SwRI Wins U.S. Grant to Assess Fracture Risk in Aircraft Engines

Blade view of the DARWIN user interface. Image courtesy of SwRI.

The Southwest Research Institute (San Antonio, Texas, USA) was awarded a grant from the U.S. Federal Aviation Administration (FAA) to continue a research program to improve the safety of commercial aircraft engines. The grant is planned to be a four-year, $4.5 million cooperative agreement to develop improved damage tolerance and risk assessment methods for critical engine parts.

SwRI has collaborated with the FAA since 1995 to improve the safety of commercial aircraft, particularly in assessing the risk of fracture of engine rotors. The Probabilistic Integrity and Risk Assessment of Turbine Engines (PIRATE-3) program is now in its third phase after three previous research grants. The FAA started this research in response to an accident at Sioux City, Iowa, USA, in 1989 caused by the uncontained fracture of an engine fan disk, which resulted in the deaths of 112 people.

“Incidents like Sioux City are extremely rare, and for 25 years, SwRI has been working with the FAA and industry to make them even rarer,” says Craig McClung, PIRATE-3’s principal investigator (PI) for SwRI. “If there’s an anomaly that could cause a crack in the engine, we use computer simulations to determine the probability that it will lead to a failure.”

To accomplish this, McClung and his colleagues created DARWIN (Design Assessment of Reliability with INspection), which they described as fracture mechanics and reliability assessment software to support damage tolerant design and analysis of metallic structural components.

Michael Enright, co-PI of the program and Jonathan Moody, group leader, are leading SwRI’s DARWIN development team at SwRI, which also incorporates several major U.S. manufacturers of aircraft engines. The research effort includes material testing to better understand the behavior of anomalies and cracks under cyclic stress.

“DARWIN is based on what we know about how cyclic stresses and temperatures affect the cracks resulting from any material or manufacturing anomalies that might be present,” McClung says. “Starting from the probability that an anomaly occurs in the engine rotor and is not detected, we use DARWIN to predict the probability of fracture during the entire lifetime of each engine over the whole fleet of engines.”

DARWIN is utilized by aircraft manufacturers across the globe to ensure the engines they create are resistant to cracks. It helps designers to visualize and manipulate a three-dimensional component model while identifying locations where a crack might initiate. The system slices the model along the plane where a crack is most likely to grow before determining stresses that will dictate the growth rate.

“DARWIN is mainly utilized at the engine design stage,” McClung says. “The industry needs to predict what’s going to happen before they put several thousand engines into service, and the FAA requires that they show an acceptably low fracture risk before they are even allowed to put the engine into service. Anomalies are extremely rare, and fractures are even rarer, but we’re trying to prevent them from happening in the first place with better design methods. Other research teams have been developing better manufacturing methods and better inspection methods.”

The work of McClung and his colleagues has continued to evolve as new incidents occur. The Sioux City accident was caused by an anomaly in a titanium engine rotor, and early research focused on factors that led to that incident. In 2016, a disk fracture and resulting engine fire at Chicago’s O’Hare International Airport was caused by a material anomaly in a rotor made from a nickel-based alloy. That event is currently driving much of the PIRATE-3 program.

“Each time there is a new kind of threat, a different kind of anomaly that causes a serious accident, we want to learn from it and reduce the possibility that it could ever happen again,” McClung says.

Source: SwRI, www.swri.org.