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Premium grade titanium alloys, formerly processed by double vacuum arc remelting (VAR) and now processed by triple VAR, are used for fan and compressor rotors and disks in aircraft jet engines. Occasional upsets during processing can result in the formation of metallurgical anomalies referred to as hard alpha (HA). These anomalies are nitrogen-rich alpha titanium that are brittle and often have microcracks and microvoids associated with them. Although rare, these anomalies have led to uncontained engine failures that resulted in fatal accidents such as the incident at Sioux City in 1989. In a report issued by the FAA after the accident at Sioux City, it was recommended that a damage tolerance approach be implemented to explicitly address HA anomalies, with the objective of enhancing conventional rotor life management methodology. The approach adopted is based on probabilistic fracture mechanics. This enhancement is intended to supplement, not replace, the current safe-life design methodology.
The probabilistic, damage tolerance code developed in this program for low-cycle fatigue of titanium rotors/disks containing HA anomalies is called Design Assessment of Reliability With INspection (DARWIN). It was developed in collaboration with Honeywell, General Electric Aircraft Engines, Pratt & Whitney, and Rolls-Royce Allison. DARWIN is a computer program that integrates finite element stress analysis, fracture mechanics analysis, non-destructive inspection simulation, and probabilistic analysis to assess the risk of rotor fracture. It computes the probability-of-fracture as a function of flight cycles, considering random defect occurrence and location, random inspection schedules, and several other random variables. Both Monte Carlo and advanced, fast integration methods are integral to the probabilistic driver. A fracture mechanics module, called FLIGHT-LIFE, is also incorporated in the code. In addition, a user-friendly graphical user interface (GUI) is available to handle the otherwise difficult task of setting up the problem for analysis and viewing the results.
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