Volume 9, Issue 3 (March 2012)
Investigation of Load Control Errors for Spectrum Fatigue Testing at High Frequencies
Ultra-high cycle fatigue (gigacycle) tests have shown that a true fatigue endurance limit does not exist for most metallic materials used in high cycle applications. These findings have significant implications for rotorcraft dynamic structural components that have traditionally been designed using endurance limit, stress-life methods. Unfortunately, the gigacycle fatigue test results generated to date cannot be easily applied to rotorcraft component design, because the interaction of different crack nucleating mechanisms under variable amplitude loading is not well understood, and must be studied using spectrum fatigue tests carried out to very long lives. To quantify the effect of spectrum load control errors on the rate of fatigue damage accumulation for a standard servo-hydraulic fatigue test machine, a damage ratio parameter for crack initiation is calculated using the high cycle fatigue portion of the Coffin-Manson strain-life curve with rainflow cycle counting. The fatigue damage parameter allows the level of controller error to be assessed as a function of test frequency, peak load levels, and test spectrum complexity. Also, different command feedback compensation schemes are tested to determine the range of control error that can be expected for a given set of test parameters. A sensitivity study was performed on the command signal response errors as a function of changes in the slope of the mean damage curve beyond 106 cycles. The test results demonstrate that rotorcraft spectrum fatigue tests can be performed at frequencies up to 150 Hz with a level of nominal damage accumulation error less that 1 % under most conditions. When electrical measurement system error is accounted for, the damage accumulation error is less than 2 % under most conditions. The results also show that it is important to consider the shape of the stress-life curves in the gigacycle fatigue range when performing spectrum tests out to long fatigue lives.