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Author Topic: What gives a material tensile strength?  (Read 754 times)

Offline thedoc

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What gives a material tensile strength?
« on: 07/04/2016 12:50:02 »
Balaji Ramanathan asked the Naked Scientists:
   What gives a material tensile strength?  Strength under compression is understandable since you are trying to push the atoms of the molecules of the material together, and there may not be any more space for them to be pressed any closer, so that manifests as resistance to compression.  But what keeps the atoms and molecules of a material together?  Initially I thought it was the crystalline structure of the material, but that can't be it because non-crystalline materials have tensile strength too.  I would be very curious to know what the answer is.
What do you think?
« Last Edit: 07/04/2016 12:50:02 by _system »


Offline chiralSPO

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Re: What gives a material tensile strength?
« Reply #1 on: 07/04/2016 14:15:55 »
The bonds holding the atoms together is responsible for the tensile strength. (And the weakest bonds determine the strength)

For crystalline salts, this is ionic bonds. For metals and alloys, this is metallic bonds. For covalent lattice structures like diamond, corundum, quartz, or boron nitride, this is covalent bonds. For molecular solids, like naphthalene, sugar, or ice, even though each molecule is held together by strong covalent bonds, it is the intermolecular bonds (hydrogen bonding, pi-stacking, and dipole-dipole or induced dipole interactions).

Offline puppypower

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Re: What gives a material tensile strength?
« Reply #2 on: 08/04/2016 12:55:21 »
One way to answer this question is to look at steel. Tool steel, such as the steel used to make hammers and wrenches, is different from spring steel, which is more flexible and better in tension. Tool steel is better at compression; hammer.

Both can have similar compositions. The difference is connected to how they are cooled and therefore the type of crystals that will form during the quenching process of manufacturing. Tool steel is quenched quickly. This results in a lot of small crystals forming. Spring steel is cooler slower. The result is fewer, but larger crystals. The higher tensile strength of spring steel, is connected to larger metal crystals.

In tool steel, the large number of small crystals create a lot of crystal boundary surface area. This boundary layer is not as strong as the crystals. This is not a problem under compression. However, this weakness can cause problems under tension. The larger crystals in spring steel has much less surface area at crystal boundaries. This allows the tension to be better distributed within the crystals.

Say you are with your red neck buddies, off roading down in Georgia. Your truck springs are being abused constantly due to the jumps and landings. The jumps are adding work to the spring, which will heat them up. If they are heated enough the spring steel crystal structure can begin to loosen up. Next, we go through a deep cold mud hole. The rapid quenching causes the hot spring steel, to reform tool steel crystals. A little while later, the spring breaks, since it can't handle tension in quite the same way. A good red neck will have spares. He will also bring the spring home to weld and thermally stabilize, so he can have a back for next weekend. I say red neck in a respectful way since they are very resourceful.

Say we take a piece of steel sheet, and hit it hard with a hammer, in one spot. The dent will be one of the first places the steel will corrode. The compression of the steel, will impact the crystal structure, adding stored potential energy fixed into the structure. The same is true if you bend or drill steel. These places will have extra potential energy making them more vulnerable to corrosion. What we have done is turn larger crystals into smaller ones, but not via a stable crystal state. We would need to heat the steel to allow the atoms to relax and release the potential. Then we cool the steel.

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Re: What gives a material tensile strength?
« Reply #2 on: 08/04/2016 12:55:21 »


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