Novel quantitative sizing of delamination in hard-to-inspect locations

This thesis presents a study on the characterisation of scattered Lamb wave field from delamination damage in structures. Composite structures are increasingly used in the manufacture of aircraft components. While they offer many mechanical advantages due to their high strength to weight ratio, there are still numerous health monitoring issues that need to be addressed. One key issue is the detection of delamination that can form with little to no signs of the damage on the surface of the structure. They may initiate during both manufacturing and the life cycle loading of the component. Structural features such as through holes favour the formation and growth of such damages around them due to stress concentration. Lamb waves provide a promising diagnostic tool to detect such damages. Assessment of the severity of such hidden damages requires information on the location and size of the damage. The study looks into the scattering of fundamental flexural Lamb wave mode by delamination along free edges of plates and around the circumference of through holes. Finite element and experimental methods are used to characterise the damage scattered wave field in the study.
   Surface bonded piezoelectric actuator elements are used to excite the incident Lamb Wave field in the experiment. The wave field generated by piezoelectric discs is largely dependent on the geometry of the actuator. Understanding of the Lamb Wave Tuning curve allowed for best choice of operation point the dispersion curve, to generate a fundamental flexural wave dominant incident field. A0 mode is used in this study as it shows greater sensitivity to delamination type laminar damages. Wave interaction with the delaminations are visualised using a 3 Dimensional laser vibrometer. The experimental wave interaction patterns showed excellent correlation with the finite element results. The contribution to the wave field from the delamination damage was isolated using the baseline subtraction method.
   Representation theorem is used to show that the scattered field from delamination type damage in beams can be reproduced from force doublet/couple solutions. These force doublet solutions are developed in 2D plane strain and beam theory. In beam theory these force doublets are represented as moments. For the case of mid plane delamination the force doublet combination has 0 net force and moment. However there still exists a wave field that is generated from it. It is also noted that both horizontal and vertical force couples in the mid-plane only produces an A0 wave field.
   Study of the scattered field from delamination along edges in aluminium plates showed a linear relationship between the damage area and the defect scattered wave field amplitude. This linear relationship was restricted to the small damage sizes. Further within the linear limit the scattered field can reproduced from a combination of force couples at the origin of the damage location using representation theory. The strength of this force doublet is scaled with the area of the damage to produce a wave field identical to that scattered by the laminar damage.
   The maximum amplitude of the scattered field from the edge delamination, for oblique incidence, is along the edge of the plate. This indicated that the edge of the plate becomes a guiding feature for the scattered field. For large delamination the forward scattered field is much stronger than the back scattered field. For mid-plane delamination A0 incidence only produced A0 in the scattered field. Off mid-plane position of the delamination produced A0 and SH0 in the scattered field through mode coupling. The SH0 mode could be used for baseline free damage detection of off mid-plane delamination.
   This mode coupling effect for off mid-plane delamination is also observed when the delamination is around a through holes in isotropic specimen. For the case of delamination around a through hole, detection of delamination with limited view angles was studied. Interestingly it was found that a direct line of sight gives the weakest scattered field and an incidence angle of incidence to 70˚ gives the strongest scattered field. These positions of maximum and minima match those of the shear stress (σ_zθ) distribution around the through hole in the baseline structure. It is shown that for small damages the scattered field can be reproduced from point moments at the location of the origin of the delamination.
   The extension of the work to composite laminates showed that even for simple case of mid-plane edge delamination in quasi-isotropic laminates the scattered field is far more complicated than the case in isotropic plates. Beam steering effects are introduced by the anisotropy of the material in composite laminates. Due to this effect the maximum amplitude in the scattered field is directed along the fibre orientation in the outer plies, especially for large delaminations. The scattered amplitude along the fibre direction of the outer plies shows a monotonic increase with damage size, in the forward direction of the scattered field, for all through thickness positions of the delamination.
   The effect of energy focusing along the fibre direction of the outer plies, in the forward scattered field, is also noticed for delamination around through holes in quasi-isotropic laminates. The scattered energy concentration along fibre directions is enhanced when the non-dimensional length scale of the damage (damage size: incident wavelength) is increased. Findings of this thesis gives insight into the scattering of A0 wave by delamination around various geometrical features. These result will aid in the development of Lamb wave based structural health monitoring techniques for qualitative and quantitative detection of the damage