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Application of an impulse hammer for structural gap detection within composite UAV wings

Creese, Aaron Thomas
Senior design project UAV Wings manufactured at the Oklahoma State University Design and Manufacturing Laboratory have had midflight delamination’s due to improperly bonded structure. These structural deficiencies are caused by gaps between the structure and inside composite skin during the bonding process. Current lab techniques implore the use of tap testing wings by hand before bonding to ensure properly fit structure. However, this approach requires significant training and lacks the ability to locate smaller gaps. Automation of testing could turn this qualitative approach into a quantitative one, increasing the ease and reliability of structural fitting. An extensive literature review determined that the application of non-destructive acoustic emission through the use of automated tap testing could be a simplistic yet effect approach. A modally tuned tap hammer could be used to determine the reduced local stiffness caused by structural gaps beneath composite wing skin. In order to investigate the application of a modally tuned tap hammer for structural gap detection, three rounds of eleven experiments were undertaken. The first round of four experiments investigated the applicability of a tap hammer for detecting structural contact through the use of metal beams. Results from this round determined that spectra analysis was not effective, however the force pulse width and maximum magnitude were key indicators of changes in stiffness. The second round of four experiments explored the application of automated tap testing with composite beams. The results demonstrated that lower stiffness of thin composite beams further enabled the proposed automated tap testing approach. The third round of three experiments explored the effects of wing geometry on automated tap testing by examining wing sections with structures that included gaps. The results indicated that shear web gap detection was much more effective than rib structures, however the composite skin thickness and curvature played a major role in the applicability of the automated tap testing approach. Although this method showed promising signs with differences seen in force pulse shape, the resolution of gap detection is rather small due to the lack of reduced local stiffness caused by structural gaps beneath composite wing skin.