Diving into a pool from a few feet up allows you to enter the water smoothly and painlessly, but jumping from a bridge can lead to a fatal impact. The water is the same in each case, so why is the effect of hitting its surface so different?
This seemingly basic question is at the heart of complex research by a team at MIT that studied how materials react to stresses, including impacts. The findings could ultimately help explain phenomena as varied as the breakdown of concrete under sudden stress and the effects of corrosion on various metal surfaces.
Using a combination of computer modeling and experimental tests, the researchers studied one specific type of stress — in a defect called a screw dislocation — in one kind of material, an iron crystal lattice. The team analyzed how the strength of a material can increase quite abruptly as the rate of strain applied to the material increases. This transition in the rate at which a material cracks or bends, called a flow-stress upturn, has been observed experimentally for many years, but its underlying mechanism has never been fully explained.
The way deformation varies, depending on the forces being applied, is similar to the way the surface of water in a pool can part gently when a diver hits the surface at a certain rate of speed, but doesn’t have time to part and behaves like a solid when the impact is too rapid, as in a jump from a great height.
The key is something called “strain localization” — that is, the way an impact or other stress is confined to a small initial location, and how rapidly the applied forces can then spread beyond that point. To understand that fully, the team had to analyze how the atoms and molecules move to produce this behavior.
The team found that, in addition to the rate at which the strain is applied, the effect depends critically — and in a highly predictable way — on the temperature of the material. The rate of change taking place within the material can suddenly change by orders of magnitude, transforming a slow erosion into a sudden catastrophic fracture. The analysis could potentially help predict the breakdown of structures as varied as concrete buildings, metal pressure vessels in power plants, and the structural components of airplane bodies.
