrasonic wave field to resonant modes of the lens.

Rapid guided wave inspection of complex stiffened composite structural components using non-contact air-coupled ultrasound

Abstract: This article demonstrates a rapid, fully non-contact inspection technique for a full-scale complex composite structural component using air-coupled ultrasonic guided waves. The presence of different features such as stiffeners , stringers and geometric variations in skin-stiffened structures makes the received guided wave signal cumbersome and difficult to interpret. Experiments, supported by three-dimensional finite element models , are used to demonstrate the physics of guided wave interaction with complex features and defect configurations. B-scans are used to detect geometric variations in skin, and also disbonds in the skin-stiffener interface. Correlation between the numerically simulated and experimentally obtained B-scans is established. Different regions in the B-scan images could be used to locate and identify the defects and geometric variations in the test sample. The size of the disbond can also be computed from the B-scan.

SAFE-PML approach for modal study of waveguides with arbitrary cross sections immersed in inviscid fluid

Abstract: Ultrasonic guided wave is an important non-destructive tool for large area inspections of immersed structures as well as fluid characterizations. In this paper, a numerical tool is developed for the modal study of immersed waveguides with arbitrary cross sections, by coupling the Semi-Analytical Finite Element (SAFE) method with Perfectly Matched Layer (PML). The model is first validated on waveguides with regular cross sections with analytical solutions. It is then applied to immersed waveguides with rectangular cross sections and L-shaped cross sections, showing the potential of guided waves for NDT applications and fluid characterizations.

Ultrasonic Waves in Solid Media

Abstract: Preface 1. Introduction 2. Dispersion principles 3. Unbounded isotropic and anisotropic media 4. Reflection and refraction 5. Oblique incidence 6. Wave scattering 7. Surface and subsurface waves 8. Waves in plates 9. Interface waves 10. Layer on a half space 11. Waves in rods 12. Waves in hollow cylinders 13. Guided waves in multiple layers 14. Source influence 15. Horizontal shear 16. Waves in an anisotropic layer 17. Elastic constant determination 18. Waves in viscoelastic media 19. Stress influence 20. Boundary element methods Bibliography Appendices A. Ultrasonic nondestructive testing principles, analysis and display technology B. Basic formulas and concepts in the theory of elasticity C. Basic formulas in complex variables D. Schlieren imaging and dynamic photoelasticity E. Key wave propagation experiments Index.

Review of z-pinned composite laminates

Abstract: This paper reviews published research into polymer composite laminates reinforced in the through-thickness direction with z-pins. Research into the manufacture, microstructure, delamination resistance, damage tolerance, joint strength and mechanical properties of z-pinned composites is described. Benefits of reinforcing composites with z-pins are assessed, including improvements to the delamination toughness, impact damage resistance, post-impact damage tolerance and through-thickness properties. Improvements to the failure strength of bonded and bearing joints due to z-pinning are also examined. The paper also reviews research into the adverse effects of z-pins on the in-plane mechanical properties, which includes reduced elastic modulus, strength and fatigue performance. Mechanisms responsible for the reduction to the in-plane properties are discussed, and techniques to minimise the adverse effect of z-pins are described. The benefits and drawbacks of z-pinning on the interlaminar toughness, damage tolerance and in-plane mechanical properties are compared against other common types of through-thickness reinforcement for composites, such as 3D weaving and stitching. Gaps in our understanding and unresolved research problems with z-pinned composites are identified to provide a road map for future research into these materials.

Finite element model for waves guided along solid systems of arbitrary section coupled to infinite solid media.

Abstract: The Semi-Analytical Finite Element (SAFE) method is becoming established as a convenient method to calculate the properties of waves which may propagate in a waveguide which has arbitrary cross-sectional shape but which is invariant in the propagation direction. A number of researchers have reported work relating to lossless elastic waves, and recently the solutions for nonpropagating waves in elastic guides and for complex waves in viscoelastic guides have been presented. This paper presents a further development, addressing the problem of attenuating waves in which the attenuation is caused by leakage from the waveguide into a surrounding material. This has broad relevance to many practical problems in which a waveguide is immersed in a fluid or embedded in a solid. The paper presents the principles of a procedure and then validates and illustrates its use on some examples. The procedure makes use of absorbing regions of material at the exterior bounds of the discretized domain.

On the use of absorbing layers to simulate the propagation of elastic waves in unbounded isotropic media using commercially available Finite Element packages

Abstract: Finite Element models for simulating wave propagation and scattering from defects are vital for ultrasonic methods in NDE. This article addresses methods to dramatically enhance computational efficiency by only meshing a local region of the material surrounding the defect; this reduction requires some kind of boundary, or boundary condition, which absorbs, rather than reflects, any waves arriving at the exterior of the modelled domain. A variety of approaches exist and we take two approaches, Perfectly Matched Layers (PML) and Absorbing Regions, selected specifically as they are readily implemented in commercially available Finite Element packages without requiring the source code. We illustrate both bulk and guided waves, and analysis is used to guide the performance, and thus to plan the use, of each of them. Finally, application examples illustrate the gains yielded by absorbing layer methods in terms of reducing both model size and unwanted reflections.

Wave propagation along transversely periodic structures

Abstract: The dispersion curves for guided waves have been of constant interest in the last decades, because they constitute the starting point for NDE ultrasonic applications. This paper presents an evolution of the semianalytical finite element method, and gives examples that illustrate new improvements and their importance for studying the propagation of waves along periodic structures of infinite width. Periodic boundary conditions are in fact used to model the infinite periodicity of the geometry in the direction normal to the direction of propagation. This method allows a complete investigation of the dispersion curves and of displacement ∕ stress fields for guided modes in anisotropic and absorbing periodic structures. Among other examples, that of a grooved aluminum plate is theoretically and experimentally investigated, indicating the presence of specific and original guided modes.