Experimental investigation of delamination effects on modal damping of a CFRP laminate, using a statistical rationalization approach

As an attempt towards damage detection in composites, in this chapter, an experimental investigation is presented aiming at extending the current understanding of how delamination will affect the vibration characteristics of carbon fiber-reinforced plastics (CFRP). Different percentages of delamination have been applied artificially on CFRPs, and experimental modal analysis has been performed for both natural frequency and modal loss factor parameters. It is a well-established fact that, due to the global nature of lower modes’ natural frequencies, small defects have small effects on these modal parameters and hence, generally speaking, natural frequency is not a good feature for damage indication. On the other hand, modal loss factor can be considered as a good damage index (DI), especially, if the healthy structure is of low damping characteristic. However, the problem with modal loss factor is that it is hard to identify reliable values for this parameter and usually identified values have high scatter. In order to be able to identify reliable modal loss factors, the modal parameter extraction techniques with high order of accuracy in damping identification, i.e. circle fit (CF) and line fit (LF) methods, are used, leading to the rationalization process being optimized, resulting in the extended line fit method, (ELFM). Using ELFM, it has been shown that both natural frequency and modal loss factor have changed due to delamination. While, as expected, natural frequency has experienced insignificant changes, modal loss factor has proved to be a highly sensitive indicator, undergoing major changes even at initial damage stages. Modal damping mechanisms and their relationship with mode shapes have been examined. The results reveal that delamination severity can be detected using modal loss factor variations.