What is the purpose of the yield point or yield strength/proof stress/offsett yield point?

[richard4556]

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.

 

[stanweld]

From an Engineering design standpoint, the Yield Strength, measured by the offset method, is applicable. A defined Yield Point may or may not be observed during tensile testing, even on ferritic steels. A yield point will not be observed with face centered cubic alloys. In reality, design is based on the specified minimum yield strength of the materials - not the actual yield strength of the material. And design stress may be slightly beyond actual yield.

A material may or may not "hold" when stressed beyond its Yield yet below its Ultimate Tensile Strength.

 

[richard4556]

I'm not actually studying materials engineering as such, just need to know something about the subject. As a layman, it seems intuitive that you would not allow a material to be subject to a stress that would leave it permanently deformed. So, one tends to "get" the purpose of Yield Point. One the other hand it seems counter intuitive to stress a metal leaving it permanently deformed. But, that is precisely what is allowed with the Yield Strength figure. And so is hard to "get" at first.

 

[EdStainless]

The offset yield strength is a convenient shorthand.
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.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[TVP]

Richard,

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.

 

[richard4556]

Okay, I think I understand now. If you are a manufacturer, Yield Point and Yield Strength/Offset Yield Point will have some significant and practical meaning, but if it's a matter of engaging in structural engineering, although it may be taken note of, you, more-or-less, arrange stress to be significantly below these points. So, for the structural engineer it is certainly *not* kosher to allow for any stress to permanently deform the metal. So, the structural engineer does not allow stress anywhere near the Yield Point or the Offset Yield Point/Yield Strength value.(Yield Strength being a convention, not a "real" property). Manufacturers on the other hand want deformity.

 

[stanweld]

You're getting the picture. Manufacturers of items made by cold forming need to precisely know the tensile properties (YS, UTS, %Elongation, %Reduction of Area) of the raw stock in order to produce the item with the least amount of defective parts. It has been my experience that a 1/2 point of Rockwell superficial hardness on a particular grade (temper designation) of cold-rolled, raw product can lead to a 50% rejection rate of the items made therefrom.