Paul C. Paris, pioneer of fracture mechanics, honored for his work

November 29, 2010

On Nov. 18, Paul C. Paris, PhD, professor emeritus of mechanics in the School of Engineering & Applied Science at Washington University in St. Louis, received an honorary doctorate from the University of Paris West, one of many awards to cap his career in fracture mechanics.

The honorary degree, which was conferred on five recipients, including Sonia Sotomayor, JD, associate justice of the U.S. Supreme Court, is given to "exceptional individuals" who "best reflect humanist values and the ideals of universal knowledge," according to the congratulatory letter Paris received from the president of the university.

He began his career at a crucial moment in the history of engineering.

In January 1954, a de Havilland Comet, an early commercial jet airliner, broke up in flight and crashed into the Mediterranean Sea. In April of the same year, another Comet crashed in the waters near Naples, Italy.

Investigation was delayed by the difficulty of recovering debris, and once the wreckage was recovered, hypothesis after hypothesis -- ranging from sabotage to dynamic instability to control failure -- fell before the evidence. A deceptively simple but disturbing conclusion finally was reached by experimentation.

Stresses in the metal skin had concentrated at the corners of square "windows," and, following repeated cycling of cabin pressure, these stress concentrations opened cracks that suddenly grew the length of the fuselage, and the whole aircraft came apart.

Back in 1954, the mechanisms underlying this type of failure were a mystery, and engineers had no understanding of how fatigue, fracture and stress concentrations might be related, much less a predictive model.

Trained at Lehigh University in applied mechanics, Paris became a faculty associate at the Boeing Co. in the summer of 1955. Because Boeing was developing the first American commercial jet, Paris was asked to look into the Comet failures.

"I was scared to death," he says. "I had never been in industry before, and they handed me this enormous task. I didn't have the courage to tell them I didn't know anything about fracture."

"Identifying incremental crack growth as the key to fatigue failure and recognizing that the then-obscure framework of fracture mechanics would have predictive capability constitutes one of the greatest engineering breakthroughs of the last century," says Guy Genin, PhD, Paris' colleague in the Department of Mechanical Engineering & Materials Science. "Paul was clearly well ahead of his time."

In fact, Paris was 15 years ahead of his time. While the Paris law -- which describes crack growth with the aim of predicting the number of cycles to failure and thus the remaining lifetime of a part -- now is routinely used to design parts that vibrate, such as aircraft bodies and automobile cranks, Paris' first paper on fracture mechanics was rejected by all of the top journals in the field.

Industry indifference ended in 1969. Late that year, an F-111 U.S. Air Force fighter-bomber crashed after losing a wing. Almost immediately, the Air Force and the Federal Aviation Authority mandated that fracture analysis become part of the aircraft design process.

By that time, Paris had been teaching courses on a new analytic method called fracture mechanics for more than a decade at both Boeing and Lehigh, introducing the subject to many colleagues who continued to develop the field with him. Fracture mechanics now is a standard component of an engineering education.

Paris joined Washington University in St. Louis in 1976, becoming professor emeritus in 2009 but continuing to teach. His course in fracture mechanics is the longest continuously running course in the history of the field.

"My students who take Paul's fracture course are often amazed to discover that Paul, who is so accessible and unassuming, is the internationally renowned founder of this field," says Genin. "He is a wonderful and supportive mentor, colleague and friend to students, former students and colleagues alike."

Paris has received many awards for his contributions to fracture mechanics. In 2003, he was awarded the third Crichlow Trust Prize (a medal and a $100,000 honorarium) by the American Institute of Aeronautics and Astronautics.

In 2009, he was awarded the George Irwin Gold Medal by the International Conference on Fracture at Ottawa, Canada, the first gold medal ever issued by that conference.

Of all his awards, Paris says, he is proudest of his honorary degree. He attributes a lifelong interest in France to a Lehigh professor who was born in Brittany, a French province, and who taught all of his students an appreciation of French science -- and of French wine.

Washington University in St. Louis

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