Carbon fiber laminate composites are increasingly employed for aerospace structures as they offer advantages, such as a good strength to weight ratio. However, impact during the operation and servicing of the aircraft can lead to barely visible and difficult to detect damage. Depending on the severity of the impact, fiber and matrix breakage or delaminations can occur, reducing the load carrying capacity of the structure. Efficient nondestructive testing and structural health monitoring of composite panels can be achieved using guided ultrasonic waves propagating along the structure. The scattering of the A0 Lamb wave mode at delaminations was investigated using a full three-dimensional (3D) finite element (FE) analysis. The influence of the delamination geometry (size and depth) was systematically evaluated. In addition to the depth dependency, a significant influence of the delamination width due to sideways reflection of the guided waves within the delamination area was found. Mixed-mode defects were simulated using a combined model of delamination with localized material degradation. The guided wave scattering at cross-ply composite plates with impact damage was measured experimentally using a non-contact laser interferometer. Good agreement between experiments and FE predictions using the mixed-mode model for an approximation of the impact damage was found.

Failure in aerospace composites owing to low-velocity impact raises a significant maintenance concern because it can lead to invisible damage. For the aerospace industry, such defects pose a potential danger to the structural integrity of aircraft. This in turn jeopardises passenger safety and incurs high repair costs. Hence, it is important to efficiently monitor composite structures during the service life. In this study, the potential of low-frequency guided ultrasonic waves for health monitoring in laminated composite plates is investigated. This study focuses on the use of the first antisymmetric guided wave mode (A0). The first part of this study is to investigate the propagation of the A0 mode in three different undamaged composite plates experimentally. The dispersive and anisotropic behaviour are in agreement with the results of finite element simulations and semi-analytical analysis. The final part of this study presents the scattering of guided waves at the impact damage using a non-contact laser interferometer. Significant scattering activities were observed and the impact damage size can be estimated to be about 10 × 25 mm. In conclusion, these results demonstrate the potential of guided ultrasonic waves for the inspection of aerospace composite structures.

a b s t r a c tLow velocity impact can lead to barely visible and difficult to detect damage such as fibre and matrix breakage or delaminations in composite structures. Drop-weight impact damage in a cross-ply carbon fibre laminate plate was characterised using ultrasonic C-scan measurements. This was compared to the results provided by a novel X-ray imaging technique based on the detection of phase effects, which can be implemented with conventional equipment. Three representations of the sample are provided: absorption, differential phase and dark-field. The latter is of particular interest to detect cracks and voids of dimensions that are smaller than the spatial resolution of the imaging system. The ultrasonic C-scan showed a large delamination and additional damage along the fibre directions. The damage along the fibre directions and other small scale defects were detected from the X-ray imaging. As the system is sensitive to phase effects along one direction at a time, the acquisition of an additional scan, rotating the sample 90°around the beam axis, provides information in both fibre directions. These two techniques enable access to a set of complementary information, across different length scales, which can be useful in the characterisation of the defects occurring in composite structures.

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