COLUMBUS, Ohio -- Researchers here have developed a new method for predicting how well materials used in aircraft, automobiles, turbines and other machines will hold up under heavy strain without developing dangerous structural cracks.

Their aim is to help designers make vehicles and mechanical devices strong enough to withstand everyday stress without failing, said Herman Shen, an assistant professor of aerospace engineering at Ohio State University.

Fatigue cracks can form in any load-carrying structure during normal use, sometimes with catastrophic results. An April 1988 incident involving a Boeing 737 in Hawaii brought national attention to the problems of fatigue cracks in aircraft. In this accident, the upper portion of the aircraft was ripped apart in flight when a fatigue crack caused an instability in the structure.

The crack that caused the problems on this flight probably wasn't the result of specific damage but rather the normal wear on the plane, Shen said. His process will determine how the frames of aircraft, automobiles and other load-carrying

structures will respond when a fatigue crack forms.

"When we design an aircraft or an automobile, we know what the life cycle will be without fatigue cracks," Shen said. "This project examined how structures would react when a fatigue crack is present."

In designing critical components in aerospace, mechanical, and civil structures, engineers need to determine how the components will handle severe stress and what will happen when they develop fatigue cracks. Engineers try to figure out how long components with cracks can safely operate and their risk for fracture or total failure.

The problem with current methods of predicting development of fatigue cracks is that engineers assume that the pressure on the crack, initial crack conditions and other factors are known from the start. But that's not always the true in real-life conditions, Shen said.

In most studies of fatigue cracks, researchers assume the crack will follow a specific path, and the problem of multiple cracks is not addressed. Shen's research takes into account the random uncertainties in properties of metal, pressure on fatigue cracks and other factors. This allows for more accurate determination of how long the aircraft or other device will remain structurally safe, and the risk of more serious damage.

During a summer internship at the NASA Lewis Center in 1991, Shen studied fatigue cracks in aircraft and spacecraft. By employing a mathematical theory, he was able to predict the path that fatigue cracks would follow in these craft.

Shen then applied standard probability theory to see how reliable a structure would be in the presence of such a fatigue crack. "This method will provide a guide for aircraft or automobile structure designers," Shen said.

It will also aid designers who want to create a structure that will remain safe throughout its operating life, but not outlast the life expectancy of other components.

The research was published in a recent issue of AIAA Journal.

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Contact: Herman Shen, (614) 292-2691

Written by Kelli Whitlock, (614) 292-9475