By Gary Thomas
Jumping into a pool from a few steps up and jumping from a bridge create two different effects. Although the water level is the same, jumping from a few steps up allows the jumper to enter into the water painlessly and smoothly, on the other hand, the jumper can meet a fatal impact while jumping from a bridge. The impact of hitting its surface is very different.
A dislocation in a crystal lattice, a disconnected region in its structure (represented by the array of atoms shown in blue) can separate from the rest of the lattice at a rate determined by the potential energy of the system, represented by the wavy surface. To the left, the higher potential energy (shown in red) prevents the defect from moving in that direction, but to the lower right (shown in blue) the defect can glide toward a lower-energy state, if it first overcomes the higher-energy hump. Once over that hump, it can move rapidly and continuously — a condition called flow stress. Image courtesy of Yue Fan and Bilge Yildiz
A research team from MIT's Department of Nuclear Science and Engineering (NSE) is trying to answer this basic question. The team examined how materials respond to stresses, including impacts. The research findings can help explain phenomena as separate from the collapse of concrete under unexpected stress and the impacts of corrosion on different metal surfaces.
The researchers examined one specific kind of stress, in a defect termed a screw dislocation, in an iron crystal lattice utilizing a combination of experimental tests and computer modeling. But according to researchers, the underlying explanation may have wide connotation for different types of stresses in different materials. The research, executed by Yue Fan, a doctoral student, Bilge Yildiz, an associate professor, and Sidney Yip, a professor of emeritus, will be published in the “Physical Review Letters” journal this week.
The team basically reviewed how the power of a material can grow suddenly as the rate of strain activated in the material grows. This shift in the rate where a material bends or cracks, termed a flow-stress upturn, has been noted analytically for many years, however, its fundamental mechanism has never been explained fully. Fan said that flow-stress upturn is a significant aspect in materials, explaining how they crack and bend in a process named plastic deformation. But the variation in deformation depends on the forces being used.
The key is called “strain localization” and this is the way an effect or other stress is restricted to a small initial spot, and then the applied forces are rapidly expanded beyond that point. Besides the rate at which point the strain is applied, the team found that the impact critically depends on the material’s temperature. Therefore
Yildiz said that the study could help anticipate the breakdown of construction as varied as metal pressure vessels in power plants, the structural segments of airplane bodies and concrete buildings, but additional work will be required to prove how these fundamental principles can be utilized in these different materials.