| Detection and characterization of fatigue damage in metallic airframe structures are critical to life extension and prevention of catastrophic failure. Conventional nondestructive evaluation (NDE) methods, such as ultrasonics and eddy current, are capable of detecting cracks emanating from fastener holes. However, both approaches require measurements to be made in close proximity to each fastener hole, which ultimately leads to a time-consuming inspection. In some cases, fasteners must be removed from a hole to ensure that critical flaws are detected. Recent research has shown that a half power bandwidth method, a technique used to quantify damping characteristics, may be used to detect fatigue damage in homogeneous metallic materials near the point of crack initiation. The objective of the proposed research is to build on the success of previous work and develop a nondestructive damping measurement approach and apply it in a passive sensing mode to detect fatigue cracks in inhomogeneous structure. The results showed that the half power bandwidth method was capable of detecting both simulated cracks (notches) as well as actual cracks. Actual cracks appeared to be easier to detect than notches based on a greater percentage increase in modal damping factor. Also identified was the potential to discriminate between single-site and multiple site damage cases based on changes in modal damping factor. A quantitative assessment of the impact of structural variables on modal damping factor indicated that all main variables (thickness, alloy, hole diameter) and their interaction effects were all statistically significant, indicating that standardization of the measurement is critical for passive nondestructive evaluation. |
