Depth detection of surface-breaking cracks in concrete plates using fundamental Lamb modes

Surface-breaking cracks affect the material and structural properties of concrete members, e.g. concrete plates. Therefore, the non-destructive measurement of the crack depths is important to assess the structural strength and serviceability of concrete plates. In this paper, a theoretical transfer function for a pulse propagating as Lamb modes is used to study the effects from the dispersion of Lamb modes and the participation of high-order Lamb modes. It is found that the fundamental symmetric Lamb mode dominates when a vertical Morlet pulse is applied at the surface; thus, wavelet transform is used for the analysis of the surface responses to extract information of the fundamental Lamb mode. The recently proposed wavelet transmission coefficient (WTC) is sensitive to normalized crack depths up to one-third of the wavelength. The crack depth is successfully evaluated not only in numerical simulations but also in experimental tests.