The aeronautical industry is among many which have focused on material with a high strength to weight ratios in order to accomplish as much efficiency as possible during flight. Among material of high strength to weight ratios are composite materials (Matthews and Rawlings, 1999).
A composite material is defined by Lee (1989) as the combination of two or more materials of different characteristics (composition or form) which remain bonded together. This yields a material which essentially has all the beneficial attributes of its parent materials and little of their shortcomings. In the aeronautical field composite materials are used to construct components such as airplane wings spoilers and panels, vertical and horizontal stabilizers etc. (Baker et al., 2004).
The wide use and range of composites has led to the development of various techniques for damage detection. One of the common types of damage which composites experience is barely visible impact damage, which can be caused by dropped objects during construction or maintenance. Laminated composites, unlike isotropic materials such as steel, show essentially no yielding prior to complete failure. This makes it crucial to assess them for damage before use.
This research presents an investigation into the possibility of damage detection in laminated composites by making use of full field digital image correlation (DIC). The technique is often employed to assess the structural deformation characteristics of components under various loading conditions, which can then be correlated with finite element assessments. In this research three experimental methods are explored in terms of their performance in detecting barely visible impact damage: 1) modal analysis, 2) full field DIC under static loading, 3) full field DIC under dynamic loading. Modal analysis results showed no noticeable shifts in natural frequencies between an undamaged and damaged carbon/epoxy woven laminated composite, when damage was induced via static point load application at the center of the specimen.
Full field DIC under dynamic loading namely, drop impact tests designed to induce no addition damage, revealed no change in peak out-of-plane displacements between an undamaged and damaged specimen. The technique proved effective only when severe visible damage was induced, which falls out of the scope of the research. Prior to full field DIC testing under static loading a number of experimental exercises were performed to check the accuracy of the method when measuring rigid body motion and out-of-plane displacements. The acquired results were compared to independent measurements obtained using a micrometer for rigid body motion and an eddy current probe for out-of-plane displacements.
The method of full field DIC under static loading condition showed significant reduction in stiffness between the undamaged and damaged composite, with increases in out-of-plane peak displacement and von Mises strain fields. The success of full field DIC under static loading paved the way forward for the investigation of damage detection in laminated composites under various impact energies (i.e. centered impact and off-center impact) resulting in barely visible impact damage.
The finding revealed an increase in compliance of the impacted laminated composite due to the damage. The barely visible damage can be detected using full field DIC only when the loading or excitation is located around the damaged region. There was no noticeable change in out-of-plane displacement fields of the specimens that were subjected to centered impact and off-center impact. Significant changes in von Mises strain fields exist between the undamaged and damaged laminated composites with centered impact.
Dissertation (MEng)--University of Pretoria, 2020.