Testing hardness of materials

  • Thread starter DrnBrn
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Hi

I'm not a physicist but I'm looking for a simple way to test the hardness of different materials. I need to do a demonstration and collect results with a science class, the task sheet that accompanies the lesson says to place a cloth over the materials and hit them with a hammer. I would prefer a simple experiment that would allow me to calculate the force I apply to each material and then calculate the hardness based on the indentation left by the mass. I am aware of experiments involving small round objects and measuring very small indentations with a special microscope. Is there any way I can do a similar test on a larger scale? The results don't have to be very accurate, just as long as we can see a difference between the materials. Your ideas would be appreciated.

Thanks
Darren
 
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You've kind of answered your own question. You drop something with a consistent energy onto the object.

The reason impact tests are done on a small scale is because the shape and flaws of the object quickly becomes the dominant factor when you move to large scales. So you start to test stiffness and not hardness. So you need to make sure your samples are the same size.
 
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You could also follow Mohs and simple test what scratches what.

A harder substance will scratch a softer one, but not the other way round.
Mohs simply arranged these in a 'pecking order'.
 

turbo

Gold Member
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First off: hitting things with hammers is not a very accurate way to determine hardness. Also, whacking stuff with a hammer can cause even very hard materials to fail due to brittleness, strong cleavage planes, internal stresses, etc. If you really do need to use impacts, follow the lead of opticians in eyeglass labs. To keep the impacts as consistent as possible, they drop a steel ball bearing through a tube (standard test equipment) such that it impacts a lens. Plain glass lenses will shatter quite easily. Glass lenses that have been tempered properly in a furnace are tougher and resist shattering. Standard plastic lenses can shatter, as well, producing sharp shards. Polycarbonate lenses are very shatter resistant, in contrast. As you can see, the impact test is NOT testing hardness, because glass is very hard, plastic is softer, and polycarbonate is softer still. The impact test simply measures shatter-resistance.
 
The OP seems to have conflated toughness with hardness. The hardness of a substance tells you how well it resists scratching. Toughness shows how it resists breaking. Too different qualities of a material. Diamond is the worlds hardest natural occurring substance, but hit one with a hammer and see what happens. Steel is very tough, one of the reasons that it is used in hammers' but most steels are softer than quartz, and some are softer than glass. Sand, which is usually mostly quartz, will scratch steel quite readily.

So perhaps the OP should redefine what quality he is trying to test.
 
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The OP seems to have conflated toughness with hardness. The hardness of a substance tells you how well it resists scratching. Toughness shows how it resists breaking. Too different qualities of a material. Diamond is the worlds hardest natural occurring substance, but hit one with a hammer and see what happens. Steel is very tough, one of the reasons that it is used in hammers' but most steels are softer than quartz, and some are softer than glass. Sand, which is usually mostly quartz, will scratch steel quite readily.

So perhaps the OP should redefine what quality he is trying to test.
Wut? Hardness isn't just about scratching, it's resistance to indentations in general.
He clearly knows at least something about what hardness testing as, as he described (roughly - though it's static) the Brinell test method.

Though a scratch test would in this case be better than a hammer test.

EDIT: I can see why you said that though, many people get toughness and hardness mixed up.
 
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