Abstract
It is important to detect cracks in the primary phase because cracks extend both in length and depth with increasing fatigue cycles. This paper proposes an index of homogeneity based on the spatio-temporal gradient analysis of Lamb wave field. The index is the normalized dimensionless number based on the rank of the covariance matrix which is composed of the vertical surface displacement, its vertical particle velocity, and the corresponding pair of shear strains. Consequently, the proposed index has the property which is independent of wave number and its velocity.
