Rough times: NIST's new approach to surface profiling

January 9, 2008
Surface Profiling; Scanning Laser Confocal Microscopy
The four images (taken with scanning laser confocal microscopy) show variations in surface roughness of an aluminum alloy as produced by increasing amounts of strain: A-1 percent, B-4 percent, C-8 percent, and D-12 percent. Credit: Mark Stoudt, Joseph Hubbard and Stanley Janet, NIST

Researchers at the National Institute of Standards and Technology have developed a novel technique for measuring the roughness of surfaces that is casting doubt on the accuracy of current procedures. Their results announced in a forthcoming paper could cut development costs for automakers as they design manufacturing tools for new, fuel-efficient, lightweight alloys.

Surface roughness is a key issue for auto manufacturers and other industries that use sheet metal, one that goes far beyond simple cosmetics. Faint striations and other marks that appear when metal is shaped can indicate residual stresses that can cause the part to fail later on. They also lead to extra wear and early retirement for the expensive stamping dies used to form sheet metal into fenders and other body parts (a typical production die can cost $2 million or more.)

And measures of surface roughness feed into models that predict friction and the metal’s “springback”—the amount it will unbend after being stamped. Springback has to be known and controlled to build accurate dies for complex metal shapes. A significant cost in introducing new lightweight alloys for cars is the trial-and-error process needed to develop a new set of dies for each new alloy.

Conventionally, roughness is measured with a profilometer, an instrument with a probe like a high-tech phonograph needle that is tracked in a line across the test surface to record the peaks and valleys. The process is repeated several times at intervals across the test surface, and the results are averaged into a “roughness” figure. (New non-contact instruments use optical probes, but the idea is the same.) But NIST researchers have found that these measures may be misleading—as both measurement uncertainties and statistical errors are compounded when the 2-D lines are extrapolated to the entire surface.

NIST’s approach uses data from a scanning laser confocal microscope (SLCM), an instrument that builds a point-by-point image of a surface in three dimensions. The data from a single SLCM image—representing an area of about 1000 X 800 micrometers by 20 micrometers in depth—are analyzed using mathematical techniques that treat every point in the image simultaneously to produce a roughness measure that effectively considers the entire 3-D surface rather than a collection of 2-D stripes.

One early finding is that the generally accepted linear relationship between surface roughness and material deformation is wrong, at least for the aluminum alloy the group studied. The more accurate data from the 3-D analysis shows that a more complicated relationship was masked by the large uncertainties of the linear profilometers. The NIST researchers hope their new technique will lead to more accurate models of the effects of strain on new alloys and, eventually, lower development and tooling costs for metalworking industries.

Citation: M.R. Stoudt, J.B. Hubbard and S.A. Janet. “Using matrix methods to characterize evolution of deformation induced surface roughness in aluminium sheet,” Materials Science and Technology, In Press.

Source: National Institute of Standards and Technology

Explore further: Biomedical 'skin-like bandage' is stretchy, durable and long lasting

Related Stories

Nanotechnology providing the tools to clean up oil spills

October 11, 2016

Oceanic oil spills are tough to clean up. They dye feathers a syrupy sepia and tan fish eggs a toxic tint. The more turbulent the waters, the farther the slick spreads, with inky droplets descending into the briny deep.

Recommended for you

Uncovering the secrets of water and ice as materials

December 7, 2016

Water is vital to life on Earth and its importance simply can't be overstated—it's also deeply rooted within our conscience that there's something extremely special about it. Yet, from a scientific point of view, much remains ...

Blocks of ice demonstrate levitated and directed motion

December 7, 2016

Resembling the Leidenfrost effect seen in rapidly boiling water droplets, a disk of ice becomes highly mobile due to a levitating layer of water between it and the smooth surface on which it rests and melts. The otherwise ...

The case for co-decaying dark matter

December 5, 2016

(Phys.org)—There isn't as much dark matter around today as there used to be. According to one of the most popular models of dark matter, the universe contained much more dark matter early on when the temperature was hotter. ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.