The invisibility behavior of initiation and growing a delamination in an aerospace structure makes it one of the most dangerous and catastrophic damages. In this paper, the non-destructive testing (NDT) and structural health monitoring (SHM) techniques were used to visualize and quantify three different sizes of simulated delaminations inserted in a crossply CFRP plate. An experiment of using RollerFORM and OmniScan equipment was conducted to verify and visualize three simulated delaminations. The guided waves-based developed imaging methods with a sufficient network of piezoelectric wafer active sensors was performed to quantify the shape and size of simulated delaminations. The groupvelocity directivity plots were determined based on the mechanical properties of the interested specimen to estimate the group velocity values of incident and scattered waves. A simple method was developed to estimate the group velocity values of incident and scattered waves at each point of interested area based on the group-velocity directivity plots of propagating Lamb modes and the coordinates of transmitter and receiver transducers. The results demonstrate the capability of the developed imaging method for quantifying the size and shape of interested delaminations. The interaction of guided waves with delaminations were visualized experimentally using scanning laser Doppler vibrometer (SLDV). The effect of the delamination severity on the trapped waves generated over the delamination region was studied experimentally. It was found that delamination size can affect trapped waves. The large delamination has strong trapped waves compared with the small delamination which has weak trapped waves. The wavenumber analysis was conducted for the experimental wavefield data to study the effect of delamination severity on wavenumber components and to identify the delaminations. The result showed that new wavenumber components can affected by the delamination severity. The significant new wavenumber components due to strong trapped waves can be observed for the large delamination case.