Investigation of contact acoustic nonlinearities on metal and composite airframe structures via intensity based health monitoring

Nonlinear Structural Intensity (NSI) and Nonlinear Structural Surface Intensity (NSSI) based damage detection techniques were improved and extended to metal and composite airframe structures. Previous work reported the initial development of both linear and nonlinear structural intensity (vibrational energy flow) techniques for airframe damage detection. In this work, the measurement of NSI maps at subharmonic frequencies were completed to provide enhanced understanding of the physical mechanism behind the characteristic contact acoustic nonlinearity (CAN) mechanism. Important results include NSI source localization visualization at subharmonic and ultra-subharmonic frequencies, and evaluation of structure damping effects for the NSSI approach. A new detection metric relying on modulated wave spectroscopy was developed and implemented using the NSSI damage detection feature. Both the single interrogation frequency NSSI, and its modulated wave extension (NSSI-MW) were shown to be more sensitive damage detection metrics than nonlinear features based solely on strain or acceleration. The active NSSI techniques were also extended to composite materials, adding a level of complexity due to the higher distributed damping present in the plate structures studied. The results show NSSI can be used to detect delaminations in the bond line of an integrally stiffened composite plate structure with a high degree of sensitivity. Initial damage detection measurements made on an OH-58 tail boom also show high sensitivity. The techniques developed can be applied to other complex airframe structures and damage types of interest.