Testing sheet metals and stamping dies costs the automotive industry tons of cash, and it's all wrong, wrong, wrong

Sheet metal isn't high-tech stuff, but a lot of work goes into getting it right when it comes to things like the automotive industry. The surface of a stamped piece of metal is an indicator to the amount of stress it can handle and how, when and where it may fail. The standard method of measurement for surface profiling is being called into question for its accuracy after findings from a group of NIST researchers.

Traditionally, the measurements in question were done with an instrument called a profilometer. The simple device, similar to a phonograph needle, is drawn in a line over the surface of a material to measure the often microscopic terrain. After repeated tests, the data from the instrument is averaged to make a "roughness" measurement.

Unfortunately, as the NIST research shows, this method may be terribly inaccurate due to uncertainties and statistical errors when the measurements are extrapolated into a three-dimensional surface.

Rather than using a two-dimensional technique like the profilometer, Mark Stoudt, Joseph Hubbard and Stanley Janet of NIST use an instrument called a scanning laser confocal microscope to make a point-by-point image of a surface. The measured area from one image is roughly 1000 x 800 micrometers long and wide by 20 micrometers deep. The data from each point is then analyzed simultaneously using mathematical techniques to give a more accurate measurement than could be obtained by two-dimensional methods.

One discovery already made on aluminum alloy used in the study is that the generally accepted linear relationship between surface roughness and material deformation for the alloy is wrong. Data from the research suggests a more complicated relationship was hidden by the uncertainties of linear profilometers.

This and the rest of their findings have been published in a paper titled "Using Matrix Methods to Characterize Evolution of Deformation Induced Surface Roughness in Aluminum Sheet" in the journal Materials Science and Technology.

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