Life Predictions for High Cycle Dynamic Components Using Damage Tolerance and Small Threshold Cracks

Dynamic components for helicopters are typically designed for fatigue using the Palmgren/Miner method. This method, also known as the safe-life methodology, determines a “safe life” for the component from an assumed usage spectrum, associated maneuver stress levels, and the S/N curve for the component. However, the inability to quantify reliability, the cost of retiring parts that probably have no damage, and the fact that the crack that usually results in aircraft part failure is not modeled, have all led toward the damage tolerance design approach being implemented into the Federal Air Regulations (FAR) by the FAA. This paper investigates the accuracy of several damage-tolerance analysis methods in determining the crack growth time from an initial detectable crack size to 25 mm for a helicopter load spectra from a helicopter dynamic component. The results of the analysis are compared to results measured experimentally. The crack growth equations used in the programs are discussed to provide a basis for understanding the results. This work was the outcome of a round-robin crack growth problem proposed at a workshop on fatigue design of helicopters in Pisa, Italy in September 2002. The round robin problem provides an excellent opportunity for comparing crack growth analysis with results from verification tests.