Downloaded from https://www.everyspec.com ADS-79D-HDBK safe life of a metal component is that usage period in flight hours when there is a low probability the strength will degrade below its design ultimate value due to fatigue cracking. The determination of the safe life of aircraft metal components depends primarily on the results of full scale fatigue tests that do not introduce other types of damage such as corrosion. The number of simulated flight hours of operational service successfully completed in the laboratory is the ―test life‖ of the metal component. The safe life also depends on the expected distribution of failures.
The distribution of failures provides the basis for factoring the test life. The factor is called the ―scatter factor.‖ The distribution of failures may be derived from past experience from similar aircraft or from the results of design development testing preceding the full scale fatigue test. The test life is divided by the scatter factor to determine the safe life. The scatter factor is supposed to account for material property and fabrication variations in the population of aircraft.
An exception to using critical metal components beyond the ―safe life‖ fatigue limits for bearings and gears is to employ vibratory CBM monitoring if the onset of the fatigue failure mode (accompanied by surface/subsurface cracking, spalling, flaking, chipping, and pitting) is detectable utilizing a validated detection system and will not result in fatigue cracking or malfunction progressing into a failed state within 2 data download intervals of the monitoring system. An example of such an exception is Contact Fatigue in transmission gears, bearings, and shafts.
Contact fatigue generally initiates as microscopic surface and subsurface cracks which develop into surface pits subjected to alternating Hertzian stresses due to concentrated loads repeated many times during normal operation. This then leads to spalled surfaces and significant subsurface cracking if the critical metal components are left to degrade to failure. Contact fatigue is usually associated with significant operational capability remaining (time on wing / remaining useful life) from the onset of spalling depending on the speeds, loads, temperatures, and lubrication present.
Bearings are especially vulnerable to this type of fatigue failure onset as they are designed using an L10 lifing approach which accepts 10% of the bearings to fail at any point prior to reaching the design life. As a result, it is accepted design and operational practice to employ chip detectors in aircraft transmissions to provide sufficient indication of impending gear, bearing, and shaft failures due to Contact Fatigue.
In other words, there is some very limited ―damage tolerance‖ accepted in the ―safe life‖ design of the rotorcraft engines and transmissions which allows the components to operate until sufficient quantities and/or size of material is captured by the chip detectors denoting an impending failure. It is an ongoing CBM objective to increase the sensitivity of sensors tracking contact fatigue progress that generates the metal degradation of the gears, bearings, and shafts. This, in turn, provides earlier impending failure indication than the chip detectors historically employed in aircraft engines and transmissions.
As a result, the US Army is able to provide an increased time estimate to the user before failure using vibration based diagnostics. This time estimate is characterized as either a time horizon to maintenance (time on wing) or RUL.
Since each gear, bearing, and shaft contact fatigue life is unique to the engine and transmission application due to the specific loads, speeds, temperature, and lubrication associated with the designs, teardown analyses (TDAs) of engines and transmissions with onset of impending failures are necessary to achieve an accurate portrayal of how contact fatigue progresses on the bearings and gears, as well as facilitating an evaluation of vibratory CBM system diagnostic performance.
Recommendations on component condition color codes based on a CBM score card may then be prepared to tie diagnostic performance to component condition as well as provide a prognostic estimate of time horizon to maintenance or RUL based on experience for each unique gear, bearing, and shaft application. This can lead to differences in acceptable damage (e.g. crack size or spall length and depth) between components from application to application. To ensure a consistent assignment of damage severity for each of the 82