Abstract
Two new ultrasonic approaches to NDT of in-plane stiffness anisotropy have been developed and applied to various composite materials. The first approach is based on polarization measurements of ultrasonic shear waves which manifest birefringence due to asymmetry of in-plane stiffness. Birefringence is applied for monitoring of fibre orientation and evaluation of in-plane stiffness anisotropy in various composites and multi-ply composite laminates. The other approach uses mode conversion of air-coupled ultrasound to zero-order plate waves for remote NDT of the in-plane stiffness anisotropy. A non-contact ultrasonic evaluation of the in-plane anisotropy of Young`s modulus is based on air-coupled measurements of the \( {a}_{0}\)-wave dispersion extrapolated to a “static” case. Depth-resolved measurements of stiffness anisotropy are demonstrated in multiple composite laminates. The methods and instrumentation developed are capable of detecting improper ply positions and orientation in composite laminates in production phase, variation in stiffness anisotropy due to material aging or damage progression in operation.
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