Delamination is a key limitation of composite materials. Delamination is attributed to intrinsically low interfacial strength of laminated composites and may result in loss of integrity of the structure. Several analytical approaches have been suggested to predict delamination. This paper presents a guided wave inspection approach to quantify the width of a delamination in a carbon fiber reinforced polymer (CFRP) composite plate.
A 2D numerical model for predicting A0 Lamb wave propagation behavior in a cross-ply [0/90]s composite laminate plate containing a delamination is simulated using a commercial FEM software. It is demonstrated from the dispersion curve of Lamb waves that the phase velocity of A0 mode in the sub-laminates above and below the delamination area are different as depicted schematically in Figure 1. The wave propagates with the same velocity in the main laminate. In the delamination area, waves propagate separately with individual velocities in the two sub-laminates above and below the delamination. After passing the delamination, the waves then propagate in the same velocity again in the main laminate. The phase difference between the waves travelling from the upper and the lower sub-laminates can be expressed as:

Figure 2(a) shows the waves travelling from upper and lower sub-laminates. Figure 2(b) shows that the phase difference between the two waves increasing linearly with the delamination width. Figure 2(c) shows the change of amplitude of the superimposed signal with the delamination width. When the phase difference is 180, the signal amplitude is smallest.

Figure 1. Scheme of the cross-ply laminate with delamination used in the simulation.

Figure 2. Time series and phase difference of the Lamb waves in the upper and lower layers after A0 wave passing the delamination for four different delamination widths sizes.
The effectiveness of the method was also verified experimentally, on a GFRP plate sample.