Collection Of Damaged Goods Aids In Aircraft Research

February 23, 1998

WEST LAFAYETTE, Ind. -- A cracked fencing foil. A fractured garlic press. A broken diaper pin.

Those damaged objects and others are all at home in Skip Grandt's office at Purdue University, where a burgeoning collection testifies to the aeronautics and astronautics professor's scholarly passion: material corrosion, fatigue and cracking.

The collection got its start some 25 years ago, when Grandt, then a research engineer with the Air Force Materials Laboratory, taught a course on fracture mechanics at the Air Force Institute of Technology. Graduates of the class took positions with Air Force maintenance and repair centers and began sending him failed aircraft parts to use as examples for his students.

The collection grew, accompanying Grandt in 1979 to Purdue, where he and five other faculty members have just completed a $3.3 million, four-year project for the Air Force Office of Scientific Research. The project's goal: to improve the safety and extend the operational life of aging planes.

In labs across campus, Grandt and his colleagues were cracking, corroding and stressing pieces of aluminum alloy and parts of old planes to simulate various environmental conditions, fatigue and procedures that aircraft endure. Areas of investigation included how cracks and corrosion form over time and how tiny cracks in an airplane's structure and skin grow and interact with larger cracks.

As the project progressed, Grandt asked other Air Force colleagues to send him old aircraft components to cut up and examine. They responded with pieces of corroded fuselages and wing skins from several retired aircraft. Test specimens from these components provided Grandt and his students valuable information on how materials degrade during extended periods of service.

Then, a doctoral student conducting fatigue tests of plastic specimens gave his own plastic hairbrush to Grandt -- the handle had broken from a crack similar to ones he had been studying in the lab. Someone else contributed a toilet seat ("It was a classic fatigue failure," Grandt notes), and the professor's office soon became known as a depository for damaged goods.

Grandt has contributed a few items of his own: the diaper pin was used by his son, and the professor nabbed an office stapler with a broken handle after spotting several cracks in the object and biding his time. The cracks coalesced, the handle broke, and the stapler is now a specimen.

As the collection has grown, Grandt's recently concluded research project has provided valuable insight into how much cracking a plane can tolerate before it becomes unsafe.

"We have developed a good numerical analysis that predicts how these small cracks grow and interact when they originate around fastener holes or small pits that form from corrosion," says Grandt, who expects that a variation of his model may be used someday by inspectors to more reliably estimate the remaining life of a plane by considering the amount of cracking found on the craft.

"Aging aircraft are a precious resource that we need to learn how to use safely," he says. As for his office specimens, "They make good show-and-tell materials for students."

"But," he adds, surveying his cluttered office, "it's starting to get out of control."

Purdue University

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