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LibraLogic
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| Joined: | Wed May 14th, 2008 |
| Location: | West Palm Beach, FL, USA |
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Posted: Fri Aug 8th, 2008 03:50 pm |
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When we learn about the Modulus of Elasticity of a material, lets say STEEL for example which is 29,000,000 psi (29,000 ksi); Would this properly be referred to as the force in TENSION (or Bending?) that the steel can handle, before it surpasses ELASTIC LIMIT?
If so, the ULTIMATE LOAD is that point whereas permanent damage to the member exists, but occurs just before failure, right? But what type of failure occurs? (Shear? Tensile? If we were watching this failure, would it look like a stretched piece of gum, or a snapped pretzel stick?
I’ve found in my readings that ULTIMATE LOAD of steel occurs at “58 ksi” but if the steel hasn’t reached its ELASTIC LIMIT till 29,000 ksi, how is only 58 ksi the point where failure begins to occur? Is ULTIMATE LOAD supposed to be read as (58,000 ksi)?
Thanks for anybodies help in clearing this up for me.
Last edited on Fri Aug 8th, 2008 03:52 pm by LibraLogic
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ghack
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| Joined: | Mon Jul 30th, 2007 |
| Location: | New Orleans, Louisiana USA |
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Posted: Fri Aug 8th, 2008 11:46 pm |
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E is a ratio of stress (the force) over strain (the physical deformation of the material). It is not a measure of strength, it is a property of the material.
The ratio holds until the elastic limit is reached. After that, the material can support additional load (force) but the deformation (strain) is permanent.
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StrongWilled
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| Joined: | Mon Oct 15th, 2007 |
| Location: | Virginia USA |
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Posted: Sun Aug 10th, 2008 01:17 am |
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LibraLogic wrote: When we learn about the Modulus of Elasticity of a material, lets say STEEL for example which is 29,000,000 psi (29,000 ksi); Would this properly be referred to as the force in TENSION (or Bending?) that the steel can handle, before it surpasses ELASTIC LIMIT?
If so, the ULTIMATE LOAD is that point whereas permanent damage to the member exists, but occurs just before failure, right? But what type of failure occurs? (Shear? Tensile? If we were watching this failure, would it look like a stretched piece of gum, or a snapped pretzel stick?
I’ve found in my readings that ULTIMATE LOAD of steel occurs at “58 ksi” but if the steel hasn’t reached its ELASTIC LIMIT till 29,000 ksi, how is only 58 ksi the point where failure begins to occur? Is ULTIMATE LOAD supposed to be read as (58,000 ksi)?
Thanks for anybodies help in clearing this up for me.
Modulus of Elasticity (Young's Modulus) represented by 'E' .
Modulus of Elasticity is graphed as a relationship of stress to strain.
Each material has a different graph, thus a different measure of "stiffness".
There's a point on that graph where steel reaches its elastic limit. The elastic limit is represents the stress value below which a material will return to its original length after that stress is removed.
Ultimate load is another point on the graph that represents the highest level of stress that a material can be subjected.
When we are dealing with steel, ultimate load is not such an important concern, because steel demonstrates a definitive yield point (that point where stress stays constant, but strain increases) and design takes this into account. Ultimate load is more critical with use of ductile materials like concrete. The points on the stress/strain curve for concrete illustrate that concrete exhibits very different behavior under load than steel.
Failure for steel looks like the stretched out rubber band. It can be the result of whatever stress a particular member is subject to. The ductile material has an extra reserve of oomph.
Failure for concrete is the snapped pretzel stick. Concrete has no extra reserves or means of early warning of failure. Snap * it's over.
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