Catastrophic Failures Propagate Field of Fracture Mechanics
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Catastrophic Failures Propagate Field of Fracture Mechanics The development of fracture mechanics growth, concluding that for a given dates back five centuries to Leonardo da stress, there is a critical size above which Vinci, who carried out experiments on the a crack will propagate in an unstable fracture of iron wire. He demonstrated fashion. For a crack of critical size, crack that the strength of a piece of wire varies propagation releases just enough strain inversely with its length; long wires are energy to cancel out the surface energy weaker, on average, while short wires are cost. This work drew on an earlier result stronger. This behavior, better known of C.E. Ingliss who performed a stress today as the "size effect," supported the analysis for an elastic medium containing theory that wire fracture is initiated by an elliptical hole. Griffith's model applied rare flaws. A longer piece of wire is more well to glasses, and could predict the fraclikely to contain a larger flaw that causes ture stress of a glass sample as a function fracture at a lower critical stress. of the size of its largest flaw. However, More systematic investigations of frac- because Griffith's model relied on the ture phenomena came only in the nine- assumption of linear elasticity, it did not teenth and twentieth centuries. Not sur- properly describe the fracture behavior of ductile materials, such as many metals, prisingly, much of the development of modern fracture mechanics was motivat- where plastic deformation plays an ed by catastrophic fracture-related acci- important role. dents. In the 1800s more than two hunIn the late 1940s, G.R. Irwin of the U.S. dred steamboats suffered boiler explo- Naval Research Laboratory and E. sions, including the Soltana, a Civil War Orowan of MIT both independently steamboat whose explosion resulted in reformulated Griffith's theory to include the deaths of over a thousand Union solthe effects of plastic deformation. Later, diers. In 1919, a Boston molasses tank Irwin introduced the concept of the enerruptured, and a two-million-gallon flood gy release rate, and developed the idea of of the viscous, sticky fluid drowned a stress intensity factor characterizing the twelve people. stress concentration singularity near a crack tip. Irwin's contributions formed In the 1940s, 2,500 Liberty ships were the backbone of modern fracture mechanbuilt to aid Britain in World War II; of these, about 700 sustained serious cracks ics, and made possible for the first time the practical application of fracture and 145 broke completely in half. In the mechanics to engineering design. 1950s two Comet jet aircraft experienced catastrophic failures, breaking apart at In 1956, A.A. Wells successfully high altitude; losing the Comet meant applied these ideas to explain the structhat Britain lost its early competitive positural failure of the Comet jet aircraft. He tion in commercial jet aviation. showed that stress concentration near the sharp corner of a square window led to Each of these disasters pointed out initiat
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