The Robust Detection of Cracks in Complex Aerospace Structures Using Nonlinear Vibro-Acoustic Modulation
Abstract
The assessment of the state of a structure through changes in the structure's dynamic response while the structure is in use has become a widely researched topic in recent years and is commonly referred to as structural health monitoring. Although structural health monitoring has been extensively demonstrated on simple structures in laboratory settings, few implementations exist on geometrically complex structures that experience variations in the surrounding environment. This work has focused on the development of a damage detection methodology for detecting the presence of cracks in geometrically complex components despite varying environmental and boundary conditions. In this work, the difficulty associated with differentiating the changes in a structure's linear dynamic response characteristics due to damage from changes caused by variations in either the boundary conditions or the surrounding environment of the structure was demonstrated numerically using both linear and nonlinear finite element models. Because many types of damage introduce nonlinear stiffness and damping restoring forces, it was postulated that this increased nonlinearity may be able to be used to detect damage even in the midst of these varying conditions. It was demonstrated that by utilizing a multi-frequency excitation and observing the amount of modulation in the structural response, the increased structural nonlinearity due to damage could be detected despite varying boundary and environmental conditions. However, it was also observed both theoretically and experimentally that the modal vibration characteristics of the structure play an underlying role in the amount of modulation that is measured using this method. Consequently, a form of vibro-acoustic modulation that utilized a swept probing signal was developed to decrease the reliance on any specific response frequency. Furthermore, it was established that this vibro-acoustic damage detection method remained effective when random vibrations were used to exercise the crack, thereby eliminating the need for a high-energy actuator if the ambient vibrations were sufficient to exercise the crack. While experiments showed that preexisting nonlinearities in the structure's boundary conditions could provide false indications of damage, if the structure behaved relatively linearly prior to damage, vibro-acoustic modulation was found to be a relatively sensitive and robust damage detection method that could be applied to structures with complex geometries.
Degree
Ph.D.
Advisors
Adams, Purdue University.
Subject Area
Aerospace engineering|Mechanical engineering
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