The focus of this research is geared towards the vibrational behavior, aeroelasticity and detection of damaged composite wings. The free vibration modes are evaluated using Dynamic Finite Elements (DFE), Finite Elements (FE) and Dynamic Stiffness Matrix (DSM). The wing damage is considered as a through thickness edge crack for all proposed formulations and numerical tests.
Wang (2004) used a Dynamic Stiffness Matrix (DSM), which is well suited for this analysis as it provides exact fundamental natural frequencies for a uniform wing. The motivation of implementing the DFE methodology in this research is that the technique has shown to generally have higher accuracy and convergence rates in the calculated natural frequencies and modes of beam structures, when compared to other existing methods and more specifically for cases with a higher degree of complexity (e.g., tapered wings). The DFE has proven to be an excellent preliminary tool in the free vibration analysis of homogeneous metallic and composite beam and blades (Hashemi, 1998; Borneman, 2004).
Applying the Cawley-Adams Criterion for the single crack analysis of a defective wing has been shown by a number of researchers to be successful. Using natural frequency data to detect defects in structures is highly attractive since frequency data can be easily measured. Multiple cracks can be caused by impact damage or material defects in manufacturing. The propagation of two cracks simultaneously is impractical for cantilevered structures. The crack with the higher stress concentration would propagate before the second crack. With this in mind, a second crack near the tip of the wing where the stress is less intense may not propagate significantly; however the detection of this crack is essential.
A modest number of research papers have been published in the area of multiple crack detection using frequency data. Patil (2005) uses experimental verification of multiple cracks in homogeneous cantilevered beams using frequency measurements. The detection scheme is extended to multiple cracks by implementing a new strategy. This new strategy involves the implementation of a second indicator. The technique provides highly sensitive results and successfully detects more than one crack for location and size with uncertainty in measured frequency data.