Initial yield stress Vs. tensile strength

In summary, the person is trying to extract the initial yield strength from a stress-strain curve for non-linear FEA analysis. They are unsure if the initial yield strength is the same as the yield strength and are wondering if the terms have been confused in translation. They are asking for clarification and guidance on how to properly define the yield point in their analysis. They mention that plastics do not have a well-defined yield point like metals and suggest using the CAMPUS database or material supplier's website for stress-strain curves. They also mention that yield strength and tensile strength are different and cannot be calculated using a formula.
  • #1
assafwei
20
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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.
 
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  • #2
Why do you think that the usually specified property is the tensile strength?
 
  • #3
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.
 
  • #4
I suppose it's because plastics harden by deformation, so subsequent tests would obtain a higher yield.
 
  • #5
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.
 
  • #6
Do you not think you should study and understand the basic general load/extension graph of ductile materials before messing with FE analysis?
 
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Likes Joe591
  • #7
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.
 
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  • #8
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.
 
  • #9
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 :)
 

1. What is initial yield stress and how does it differ from tensile strength?

Initial yield stress is the amount of stress a material can withstand before it starts to deform plastically. This means that the material can return to its original shape once the stress is removed. Tensile strength, on the other hand, is the maximum stress a material can withstand before it breaks. In simple terms, initial yield stress is the point at which a material starts to permanently deform, while tensile strength is the point at which it breaks.

2. How are initial yield stress and tensile strength measured?

Initial yield stress and tensile strength are both measured using a tensile test. This involves applying a gradually increasing load to a material until it reaches its breaking point. The amount of load and the corresponding deformation are recorded, and the initial yield stress and tensile strength can be calculated from these values.

3. Is initial yield stress always lower than tensile strength?

No, this is not always the case. Some materials, such as ductile metals, may have a higher initial yield stress than tensile strength. This is because these materials can undergo significant plastic deformation before reaching their breaking point. On the other hand, brittle materials may have a lower initial yield stress than tensile strength, as they have a lower ability to withstand plastic deformation.

4. How do initial yield stress and tensile strength affect material selection?

The initial yield stress and tensile strength of a material are important factors to consider when selecting a material for a specific application. Materials with a high initial yield stress and tensile strength are more suitable for applications where they will be subjected to high levels of stress or loads. On the other hand, materials with a lower initial yield stress and tensile strength may be better suited for applications where flexibility and ductility are required.

5. Can initial yield stress and tensile strength change over time?

Yes, initial yield stress and tensile strength can change over time due to factors such as environmental conditions, exposure to stress or load, and aging. For example, exposure to high temperatures can cause a decrease in initial yield stress and tensile strength, while certain treatments, such as heat treatment, can increase these values. It is important to consider these factors when selecting a material for a specific application.

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