Hook`s diagram, criteria of failure

  • Last Post 05 June 2018
Adisa posted this 05 June 2018

Hi everyone,

I have found the data sheet of PA6-GF30, and I have question about the Ultimate Stress and criteria of failure of the material.

In the following pic. whether capital letter Y means the Ultimate Stress (the largest stress)  and can i put that value of stress (convert to true stress) as criteria of failure.

OR the failure will occur after the Ultimate Stress the end of necking (The necking is not showed in the below pic.) and because that U.Stress and strain can not be the failure criteria?





I read that the secant  module is used for the plastic material, but i do not know when i can utilize that. The secant model of PA6-GF30 is shown the next pic.

If someone knows answer, please tell me.

Best regards.



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srangelova posted this 05 June 2018


As far as I know in the above diagram Y indicates the point of yielding, whereas B (Bruch) is the point of fracture of the material. The yield point is usually used for serviceability limit state; the point of fracture is when the material will truly fail, but depending on the design you are making different standards place different points as limit states: some consider already plastic behavior as critical. In the diagrams above it seems like true stress-strain  rather than engineering stress-strain is depicted, so probably that is why the necking element is missing. 


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Adisa posted this 05 June 2018


Thanks for the answer.

For the metal, the yield point means pass from elastic to plastic and for the plastic it is used for serviceability limit, i.e. after which will occur the necking and after that the part will break. I need break criterial therefor from the above the pic  I can not find that point. I think the buch is the point of fracture and in the pic it is fragile  fracture (T -40,0 C).

Thanks, If You have some experiments with the plastic material,  please send me.

If have anything, what can help me that improve understanding the failure (books, links, analysis), or has anybody.

If somebody has real life test  and fem analysis i.e verification fem analysis.


Best regards.