What is the purpose of the yield point or yield strength/proof stress/offsett yield point?
What is the purpose of the yield point or yield strength/proof stress/offsett yield point?
(OP)
I'm a novice. I presume that normally you make sure that the metal in a structure does not undergo stress beyond it's yield point, or elastic limit. Although, I suppose, if the metal is stressed anywhere between the yield point and Ultimate Tensile Strength, the material is holding, it's just that if stress is relieved, it's permanently deformed. So, I guess you could have a rule where you never put or allow stress on a member, where, if you were to remove that stress, you would find permanent deformation.
If that is a general rule about stress you put on a structural member, then I can see the purpose of the yield point. That understanding of purpose of the yield point figure is based simply on the fact that the metal is not deformed - if stress is removed. Like that fact equates with design safety.
But what about the yield strength, known as proof stress, or offset yield point, which you use when the metal has no well-defined yield point? As I understand it, for a proof stress of 0.2, that means the metal would stay permanently deformed at 0.2% after having been subject to the proof stress value. (I've seen the method of getting the value of the 0.1 or 0.2 proof stress). Well, you would be saying it's okay to stress the member, even if after the stress is removed, there's a 0.2% permanent deformation. I suppose it's kosher to do that? Just don't go beyond the proof stress.
Is yield point valid for some materials, yield strength for others? Thanks.
If that is a general rule about stress you put on a structural member, then I can see the purpose of the yield point. That understanding of purpose of the yield point figure is based simply on the fact that the metal is not deformed - if stress is removed. Like that fact equates with design safety.
But what about the yield strength, known as proof stress, or offset yield point, which you use when the metal has no well-defined yield point? As I understand it, for a proof stress of 0.2, that means the metal would stay permanently deformed at 0.2% after having been subject to the proof stress value. (I've seen the method of getting the value of the 0.1 or 0.2 proof stress). Well, you would be saying it's okay to stress the member, even if after the stress is removed, there's a 0.2% permanent deformation. I suppose it's kosher to do that? Just don't go beyond the proof stress.
Is yield point valid for some materials, yield strength for others? Thanks.





RE: What is the purpose of the yield point or yield strength/proof stress/offsett yield point?
A material may or may not "hold" when stressed beyond its Yield yet below its Ultimate Tensile Strength.
RE: What is the purpose of the yield point or yield strength/proof stress/offsett yield point?
RE: What is the purpose of the yield point or yield strength/proof stress/offsett yield point?
It isn't a 'real' property, it is a convention.
In most cases you design to a fraction of the yield strength.
Some codes use 25% (or 30%) of UTS; or 50% (or 66%) of Yield, whichever is less as a design limit.
Once you get near to yield you will find things out of alignment and joints working loose.
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Plymouth Tube
RE: What is the purpose of the yield point or yield strength/proof stress/offsett yield point?
As EdStainless noted, most applications do not stress a material near its yield strength. Even highly stressed parts like fasteners, springs, gears, and bearings generally are designed so that the stresses under the most severe conditions do not cause permanent deformation.
I believe a useful example for you to think about is the manufacturing process used for creating shaped metal articles like automotive fenders, refrigerator doors, and zillions of other parts. The process is usually called stamping, and entails deforming a flat metal blank (steel, aluminum, etc.) beyond its yield point so that it assumes a new, permanently altered shape. The metal must be permanently deformed so that it can work in its intended application, but once it is put into service, it is no longer intended to be stressed such that additional permanent deformation takes place. So, understanding the yield behavior is very important for the manufacturing process, and may also be important for the final part if it something like an automotive fender or door panel that may have to resist denting and other forces that can cause additional permanent deformation.
RE: What is the purpose of the yield point or yield strength/proof stress/offsett yield point?
RE: What is the purpose of the yield point or yield strength/proof stress/offsett yield point?