What is Elastic Limit?
The elastic limit of a material is an important consideration in civil, mechanical, and aerospace engineering and design. Elastic limit, also referred to as yield point, is an upper limit for the stress that can be applied to a material before it permanently deforms. This limit is measured in pounds per square inch (psi) or Newtons per square meter, also known as pascals (Pa).
Elastic limit is a function of the elasticity of a material. Elasticity is the ability of a material to return to its original shape, or dimensions, after a load or stress is removed. All materials will deform when a stress or load is applied. Strain is a measure of the amount of deformation that occurs when a material is under stress.
Elastic strain occurs when a material is exposed to low stress. It will disappear after the stress is removed, and the material will return to its original state. Plastic strain will occur at stresses above the elastic limit. A material that experiences plastic strain will not fully recover and return to its original dimensions after the stress is removed.
This property can be illustrated using the example of a spring. If a weight is hung onto one end of a spring, with the opposite end fixed, it will extend the spring. If a small amount of weight is applied and then removed, the spring will return to its original length. If too much weight is applied to the spring, it will permanently deform and will not return to its original length when the weight is removed. The spring has undergone plastic deformation because the stress caused by the weight exceeded the elastic limit.
Materials have a measurable relationship between applied stress and the resulting strain. This relationship can be plotted in a stress-strain curve. The slope of the stress-strain curve remains constant in the region where elastic strain occurs. The elastic limit is the point where the applied stress causes an onset of permanent deformation, and the slope of the stress-strain curve changes.
Not all materials have an elastic limit. Ductility is a measure of the amount of permanent deformation before complete failure. Ductile materials, like steel and brass, will experience a large amount of plastic deformation before ultimate failure occurs. Brittle materials, like glass and concrete, will show little or no plastic deformation, and complete failure often occurs immediately after a critical value of stress has been reached. For this reason, brittle materials typically do not have a yield point.
@anon281238: An object's potential energy = mass x height. If you release it, that gets turned into kinetic energy. (mass x the distance it fell)
When a falling object strikes something the force = mass x acceleration (or in this case deceleration).
So yes, when the upper floors fell a distance equal to the height of the burned floors, the force of impact with the lower floors would be greater than their static weight.
On the previous post about accelerating mass due to gravity I meant to write that it exerts less downward force than it does simply sitting static. E=t+v. Cons of energy for gravitational mechanical energy. There must be a jolt, momentary deceleration, to exert a force greater than its' own static weight. Please do not just remove this without thinking it through. It is very fundamental physics. To suggest anything else is an embarrassment to our intelligence as a nation.
Conspiracy theorists believe the World Trade Centers shouldn't have collapsed. It is not a theory. The fire did not have any effect on the lower 95 floors of the North Tower. The top 15 floors are said to have crushed the lower 95 progressively strong/thick steel columns below, constantly accelerating. Also, there was uniform acceleration at 2/3 of g.
Fundamental physics tells you that any mass, building, bowling ball, book, or a bb, exerts less force while it accelerates down. This is not theory. It's the law of cons. of momentum and energy cannot be suspended by terrorist hatred. There is simply not any way this is accounted for by gravity and fire. The symmetry is staggeringly telling as well.
@MrMoody - One thing that I think is potentially misleading is the use of the word “plastic strain.”
I read that and I thought of plastic bowls being bent out of shape. The word plastic here does not refer to the substance called plastic. It’s a technical term meaning, more or less, bent out of shape permanently.
In that sense, just about any substance can be subject to plastic strain. This would include not only steel and (of course) actual plastic, but concrete and things like that as well.
Your house’s foundations could be subject to plastic strain, in which case there would be a crack in the foundation. That would not be good news at all.
@allenJo - I think tensile testing is one area where they do a lot of experiments with elasticity. If you are building a steel structure that is going to withstand a lot of weight, you want the building to be able to resume its original structure after the weight is removed. Otherwise, in a worst case scenario, you will wind up with a structure like the leaning tower of Pisa in Italy.
Still, I don’t think tensile testing accounts for every scenario. For example, I don’t think any architect could have anticipated the terrorist attacks of September 11, 2001. The weight of those airplanes as they crashed into the Twin Towers, and the ensuing fire, created stress loads beyond what the steel structures could withstand.
Finally, the Towers collapsed. Conspiracy theorists think that they should not have collapsed. But again, I don't believe that scenario was accounted for during the testing phase.
It’s a good elastic limit definition, and immediately brings to mind the idea of a spring mattress. A good spring mattress can experience strain, and then the spring will return the mattress to its usual shape when you’re off the bed.
In other words, it will retain the original shape. It strikes a balance between firmness and the ability to shape and curve to your body when sleeping. It follows then that bad mattresses will deform and stay deformed.
My first mattress was of the latter variety. It was cheap, and I guess you get what you pay for. The first year it was okay but after awhile it just sunk, never returning to its original shape.
It was bad for my back. Eventually I spent a little extra money and got a good mattress that would spring back to its original shape after extended use.
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