Initial yield stress Vs. tensile strength

[assafwei]

Are those the same? if not, how do I calculate the initial yield stress from the tensile strength? all the materials properties I see on the web only specify tensile strength, and I need the initial yield stress as an input on non-linear FEA analysis. Is it possible to extract the initial yield strength from a stress-strain curve?

Thanks.

 

[Studiot]

Why do you think that the usually specified property is the tensile strength?

 

[assafwei]

Let me rephrase (made some mess with the terms):

Usually I get the tensile stress at yield (or yield strength).
I need the initial yield strength, are they the same? or is the initial yield the end of the elastic portion of the curve?

Forgot to mention that I am dealing with plastics, so the linear portion is somewhat vague when I plot the results of a tensile test.

 

[Enthalpy]

I suppose it's because plastics harden by deformation, so subsequent tests would obtain a higher yield.

 

[assafwei]

So, Is "initial yield strength" the same as "yield strength"?

Currently I get good results when using the "yield strength" as the "initial yield strength" in the FEA software (Femap) so I guess its the same, but could someone please shed a light on the subject?

Thanks.

 

[Studiot]

Do you not think you should study and understand the basic general load/extension graph of ductile materials before messing with FE analysis?

 

[assafwei]

I will be happy if you could give me some pointers instead of riddles...

I will also be happy if you could explain how did you come to think that I don't understand/know the general concepts of the stress/strain curve.

Please understand - I don't pretend that I do understand the concept fully these days (last time I studied it was 10 years ago) but I am not completely clueless, there is a language barrier and it may be that some terms are being confused when translated to English, but it seems to me that this is the reason such a forum exists, in my point of view it is OK that you are trying to make me do my home work before I ask a question, but not giving the answer and patronizing misses the point of such a forum.

 

[davidm1732]

Ouch...
If you don't have access to the full stress/strain curve for your material, see if you can download the CAMPUS database...it has material data from a number of polymer suppliers, for many of their specific resin types, and will have full stress /strain curves.
If I understand where you're going with this, you need to define the elastic and plastic regions separately in your non-linear analysis code, so you need to know where your yield point is (the transition point between the elastic and plastic portion of the curve).

I would define yield point as the limit of the elastic region. IE: the point at the onset of plastic deformation. Up to this point, if you stretch the material, then release the load, the material will return to it's unloaded shape. If you stretch the material beyond this point, you will have permanent deformation of the part. Plastics don't have as well defined a yield point as metals, so you can estimate the value if you have the stress/strain curve to look at. I would define the tensile strength as the ultimate tensile strength of the material, the point at which it breaks under load. This is often shown on data sheets, but isn't as useful if you have to design for deflection, not structural failure (breakage).

Without having a good idea of the general shape of your curve for the resin type you're working with, I wouldn't feel that comfortable with estimating the yield point if all you are given is the ultimate tensile strength (IE: from a material data sheet). If you can't get access to the CAMPUS data base, then try the material suppliers web site...they may have stress strain curves you can use, or if you're careful, you may be able to scale the yield point if you find a material curve of the same shape...but this is not very reliable if you don't have some experience doing this.

 

[mzain]

yield strength and tensile strength are different.
there is no such formula to calculate the yield stress.
yield stress can be obtained from stress strain curve.
usually yield strength is taken at strain offset of 0.002.

i hope it will be helpful to u
good luck :)