But the measurements also show that averages can be deceiving. They mask extremely large variations in stresses that, until now, had gone on undetected. The experiments have implications for important practical problems in sheet metal forming and control of metal fatigue, which is responsible for many structural materials failures.
When metals deform, the neat crystal structure breaks into a complex three-dimensional web of crystal defects called dislocation walls that enclose cells of dislocation-free material. These complex dislocation structures are directly responsible for the mechanical properties of virtually all metals, and yet they remain very poorly understood.
Twenty years ago, the German researcher Häel Mughrabi theorized that the stresses in the dislocation walls and the cell interiors would be different and have opposite signs - an important result for modelling the effects of shaping and working metal on its properties. Until now there has only been indirect evidence for Mughrabi's model because of the problem of precisely measuring stress at the micron level in individual cells in the dislocation structure.
At that level, in fact, stresses can vary greatly. "Scientifically, these stress fluctuations are probably the single most significant finding of the work since no previous measurements even hinted at their existence," explains NIST physicist and lead author Lyle Levine.
The NIST/ORNL/USC team used intense X-ray microbeams to scan samples of single-crystal copper that had been deliberately stressed. The diffracted X-rays revealed the local crystal lattice spacing, a measure of stress, at each point. As this happens, a thin platinum wire is moved across the face of the crystal. By noting which diffracted rays are blocked by the wire at which point, the team calculated the depth of the region diffracting the beam. They determined cell positions in three dimensions to within half a micron.
The experiments on both compressed and tensioned copper crystals agreed with Mughrabi's model.
COMPAMED.de; Source: National Institute of Standards and Technology (NIST)