Seeing Through Carbon

Background

Fuselages, wing spars, Formula 1 monocoques, high-end bicycles, wind turbine blades, and other parts that require strength without weight are made of carbon fiber reinforced plastic (CFRP). Plies with different fiber orientations are stacked into curved, variable-thickness laminates, often joined to the substructure through adhesive bondlines of uncertain thickness, which makes ultrasonic inspection challenging in parts with complex geometry. The material is fully anisotropic—the direction of propagation and the ply being traversed determine the velocity, attenuation, and mode conversion. That anisotropy is the central obstacle to reliable nondestructive testing (NDT) of composite structures.

A missed delamination or disbond at a wing spar is a safety-of-flight issue, and any technique that cannot resolve indications in complex regions, such as under stringers, is only doing part of the job. In practice, A-scan and image interpretation remain largely manual and operator-dependent, carrying a real risk of missed indications under time pressure. Fiber waviness, one of the more strength-critical defect types in a laminate, is close to acoustically invisible under conventional inspection.

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