14 Monitoring Fatigue Life Expenditure & Detecting Crack Initiation 153 2298 a b Control Surface Control Section 766 764 762 760 Frequency Frequency 758 756 2296 2294 2292 2290 2288 0 20 40 Time - Hours Time - Hours 80 60 100 120 0 20 40 60 80 100 120 Fig. 14.11 (a) The honeycomb laminate control surface habited small step decrease in the 2,300 Hz mode of 2 Hz (b) No step shift in the frequency of the 763 Hz mode is exhibited 14.8 Conclusions The periodic logging of frequency and damping of higher order modes can provide a window into fatigue life expenditure of materials and changes in structural properties. Monitoring damping would benefit a broad range of industries that need an ability to track fatigue life expenditure to predict and prevent catastrophic structural failures. A small field portable data acquisition and measurement system could handle most structures. A Dynamic Signal Analyzer with inputs for an impact hammer/force sensor and accelerometers would be appropriate for small structures and production QA. A field portable hand held system would be suitable for monitoring critical piping systems, bridge I bars (eyebars) and other critical structural members. Air Frame Testing: For larger structure such as airframes an X, Y, Z coordinate positioning gantry system would position the measurement head to make Frequency response measurements around an impact force point. An array of non-contact laser vibrometers mounted on the positioning head would allow automated measurements. SteamTurbine Generator: Damage to turbine shafts due to transients such as loss of load or step increases in load can cause crack initiation. Measuring frequency and damping of the shaft’s torsional modes can provide a window into the health of the shaft. The frequency and damping can be measured while the turbine generator remains in operation. If a step increase in damping in the torsional modes is seen after a transient event then crack initiation should be suspected. If no step increase in damping is seen then no damage occurred. However the transient event may have used up some fatigue life. Ongoing monitoring of damping values can provide the lead time to head off catastrophic failures and unnecessary down time. I-Bars: Eyebars are non-redundant structural elements that hold up bridges. Eyebars (I-bars) usually consist of several bars stacked side by side and pinned. They are subject to corrosion and fatigue cracking. The inner most elements in an eyebar (I-bar) stack are at best very difficult to inspect. Seeingcorrosionpitting and fatigue cracks on the inner I-bars is very difficult. When I-bar faults are detected their fatigue life can be restored through welding, heat treatment, and shot peening. Piping: Catastrophic piping failures due to fatigue, thermal stress, vibration, hydrogen imbrittlement and corrosion can generate headlines and can be very costly in life and collateral damage. The key to preventing catastrophic failures is advanced warning and periodic damping measurements could provide that warning. Fiber/resin composites: Production testing of fiber/resin would allow another level QA over the tap and lessen approach. Frequency and damping values would allow associating numbers to different types of failure mechanisms. 14.9 Opportunities for Further Investigation The time constraints and limited shaker force did not allow a more detailed investigation. The results identified many questions and opportunities for further investigation.
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