Probing the mysteries of cracks and stresses

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 in MIT's Department of Nuclear Science and Engineering (NSE) 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 modelling 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. But the underlying explanation, the researchers say, may have broad implications for many kinds of stresses in many different materials.

The research, carried out by doctoral student Yue Fan, associate professor Bilge Yildiz, and professor emeritus Sidney Yip, is being published this week in the journal Physical Review Letters.

Essentially, the team analysed 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 researchers say.