Investigation of Contact Acoustic Nonlinearities on Metal and Composite Structures via Intensity Based Health Monitoring Systems
Open Access
- Author:
- Romano, Peter Quinn
- Graduate Program:
- Aerospace Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- July 08, 2011
- Committee Members:
- Stephen Clarke Conlon, Thesis Advisor/Co-Advisor
Stephen Clarke Conlon, Thesis Advisor/Co-Advisor
Edward Smith, Thesis Advisor/Co-Advisor - Keywords:
- Health Monitoring
Structural Dynamics
CAN
Contact Acoustic Nonlinearity
Structural Acoustics
Vibration
Dynamics - Abstract:
- The detection and monitoring of fatigue cracks on rotorcraft airframes is a major concern for maintenance personnel. While current practice relies on a schedule-based approach, this method creates large periods of time where the vehicle is grounded, and not mission capable. Structural Health Monitoring (SHM) systems strive to solve this problem by providing critical information to maintenance personnel based on the actual condition of the structure. In this thesis, Nonlinear Structural Intensity (NSI) and Nonlinear Structural Surface Intensity (NSSI) based damage detection techniques were improved for metallic structures and extended to composite airframe structures. Several test beds were developed to facilitate experiments characterizing various types of relevant structural damage. These include an aluminum stiffened panel, integrally stiffened composite panels, and an OH-58D “Kiowa Warrior” tail boom. Measurement of NSI maps at sub-harmonic frequencies was completed on the aluminum stiffened panel to provide enhanced understanding of the physical mechanism behind the characteristic contact acoustic nonlinearity (CAN) mechanism. An energy “source” at an ultra-subharmonic frequency was localized within the damaged footprint for the aluminum stiffened panel, a result not explicitly shown in previous studies. Additionally, a different application for the formulation of SSI was developed. NSSI as formulated in the frequency domain had a higher signal-to-noise ratio (60 dB difference in noise floor), as well as a greater overall sensitivity to damage in the form of loose fasteners (11.6 dB increase). NSSI was also evaluated to determine the sensitivity to structural damping levels, and proved to be relatively independent of the damping condition present on the structure. A new detection metric relying on modulated wave spectroscopy was developed and implemented using the NSSI damage detection feature. This technique, denoted NSSI-MW, relied on the interaction between two interrogation waves (in the form of combinational frequencies) to form a more stable method for damage detection while still providing a high sensitivity to damage. Results comparing NSSI to NSSI-MW showed that the single-tone approach had a higher sensitivity to damage (43.1 dB detection strength), but required judicious selection of drive force levels and frequency. NSSI-MW was able to characterize the damage response with high detection strength (27.2 dB), but did not require optimization of force or frequency. The active NSSI techniques were also extended to composite materials, adding a level of complexity due to their construction (orthotropic / anisotropic designs), as well as the higher distributed damping present in the plate structures studied. Each metric was able to detect damage in the form of a delamination in the bond line of the integral stiffener to a high degree of sensitivity. Measurements transitioning NSSI and NSSI-MW damage detection metrics to an OH-58 tail boom flight structure showed promise for intensity-based health monitoring techniques to be applied to complex airframe structures. Consistent with measurements from both the aluminum and composite stiffened panels, NSSI was shown to have a higher sensitivity (45 dB detection strength) than NSSI-MW (22 dB detection strength) when detecting simulated damage in the form of a loose hanger bearing bracket. In addition, Both NSSI and NSSI-MW were shown to be more sensitive damage detection features than nonlinear metrics based solely on strain or acceleration.