154 J.H. Jensen What happens to the damping of test components driven through crack initiation, crack propagation, and failure? More detailed sampling of frequency and damping data in the transition from crack initiation through critical crack length is needed. Does the slope in the damping values verses shake time continue in a straight line slope. How large of a step decrease in damping occurs at crack initiation? Can the damping values be related to stress strain and hardiness measurements? Does the damping data differ for test specimens under high static loads and light dynamic cycling verses no static loading with high levels of dynamic cycling? What is the damping characteristics of fiber/resin composites with built in flaws such as entrapped air and weak inter layer bonding? Howdoes corrosion pitting affect the frequency and damping values? How do flaws in welds affect the frequency and damping data? How do missing or loose rivets affect the frequency and damping data? How does hydrogen imbrittlement affect the frequency and damping? Can “learning algorithms” be applied to frequency response measurements to detect very subtle flaws not visible to the eye? 14.10 Experimental Issues The first bending mode of the cowling was very lightly damped at 0.5 % and required manual tuning to stay on resonance. Using a shaker/power amplifier that can be locked onto and track the first bending mode would have greatly reduce the time to produce crack initiation. Summing in random noise into the power amplifier would allow continuous measurement and monitoring of the modes. Care must be taken in the measurement of frequency and damping. One needs at least 15 frequency lines in the resonant bandwidth to avoid bias errors in the damping estimates. Appendix: Mode Shapes Engine cowling mode shapes for 25 Hz and 766 Hz modes.
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