Hardness Vs Mises stress

[Bhask]

Hi all,
i am new to this group. Hopefully i find answer to my question. I want to know how i can compare Hardness of material with the Misess stress ! Or any other stress for that mater.

Thank you,
Bhaskar.

 

[EricZhao]

Hardness has nothing to do with von mises stress. Consider a piece of ruber and a cutting tool. They have different hardness. but if there is no load on them. Their Von Mises stress are all zero.

Are you trying to say the relationship between hardness and yield strength? For the same material, usually higher hardness means higher yield strength. But, it is not so cross different materials.

 

[feadude]

Correlation between hardness and tensile strength: Hardness is linearly related to the strength of many materials. For steels, the relationship is given by the equation:

TS(MPa) = 3.1xHB or TS(psi) = 450xHB





good57morning@netzero.com

 

[Bhask]

Hi,
Thank you eric and feadude.
i have a FE simulation in which i have stress components as output. i have measured the hardness of the same part experimentally, now i want to compare the hardness with the anyone of the stress components. actually i found the relation btw yield strength and hardness which you mentioned.
According to strain energy theory ! the yielding of material occurs when von mises stress exceeds uniaxial tensile yield stress. if we see the relation of von mises in terms of principal stresses, for a uniaxial tensile test sigma1 is sigma y (yield stress), sigma2 and sigma3 are zero's. In this case vons mises stress is equal to sigma y (yield stress).

so is it right if i consider the mises stress to be equivalent to yield strength. ! and say mises stress is 1/3 of HB....

Thank you,
bhaskar.

 

[feadude]

No that assumption is not right.
The formula that I mentioned in my other response is a relation between hardness and ultimate tensile strength of steel. It has no relation whatsoever to yield strenth, yield stress or von mises stress.

good57morning@netzero.com

 

[Bhask]

Hi feadude,

can you please check the relation which i found in this link:

http://www.calce.umd.edu/general/Facilities/Hardness_ad_.htm

under the topic
7. RELATION OF HARDNESS TO OTHER MATERIAL PROPERTIES

which states the relation is trues for yield strength is 1/3 rd of hardness ! may be this is wrong, can you please give some reference where i can find the exact correlation which you mentioned.

Thank you,
bhaskar.

 

[EricZhao]

Just clarify that feadude's formula is right. I made a mistake on "yield strength". Similar formulas can be found in Machinery's Handbook.
I am confused about you claim that "von mises stress to be equivalent to yield strength". They are not equivalent anyway. yield strength is a material mechanical property regardless your application. von mises stress is the stress level for your specific part and load. You can compare them to see if your part is safe, but they are not the same thing.

"von mises stress is 1/3 of HB"... Nobody in the engineer community will say this. safety factor might serve your intension. BTW, only von mises stress failure criteria only fits for ductile material.

 

[EricZhao]

Machinery's Handbook P551 has the equation. They are the samething except for the different unit (psi vs. Ksi). But I never trust these equations. Anybody has an idea how accurate they are?

 

[Bhask]

Hi Eric and feadude,

Thank you for your replies. Well i have checked some other sites now.
Now i am with you guys that there is no direct correlation of von mises stress with HB. Now i would be thankfull if can suggest me which parameter i can compare with hardness. i have FE results where in have only data of stresses in the component.
Thank you
regards
bhaskar.

 

[feadude]

i got the formula from page 197 of Mechanical engineering design, fifth edition, by shigley and mischke, except it is in kpsi instead of psi.

good57morning@netzero.com

 

[Bhask]

Hi feadude,
Thank you for the information. If any can give some information which is relevant to what i have asked it would be appreciated.
Thank you all again.
bhaskar.

 

[Drej]

> Now i would be thankfull if can suggest me which parameter i can compare with hardness. i have FE results where in have only data of stresses in the component.

I think you've missed the point somewhat. You cannot compare any stress component (from a load system) in this manner with a mechanical property - there is no correlation, and the questions you are posing don't make sense. Try explaining in more detail what you're trying to do/achieve.


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[EricZhao]

I think he is trying to say how safe the structure is. Why not use safety factor?

 

[Bhask]

Hi,

i have a machining process simulation. i have some chips formed during the machining. i have collected chips from experiments and i have measured the hardness of the chips formed ! i want to check if the FE model is comparable to real time process !but i cannt find hardness of the chips formed FE model. i need to somehow compare the Hardness i measured in experiments with FE model.

Thank you,
bhaskar

 

[Jordonlaw]

Bhask,

I think this is a non-trivial question. The mechanical properties of the chips you have machined off will have undergone hot working, thermal environment and high strain rate loading at least. The subsequent hardness of such chips, whilst easy to measure, probably don't relate ONLY to the metallurgical treatment of your process; the same hardness could occur due to other combinations of events on the same base material.

I'm aware that MSC MARC (and probably other codes) can perform machining simulations and do consider all of the effects I've noted above so there is some chance an FEA simulation could (and there is a very big "if" involved) predict the mechanical properties of the chips which I guess is actually you are after. Otherwise you need to do miniture mechanical tests on the chips themeselves. Maybe someone else out there has done such tests ?