Stress-Strain Diagrams

Stress-Strain Diagrams


In this video we will provide an overview of engineering stress-strain
diagram instrumentals Engineering stress-strain diagrams are developed from physical testing. A carefully
prepared test specimen is subjected to a tensile test in a universal testing
machine A testing procedures is typically guided by
a standard such as ASTM American Society for
Testing and Materials This ensures that everyone who’s performing the tests follows the same procedure Stress-strain diagrams allow us to plot the
results from tensile tests and graphically identify important
mechanical properties As a reminder, stress is force per unit area that results from an
applied load. Now these these applied loads could be in tension, compression, shear, torsion or any combination Strain is the physical deformation
response of a material to stress So an example of strain would be elongation of material When a test specimen is first placed
in a tension testing machine the force is increased and the strain is proportionally increased If we look on our stress-strain diagram
this is the linear portion of our graph If you were to release the specimen in this
region the specimen would return to its original shape, so no deformation has
taken place If we think about the specimen at the
micro-level the bonds are stretching This is considered the elastic region
and the stress-strain diagram The slope of the line in the elastic
region is the modulus of elasticity, also know as the material stiffness As we increases the load being applied to the test specimen we willl reach a point where there is no longer linear behavior this is our proportional limit. With a
little more loading past our proportional limit we will have noticeable permanent
deformation that takes place in our test specimen This point is our yield stress. Yield stress is the stress associated with this point. Yield starts at 0.2 percent strain for most metals What this means is if the stop our
tensile test and unload the specimen we find the
engineering strain is 0.002 Note since there is a degree of recovery we do not draw the unload line straight down. We draw parallel to the elastic curve. One thing to also realize is at a
microscopic level we have dislocation motion and permanent deformation that has
occurred that is why we are now in what we would
call our plastic region With additional force the test specimen
will eventually begin to neck or rather the diameter or thickness of the test specimen decreases in size We call this point the ultimate tensile
strength or UTS. This is the maximum possible engineering stress that the specimen can take in tension If we continue to load our specimen with additional force we will find that this system will
eventually fail At this point our bonds have completely
broken. As we did with the yield stress if we want to understand engineering
strain at fracture we need to unload parallel to our modulus of elasticity line For designers modulus of elasticity yield strength and ultimate tensile strength are all important mechanical properties We need to make sure we are designing
components as well as products that can withstand the strength of the materials that are being used within the product We need to make sure our component as well as our products do not go beyond the yield strength
or ultimate tensile strength when they’re in service or failure will end up occurring. These are some of the elements that design engineers need to
take into account from mechanical properties

46 comments on “Stress-Strain Diagrams

  1. Agamemnon1002 Post author

    In some materials it is not obvious from the stress strain diagram where yielding begins. Typical of this is Aluminium. So in order to set a standard they take 0.2% strain as the point where yield occurs. 0.2% strain simply means that the specimen has stretched by 0.2% of its original length (strain = elongation/original length). Even if the material is slightly in the plastic region with 0.2% strain, next time it is loaded it will be linear until this strain is reached. This work hardening.

    Reply
  2. Agamemnon1002 Post author

    You are very welcome. I am a practicing stress/structural engineer with specialism in Finite Element Analysis using primarily NASTRAN. Feel free to reply to me on this post if you ever have any other questions and I will try and help.

    I wish you luck in your studies.

    Reply
  3. niema30 Post author

    Why does the line slope downwards after it has reached its ultimate tensile strength? Why does the stress decrease after this point?

    Reply
  4. Agamemnon1002 Post author

    OK, the downward sloping of the curve is because once the load reaches a certain level, the specimen begins to exhibit noticeable "necking", by which I mean the section area begins to reduce (Poisson's effect). However, when the stress is being calculated at this level of load the original crossectional area is used whic means the stress is calculated to be lower than what it would be if the "true" are was used. If the true area was used you will not see this downward slope.

    Reply
  5. Agamemnon1002 Post author

    Your video is very informative. It is not easy to cover everything of such a vast subject in a limited time. Besides that is the beauty of this type of forum, in that it presents the opportunity for other to add to the information.
    I had very good teachers who taught me a lot both at university and in my working career, so I feel it is the least I can do to pass it on.

    Reply
  6. Dana Plant Post author

    Can you explain this a little more? Do you mean the radius of the material is shrinking faster than the load is being applied? i.o.w. necking is happening faster than loading?

    Reply
  7. Agamemnon1002 Post author

    No not really. What is simply happening is that after plasticity has been exceeded by a certain amout the specimen begins to adopt a kind of hour-glass profile. Imagine if you will taking a roll of plasticine and pulling it. You will soon notice that it will begin to thin. this will happen whether you do quickly or slowly. However, afetr yielding the amount of force required to produce a given strain is reduced. Dividing by the original area thus gives a lower stress and the curve slopes down.

    Reply
  8. daLuckyClovers Post author

    the graph looks like it starts aty pooint 0.0 , but that is not correct is it? doesnt strain start after a certain amount of stress? if i am correct what is the name of that stress?

    sorry for all the questions

    Reply
  9. Karthikeyan Subramanian Post author

    So many errors!

    Stress is not applied in a test…displacement (or in some cases strain) is applied either a constant displacement (or strain rate)

    "Yield stress starts at 0.2% strain" is just utter nonsense! Yield begins at yield stress. The engineering strain at which yield begins depends on both the yield stress and the elastic modulus. 0.2% strain is simply a well agreed upon convention for identifying the approximate yield stress, i.e, where the total plastic strain in 0.2%.

    Reply
  10. Joshua Hays Post author

    after watching multiple other videos on this same subject, this video summed it up and also made sense in a portion of the time. Thank you

    Reply
  11. Au Kit Wa Post author

    I would like to ask, if we unload in the plastic region, how should we get the permanent strain? Should we follow the same method that drawing the line parallel to the elastic modules ? Thank you very much.

    Reply
  12. Kelly Chng Post author

    Hi there,

    I am writing to you on behalf of Nanyang Technological University (NTU), Singapore to seek permission to use the video published at the link below for teaching purpose.
    https://www.youtube.com/watch?v=t9eB0PKYAt8 Stress-Strain Diagrams

    Currently, I am working with a Professor teaching "MA4807 Marine Structural Integrity" and to better explain the material, the professor would like to use your video via online learning.

    In order to ensure that we do not infringe on copyright, NTU is requesting for permission to use your video strictly for educational purpose and will only be accessible to a closed group of students. Please let me know if there are any clearance procedure(s) to allow the use of material for the above-stated purpose.

    Kindly response by 30 Nov 2017. If there is no response by the cut-off date, we will take it as approval of use and credit the material to you.

    Other details of the course are as follow:
    Requesting Party: eLC Pte Ltd (Authorised Content Partner for NTU TEL Project)
    Institute: Nanyang Technological University
    School: School of Mechanical & Aerospace Engineering
    Course Name/ Course Code: Marine Structural Integrity / MA4807
    Faculty: Associate Professor Huang Weimin
    Email: [email protected]
    Contact: (+65)6790 4859

    Best Regards,
    Kelly Chng
    Consultant, Instructional Design Team (IDT)
    eLC Pte Ltd | 16 Tannery Lane, Crystal Time Building, #06-00, S347778
    Tel: +65 6846 9040 | Fax: +65 6846 9030 | Web: www.elc.com.sg

    Reply
  13. Bartosz B Post author

    Is it only me who has a feeling that some things are mixed up here?
    proportional limit should be probably called elasticity limit instead

    Reply
  14. Mohit Gaur Post author

    Actually stress is not any normal force, you should particularly mention that It is the resisting normal force acting per unit area.

    Reply

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