stiffness

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Can someone explains to me the relationship between modulus of elasticity and hardness. When a material has high value of E, doesn't it means that the material is more brittle and easy to crack ?

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Elasticity measures an objects ability to be deformed and then regain it's original shape back. It is directly and quantifyable measurable.

Hardness though... it's more like a materials ability to withstand a sharp blow to it ( ok. that's in my mind, no formal definition here ). I think that hardness is often measured using some kind of an impact test. It doesn't tell you as much about the material for structural purposes as the materials elasticity does. I can't say much more. As a structural designer, I really don't have much need to know a material's hardness values.

Structural steel has a higher elastic modulus than concrete does ( by quite a bit ). Yet, concrete is MUCH easier to crack and is MUCH more brittle. I'm not sure which is considered to be harder. Hmmmm?

In any case, they are totally different concepts.

Dan :-)

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No, not in general. Hardness is related to ultimate strength. Approximate relationships between hardness and strength for a particular alloy, such as steel can be developed.

Brittleness is due to a lack on plastic deformation, not modulus. Basically the stress-strain curve stops near the yield. A cast iron could be considered brittle, but a steel with a similar modulus would be considered ductile.

Read through a basic materials/metallurgy type book for more background.

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You are comparing apples and oranges.

The modulus of elasticity for a material is how much it deforms (strain) for a given applied stress. Hardness is how deeply a ball (or other standard stylus) can be pressed into the surface of a material by a specific force. The harder the material, the shallower the indentation produced.

The two are basically unrelated. For example, virtually all ordinary carbon steels have the same modulus of elasticity. But you can harden a steel rod to many times greater than its annealed hardness and the modulus remains the same. In other words, the same amount of force will still elongate the bar by the same amount even though it may be much harder.

Young's modulus is also virtually unrelated to tensile strength. Music wire has a much higher ultimate (and yield) strength than a comparable low-carbon annealed wire, yet they have the same modulus of elasticity. So the music wire will bend farther before it yields or breaks, but it takes the same amount of force to do the bending.

Hope this helps!

Don

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Hi all

Although there is no relation between the two. If you think about 2 type of materials where one is really hard(for instant carbide) and the other is really soft(elastomer). Rubber tends to be more elastic where I would say the modulus is very low and the material is not easy or almost impossible to be cracked. Where carbide for instant has higher modulus and is extremely hard and very easy to crack if subject to impact. What always comes to my mind is highest modulus is easy to crack and vice versa. I may be made a wrong assumption. Most of material supplier doesn't provide full mech. properties. When comes to selection for impact resistance where I only have modulus to compare, I will choose the one with lower modulus. I'm not sure whether the way I did is correct. Anyway, thank a lot for all your input. really appreciate it.

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snipped-for-privacy@aol.com (Dan Tex1) wrote in news: snipped-for-privacy@mb-m03.aol.com:

I would think hardness could be defined as a materials resistance to a change in shape. The hardness test is done with a diamond tip, the hardness reading is how far a diamond tip can be pressed into a material with a specific force applied. (Basically, how much force is required to displace a certian amount of the material.)

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-- No, that's stiffness.

-- There are several different types of hardness tests; rockwell, brinell, and others.

But it's local and leaves a permanent indent. So it measures the load to plastically deform the material over a small area.

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That is what I was trying to say, just couldn't get the right words.

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