| Summary: | The characteristics of carbon fiber composites have enabled these materials to be accepted as replacements for metal parts in industry. However, due to their unsymmetrical material properties, carbon fiber composites are susceptible to damage, such as a delamination, which can cause premature failure in the structure. This has resulted in the need for nondestructive testing methods that can provide quick, reliable results so that these parts can be tested while in service. In this study, an approach was examined that involved a pencil lead break to excite multiple wave modes in a composite plate in an effort to identify key characteristics based on the wavespeed and frequency. These characteristics were then compared to models based on boundary conditions to generate dispersion curves using the transfer matrix method for whole composite plates that were either undamaged or damaged. To first test this approach, experiments were performed on multilayer isotropic plates and then on a composite plate. The results for all cases showed that modes could be excited by the pencil lead break in the undamaged region of the plates that were not theoretical possible in a delaminated region. Also modes that were specific to the delaminated region were excited and this allowed for a clear comparison between the two regions. This approach could be placed into practice to provide routine testing to detect delamination for in-service, carbon fiber composite parts. === Master of Science === The physical properties of high strength and low weight and the economic benefits of carbon fiber composites has resulted in these materials replacing metals in several industries. It is important, however, to be aware that the change in materials used impacts the different types of damage composites experience compared to conventional metals. One type of damage that could cause a composite part to fail is a delamination or a separation of layers. In order to identify if this damage has occurred, it is beneficial to have an inspection technique that will not damage the part. In this study, a technique was tested that involved breaking a piece of pencil lead on a plate in order to generate multiple wave modes that would propagate in the plate. Based on boundary conditions caused by the damage in the plate, the speed of the wave and frequency content could be compared to an undamaged plate to identify a delamination. A model was created to compare experimental results and demonstrated that using wavespeed and frequency could identify a delamination. The experimental results compared well with the model dispersion curves for a plate with and without a delamination suggesting this approach could be placed into practice to provide routine testing to detect delamination for in-service, carbon fiber composite parts.
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