Eigenvalue imaging method for surface inspection over the Lamb-wave field

Ultrasonic guided waves, because of their long range inspection ability, are now being used more and more as a very efficient and economical NDE method for large area inspection. This paper proposes an eigenvalue imaging method which has an ability to classify the Lamb wave fronts scattered by sub-surface defects and to localize the defects. The eigenvalues are derived from the covariance matrix defined over the 4-dimensional vector space which is spanned by following components: (1)a vertical (z-directional) displacement, (2)its vertical particle velocity, (3)the orthogonal pair of out of strains. Therefore, the 3rd eigenvalue detects the existence of the defects and the 4th one shows location of the defects beneath the surface of the plate.