Monitoring of structural integrity is an essential issue in enhancing affordability as well safety of modern aircraft and spacecraft structures. High strength and stiffness to weight ratios, which facilitate high load-carrying capacity, are making fiber reinforced composites an attractive alternative construction material for primary aircraft components like flaps, vertical and horizontal stabilizers, wings, tails and fuselage skins. However, due to the brittle epoxy matrix in which the carbon or graphite fibres are embedded, the laminates are susceptible to internal damage caused by impact during their loading use. Damage can be caused by dropping a tool during maintenance, hailstones, bird strikes, stones, and so on. This may lead to delamination, sub-surface matrix cracking, fiber/matrix debonding and fiber fracture. Due to stress and strain, their effects propagate during use altering the strength or the stiffness of the component. At low energy impact, the damage is not always visible on the hit surface but affects internally the sample and has the most effectiveness on the opposite surface. This makes the defect detection from the impacted surface, often the only accessible for investigation, a challenging task. For an early and accurate detection of these defects, especially the ones related to low-velocity impact, probes with extremely high magnetic field sensitivity are needed. The high sensitivity, on a wide frequency range offered by superconducting quantum interference device (SQUID) magnetometers, make them a powerful probe for eddy-current inspection of both metallic and composite material where other nondestructive techniques fail. We will present eddy-current nondestructive testing which use high-T_c SQUIDs, with emphasis on extremely lightweight graphite/epoxy composites damaged by low-velocity impact.

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