To add on to Dr. Franz Berge. Yield stress (YS) is the engineering way to define the onset of plastic deformation in terms of dislocation motion. The tensile strength (or ultimate tensile strength; UTS) is (almost) the point at which necking initiates, i.e. plastic instability sets in. That means the geometric softening dominates the work-hardening beyond UTS. (You may be knowing for onset of necking; the 'Considere Criterion').
Hence, the difference between UTS and YS (i.e., expressed by their ratio) tells you how much the material work-hardens. If it is large, work-hardening is more and the material is more ductile. Also this gives an indication for the resistance to crack propagation.
Relationship between YS and UTS: The ratio YS/UTS can be related to YS and one can calculate UTS by knowing YS, but it will have lower bound estimated values. please see link: http://www.twi-global.com/technical-knowledge/faqs/structural-integrity-faqs/faq-can-the-ultimate-tensile-strength-of-steels-be-estimated-from-the-yield-strength-or-02-proof-strength-values/
The maximum load that the specimen withstands defines a common engineering property, the ultimate tensile strength, UTS= Pmax/ Ao. Ao is the initial cross section of the specimen.
The applied tensile stress required to induce plastic behivor is known as the elastic limit or yield stress. For soft material like aluminum boundary between elastic and plastic region is not well defined the yield stress is taken to be the stress required to produced a specified , a small value of permanent strain, usually 0.2%.
The hard materials or those materials showing sharp yield point, the yield stress is usually taken as the minimum, or plateau, in stress that occurs just after the yield point. This stress is referred to as the lower yield stress.
Dear Jimmy,
I'd like to add a small point of concern to the remarks of Franz Berge and Phaniraj C - as both implicitly mention, knowing a value of the ratio between YS and UTS also implies that you already know a lot about the material in question, e.g. in terms of its failure characteristics etc. Thus if you are studying a "new" material, as seems to be the case with respect to the composite you mention, such information might not be available at first. Thus an option could be to look at similar materials - you have another question open here on Al matrix MMCs, so I guess these form the background of this question here, too - and transfer available data on the respective relationship to your case.
Hope I got your intention right: My understanding was that you have just UTS data available and want to derive some insights on yield strength from it - please correct me should I be mistaken.
Kind regards,
Dirk
Thanks Dirk, you are absolutely right. I have the results for the UTS and i am thinking i can get close values of the yield strengths of the materials with some formulas if they actually exist. And you mentioned close materials, please can you suggest some.
Dear Jimmy,
could you maybe describe your materials and process a little more? What matrix, which reinforcement, how is the material actually made, and what consequences does this have for its structure?
Kind regards,
Dirk
In order to have some feelings about the mechanical properties of engineering materials, one should take at least two undergraduate courses related to this topic. These are compulsory courses almost for all Engineering Departments in METU.
Especially the phenomenon of upper and lower yield points in steel involve very sophisticated dislocation point defect (paraelastic interstitial carbon atmosphere ) interaction mechanism, and it is also very sensitive to strain rates, and test temperaturs.
Good Luck
YS and UYS may have only some practical significance in structural engineering for those static load carrying elements having exposed to mainly uni and bi-axial tension, in the estimation of the safety factors.
I agree with Prof. Tarik Ömer Oğurtani that the ratio of YS/UTS (generally called as Y/T) has implications or significance in structural integrity.
Please see if you have access the following inks:
1) "THE SIGNIFICANCE OF THE YIELD STRESS/TENSILE STRESS RATIO TO STRUCTURAL INTEGRITY": http://www.eurofitnet.org/sintap_BRITISH_STEEL_The_Significance_of_y-t_ratio.pdf
2) http://www.eurofitnet.org/sintap_BRITISH_STEEL_BS-25.pdf
3) "SIGNIFICANCE OF THE RATIO OF TENSILE STRENGTH TO YIELD STRESS (TS/YS) OF REINFORCING BARS":
http://www.pgatech.com.ph/documents/Significance%20of%20the%20Ratio%20of%20Tensile%20Strength%20to%20Yield%20Stress.PDF
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