Modelling wave propagation in laminated composite waveguides with imperfections

Aburakhis, Taha 2026. Modelling wave propagation in laminated composite waveguides with imperfections. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Composite laminates are widely used in high performance engineering structures due to their high strength–to–weight ratio. However, their layered architecture makes them susceptible to interlaminar degradation that is difficult to detect using conventional inspection techniques. Ultrasonic guided waves provide a promising basis for structural health monitoring of such materials, provided that their dispersive behaviour can be modelled and interpreted reliably in the presence of damage. Existing dispersion models are well established for pristine laminates and idealised delamination scenarios. However, they typically rely on binary damage assumptions or lack the ability to represent progressive interlaminar stiffness loss and its dependence on interface depth. This limits their applicability for early–stage damage characterisation and dispersion–guided feature selection. This thesis develops an extended Semi–Analytical Finite Element (SAFE) method that incorporates interlaminar stiffness degradation through a continuous degra dation parameter embedded directly within the stiffness matrices. The formulation preserves the quadratic eigenvalue structure of the classical SAFE method while enabling systematic parametric analysis of stiffness reduction and interface depth. The results demonstrate that progressive interlaminar degradation alters guided–wave dispersion in a strongly mode–dependent manner. Mid–plane degradation produces pronounced reductions in the S0 phase velocity due to loss of symmetric extensional coupling, whereas off–mid–plane degradation moderates this response as the thicker sublaminate dominates the extensional field. The SH0 mode shows increased sensitivity in asymmetric configurations due to shear localisation, while the A0 mode remains comparatively stable because bending stiffness is governedby intact outer plies. Experimental measurements confirm trend–level agreementbetweenmeasuredgroupvelocityperturbationsandspecimen–specific SAFE predictions. Direct comparison of damaged slowness is shown to be limited by finite delamination length and non–uniform wave confinement. Building on these findings, the finite element method with Floquet–Bloch boundary conditions is extended to model localised interlaminar degrada tion within periodic waveguides. This enables analysis of modal distortion and localisation effects that cannot be captured using uniform degradation assumptions. By providing a physically consistent link between interlaminar stiffness loss and guided–wave dispersion, this thesis advances dispersion–guided damage interpretation in composite structural health monitoring.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Engineering
Uncontrolled Keywords:	1. Composite Materials 2. Fibre Reinforced Composite Plates 3. Laminated Composite Waveguides 4. Structural Health Monitoring 5. Guided Wave Dispersion 6. Delamination 7. Interlaminar Stiffness Degradation 8. Semi Analytical Finite Element Method 9. Floquet Bloch Boundary Conditions 10. Slowness Curves 11. Group Velocity 12. Mode Tracking
Date of First Compliant Deposit:	20 May 2026
Last Modified:	20 May 2026 12:19
URI:	https://orca.cardiff.ac.uk/id/eprint/187126

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