Thursday, May 10, 2012
How did the Brinell hardness test get cannonized?
Some of what you read in a newspaper or a magazine may not be true. Page 58 of the latest (May-June 2012) issue of Mental Floss magazine has an article by Gary Belsky on 10 Scales Worth Printing an Article About.
His discussion of the sixth scale begins with:
“ THE SCALE THAT USES A TINY CANNON
Almost a century after Mohs left his mark on the rock world, Swiss engineer Johan August Brinell decided that a simple scratch-based test wasn’t sophisticated enough for metals. So in 1900 he developed a better hardness test using...a tiny cannon. No joke - the Brinell Hardness Test evaluates the strength of metals by shooting a 10-millimeter-diameter ball of hardened steel into whatever you want to test.”
When I read that paragraph, I was rolling on the floor laughing (ROFL). First, Mr. Brinell was Swedish not Swiss, as could be seen by glancing in Wikipedia either under his name or Brinell hardness. Second, there’s no shooting and no cannon involved. I don’t know how Mr. Belsky came up with that misinformation, or why no one at Mental Floss bothered to check his article before publishing it. Just this year there was a 17-page scholarly article on the Historical origins of indentation hardness testing that you can download here.
Usually in the Brinell hardness test the 10mm ball is pushed against the metal surface with a load of 3000 kg held for 10 to 15 seconds. Then the diameter of the resulting indentation (in millimeters) is measured using a portable microscope, and the hardness is calculated from the load divided by the surface area. The current ASTM E10 test method calls for using a tungsten carbide ball rather than hardened steel. For steels there is an excellent correlation between Brinell hardness and tensile strength, so hardness tests are commonly used for quality control of products like castings or plate.
You can watch a brief YouTube video of a simple pneumatic Brinell tester here, and a fancier German one (with a built-in microscope) here.
The image of a small cannon came from Wikimedia Commons.