Damage Size Estimation for Composite Laminates Based on an Anisotropic Wavefront Expression

The effective damage detection does not only require the location of damage but also its size. Most of the existing damage detection methods for composite structures are imaging methods, which only focus on the damage localization but provide no information about its extent. The methods which focus on damage size can accurately estimate the circular damage but cannot quantify a narrow and long damage like transverse or fatigue cracking. Furthermore, the damage size estimation in anisotropic composite laminates using the non-elliptic path loci has not been studied for any kind of damage. The Multistage Elliptical Parametric (MEP) method introduced in this research uses piezoelectric transducers (lad zirconate titanate, PZT) to excite and sense the Lamb waves signals in composite laminates which are highly sensitive to transverse cracks. The PZT transducers are arranged as a network of square monitoring cells, and damage is estimated inside each cell separately. The Gabor wavelet-based continuous wavelet transformation (CWT) is used to process the complicated wave signals, and the delay time caused by the presence of damage is calculated. In this research, a quantitative expression of anisotropic wavefront is used to draw the non-elliptic path loci between each actuator-sensor pair. The Elliptical Parametric (EP) method, which was initially developed for quasi-isotopic laminates using the elliptical path loci, is then implemented for damage estimation in anisotropic laminates in three stages using the non-elliptic path loci. The points from where the Lamb wave signals are reflected at damage boundary are calculated, and defined as damage edge points. The MEP method is based on estimating the damage with an ellipse whose parameters are calculated using the damage edge points. The maximum distance between any two damage edge points represents the length of major axis while, the minimum distance between any two edge points represents the length of minor axis of an ellipse. These method utilizes the same arrangement of transducers, and the same data processing technique at all detection stages. It also reuses the data collected in the previous stages for damage estimation in the subsequent stages. The methods for quantification of non-circular damage using the non-elliptic path loci have not been developed previously for the anisotropic laminates which requires further attention. The MEP method proposed in this research is computationally inexpensive due to a small number PZT transducers, and lesser excitation data collected as compared to the existing ellipse-based nondestructive methods, which makes it suitable for on-line SHM.