July 28, 1998 Scientists at the Commerce Department's National Institute of Standards and Technology are the first to succeed in using a new technique that shows precisely what happens to a metal when its shape is deformed. This advance is a crucial first step toward developing new computer models that could help manufacturers save hundreds of millions of dollars annually.
A single automaker alone will spend as much as $2 billion each year perfecting molds—called dies—to press sheet steel into body parts for new car models. That's because processing steel into sheets and then pressing the metal into dies to make auto parts creates a myriad of imperfections in the atomic structure of the metal.
As the sheet metal is being stamped to make auto parts, billions of unseen defects—known as dislocations—are produced that make it impossible to predict precisely how it will behave when pressed into specific dies. Current computer models do not accurately simulate what shape a die will produce. Consequently, manufacturers end up using trial and error, sometimes redesigning a die as many as 10 times before discovering the mold that forms the proper shape.
More accurate computer models would save time and money, but significant improvements cannot be made without better data about the nature of the defects. These data then require the development of a new theoretical model (also under development by the same NIST personnel) connecting the observed defect structures with the mechanical properties. The problem becomes especially bad when manufacturers try to introduce alternative materials, such as aluminum alloys and high-strength steels, whose changing properties are more difficult to predict than the standard steel presently used.
Now, 47 years after it was first proposed by a French scientist, NIST researchers have used an advanced measurement technique, known as in-situ ultrasmall-angle X-ray scattering, to study the evolution of complex defect structures in deformed metals. The underlying mathematical theory was developed by Robb Thomson, Lyle Levine, and Gabrielle Long, and the corresponding experimental techniques were developed by Lyle Levine and Gabrielle Long, all members of NIST's Materials Science and Engineering Laboratory.
The NIST scientists conducted the measurements using intense X-ray beams generated at the National Synchrotron Light Source, a particle accelerator at the Brookhaven National Laboratory. They designed a special sample holder (called a tensile stage) for deforming the samples in the X-ray beam. Scientists now are able to study minute details about the formation and evolution of defects while the metal is actually being stretched and probed by the X-rays.
The next step will be to move the experiments to a new, more powerful accelerator called the Advanced Photon Source at Argonne National Laboratory. The APS produces X-ray beams 100 times more intense than the Brookhaven facility and should lead to even more accurate measurements.
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