Nonlinear Active Vibration Based Damage Detection and Localization

Open Access
Author:
Long, Justin Allan
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 15, 2014
Committee Members:
  • Stephen Clarke Conlon, Thesis Advisor
  • Edward C Smith, Thesis Advisor
  • George A Lesieutre, Thesis Advisor
Keywords:
  • nonlinear
  • vibration
  • damage
  • detection
  • structural
  • health
  • monitoring
  • automation
  • airframe
  • rotorcraft
  • localization
Abstract:
The development of damage detection, localization, and automation procedures is critical for a practical implementation of Structural Health Monitoring (SHM) in airframe structures. Effective SHM can assure the safety and reliability of aircraft, while reducing costly maintenance time associated with traditional visual inspections. In this thesis a nonlinear spectroscopy-based detection and localization approach was developed on a stiffened aluminum plate test bed, representative of typical skin-stiffener joints on airframe structures. Damage conditions were induced to represent working rivets and loss of torque in fasteners, common precursors to fatigue crack propagation. Active vibration sources were used to excite the structure, and strain sensor rosettes were used to measure the nonlinear vibration responses from the damage. Estimates of the damage location were obtained by using a Principal Strain based Localization (PSL) approach, to estimate the direction of wave propagation associated with the nonlinear signature from the damage location. An emphasis was placed on the use of minimal sensors and actuators, for a lightweight and practical implementation on aircraft. A systematic approach was developed for evaluating the PSL estimates using acquired strain time records. Strain amplitudes were processed at the nonlinear frequency components, and potential automation methods were developed. Optimal drive frequency selections were made by a direct comparison of damaged structure nonlinear responses with a healthy baseline. The drive condition corresponding to the highest increase in nonlinear response was studied, at various active drive levels, to determine effective forcing required for accurate localization using the PSL technique. The undamped stiffened plate configuration was used for development of both single tone and modulated wave nonlinear damage detection methods. Single tone transient methods were explored, but the technique was hampered by low signal-to-noise at the nonlinear components. A steady-state single tone excitation resulted in higher signal-to-noise estimates, but results were degraded by reverberation effects from the plate boundaries. Using the modulated wave approach with a constant exercising wave (low drive) frequency of 750 Hz, high sensitivity detection (up to 15 dB) was achieved in both 3 rows loose and 1 row loose damage conditions, with probing wave (high drive) frequencies of 6000 Hz and 5000 Hz, respectively. Accurate localization results were obtained with the use of low force levels, on the order of 1 N in amplitude. PSL angle estimates showed a weak dependence on reverberation radius estimates for the plate, but showed a strong dependence on forcing amplitude. Angle estimates were most accurate at low drive levels, where the strain response showed discrete sideband amplitudes. At higher force amplitudes, angle estimates were degraded, due to apparent nonlinear damping effects, and the development of a broadband chaotic response. In the edge-damped plate configuration, localization results were enhanced in the steady-state single tone approach, due to increased absorption at the plate boundaries. However, the broadband chaotic response degraded estimates at higher force levels. The modulated wave approach showed 10-20 dB detection sensitivity in the tested damage conditions. In addition, a low nonlinear response was shown in the healthy condition. This was due to the decrease in nonlinearity from the plate boundaries, a result of the added edge damping treatment. Compared to the undamped plate configuration, adjustments in probing wave drive frequency and forcing were necessary to induce sufficient nonlinear response. Many of the optimal drive frequencies were observed to be near the critical frequency of the plate (4800 Hz), where acoustic radiation damping losses are largest. However, required force levels were considered low enough for the use of low power, lightweight actuators in both damped and undamped plate test beds (<10 N of force amplitude across all test cases). The modulated wave PSL method was successfully transitioned for use in airframe structures. An excitation corresponding to the blade passage of the UH-60 was used as a low frequency source on the stiffened plate, an approach that would reduce the weight and power requirements in an embedded application. An active drive approach was used on a laboratory UH-60 upper cabin test bed with simulated damage, showing up to 20 dB in detection sensitivity. The developed experimental approaches represent a key step in the use of nonlinear vibration based methods, in particular wave modulation, for embedded airframe SHM applications.