Machine Design

Simulation unlocks the mysteries of crack formation

Crack-propagation analysis has been going on for several years now. In the latest iteration, research physicists at Northeastern Univ. in Boston have developed large-scale computer simulations to analyze how cracks form and grow in materials including steel, glass, nanostructures, and human bone. “The simulations help us understand what path cracks follow as they propagate in a stressed material,” says team lead Alain Karma, director of Northeastern’s Center for Interdisciplinary Research on Complex Systems. “This knowledge is useful in the development of new materials for aircraft turbine blades, microelectronic circuits, and artificial bone that can better-withstand the formation of cracks.”

The researchers began by examining the combined effects of two types of stress on crack propagation: shearing and tension. Shearing forces cause two contacting layers to slide upon each other, in opposite directions parallel to the plane of their contact. Shearing happens when material is twisted out of shape. The combination of shearing and tension causes the beginning of a crack. But the mechanism for how a crack develops and spreads has remained elusive until it could be analyzed via powerful computers. Large-scale computer simulations showed the surprising result that shearing and tension cause cracks to take the shape of a helix. Based on the results, the researchers developed a theoretical equation to predict how the helix would rotate, expand, and multiply in different materials.

The research could lead to the production of lighter automobile and aircraft parts, and composite artificial bones that don’t fracture when inside the body. The results are also providing insight into the evolution of geologic faults and fractures in the earth’s crust.

Center for Interdisciplinary Research on Complex Systems, Northeastern Univ.,

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