Notices
Results 1 to 13 of 13

Thread: Mild steel yield stress

  1. #1 Mild steel yield stress 
    New Member
    Join Date
    Apr 2009
    Posts
    4
    Hi,
    i have some mild steel test sample and they are tested with tensile stress. I have obtained the Young's modulus is in the range of 290 to 320 GPa,poisson ratio is 0.27-0.29. However, those stress and strain curve of tho mild steel do not have upper and lower yield strength. So what may be the reason of this problem?thanks

    Regards,
    el-nino9


    Reply With Quote  
     

  2.  
     

  3. #2  
    Forum Isotope Bunbury's Avatar
    Join Date
    Sep 2007
    Location
    Colorado
    Posts
    2,590
    You would find Young's modulus in the elastic range. If you didn't find the yield point I'd say you didn't pull it hard enough.


    Reply With Quote  
     

  4. #3  
    New Member
    Join Date
    Apr 2009
    Posts
    4
    But those specimens were pulled until the they rupture...
    Reply With Quote  
     

  5. #4  
    Forum Isotope Bunbury's Avatar
    Join Date
    Sep 2007
    Location
    Colorado
    Posts
    2,590
    Did you get any elongation before rupture?

    Was the machine properly calibrated?

    Perhaps your specimen had a crack in it.

    Mild steel should be ductile, but unless you have good source documentation you might have been sold a pig in a poke.

    What temperature was your test done at? Steel pressure vessels are not supposed to be hydrotested below 70 deg F due to concerns about brittle fracture. It's questionable if this rrestirction is really necessary with properly documented steels.
    Reply With Quote  
     

  6. #5  
    New Member
    Join Date
    Apr 2009
    Posts
    4
    the elongation is about 30-32% and it is tested in room temperature.


    the 1st image with upper yield and lower yield,and that is suppose how a typical mild steel should behave,am i right?
    but the data i obtained from the test is like second image,just a smoth curve without the upper and lower yield.
    Reply With Quote  
     

  7. #6  
    Suspended
    Join Date
    Apr 2008
    Posts
    2,178
    One thing if nothing else you learn from welding is, that the structure of the object far outweighs what it is made of.

    Take a look at these two structures. And then decide which if pulled straight apart along the shaft, would break first and why.

    You can do all the fancy testing you like. And I know sometimes you are forced to test, to meet some crazy requirement. That forces an agency to hire useless people. That sit around requiring useless tests.

    But take a look, consider they are both mild steel.






    If you look at these up close you can see that both have a radius in the back of the notch. One obviously a smaller radius. But the larger radius goes deeper into the part. I know from actually doing this that the one with the little slot goes first. The reason is that, the surface of the metal cannot flow around the tight radius to supply the stretching part with material. So the flow stops or a lot of heat builds up right there and destroys the metal by causing a tear.

    The large radius notch allows the part to flow and stretch. Sometimes people see this for the first time and become horrified. I was brought up on this. So it is nothing new to me. But if you do not know this one. You can throw all the books and tests away.

    I take into consideration what a square inch of material will hold, with repeated use. That number is often very low. Compared to what some claim it is today.



    Sincerely,


    William McCormick
    Reply With Quote  
     

  8. #7  
    Forum Isotope Bunbury's Avatar
    Join Date
    Sep 2007
    Location
    Colorado
    Posts
    2,590
    I can't help you on this. You might try posting your question here:

    http://www.eng-tips.com/threadminder.cfm?pid=330
    Reply With Quote  
     

  9. #8  
    New Member
    Join Date
    Apr 2009
    Posts
    4
    Quote Originally Posted by Bunbury
    I can't help you on this. You might try posting your question here:

    http://www.eng-tips.com/threadminder.cfm?pid=330
    I tried to register but it stated that there are some problems with my registration and asked to contact the admin but i did not get any reply from the admin though i have emailed them.
    Reply With Quote  
     

  10. #9  
    Forum Isotope Bunbury's Avatar
    Join Date
    Sep 2007
    Location
    Colorado
    Posts
    2,590
    Well, I just registered OK.

    Try a different handle.
    Reply With Quote  
     

  11. #10 Re: Mild steel yield stress 
    Suspended
    Join Date
    Apr 2008
    Posts
    2,178
    Quote Originally Posted by el-nino9
    Hi,
    i have some mild steel test sample and they are tested with tensile stress. I have obtained the Young's modulus is in the range of 290 to 320 GPa,poisson ratio is 0.27-0.29. However, those stress and strain curve of tho mild steel do not have upper and lower yield strength. So what may be the reason of this problem?thanks

    Regards,
    el-nino9

    Here is a decent explanation of the subject. As my earlier pictures pointed out. All this stuff is great and all, but most metals have a similar ratio.





    The problem with predicting is that you need real experience in seeing metal let loose. I love destroying metal. To see where it gives. I actually do call where it will break and why. On welding forums. Often I am called on it so I make a video of it happening.

    But from many years of manufacturing I know what will happen pretty much without any calculation at all. I can almost tell you to the pound what it will take to bend just about any structure. I might have to do some jotting down of numbers to relate the size of parts I know to the ones in question. And I have to make some assumptions about what the size of the object will do to the area that is going to give. But all in all after a while you get pretty good.

    http://www.Rockwelder.com/Welding/St.../SteelTIG.html

    The reason why it breaks next to the weld has to do with the structure created by the weld. There is now a three dimensional area, a shape a "T" shape created by the weld that strengthens the area by the weld. But causes an unnatural weakness next to the weld. Where it will break just about every time. Even with poor welds.

    Poor welds that can crack with time.

    Sincerely,


    William McCormick
    Reply With Quote  
     

  12. #11  
    Forum Sophomore
    Join Date
    Sep 2008
    Posts
    128
    The upper and lower yield strength is related to the movement of dislocation.
    Only when there are few movable dislocations, the upper and lower yield strength can appear.
    So I think maybe your samples have been deformed before the test?
    I suggest annealing the samples and making the tensile test again. If the upper and lower yield strength appears, it shows the samples are deformed. :-D
    Reply With Quote  
     

  13. #12  
    WYSIWYG Moderator marnixR's Avatar
    Join Date
    Apr 2007
    Location
    Cardiff, Wales
    Posts
    5,769
    sorry, i should have seen this thread earlier, i could have cleared up any confusion that may have arisen in the meantime

    mild steel in the hot rolled condition will exhibit a visible yield point, where you can measure a lower and upper yield strength

    however, if the hot rolled material has been skinpassed (deformation of 1-2%) or cold rolled (cold deformation varying from 50 to 98%) this visible yield point will disappear and your tensile curve will be smooth as in the following wiki article - in that case the yield point is defined as the 0.2% proof stress (point 4 on the graph)

    the reason why there is a visible yield point in the first place is that plastic deformation happens through the movement of dislocations, imperfections in the iron lattice structure, and that interstitial alloying elements such as carbon and nitrogen pin these dislocations until the plastic deformation is strong enough to separate them from one another

    after plastic deformation has taken place that has dragged all the dislocation loose from the interstitial elements any subsequent deformation will be smooth - that is, until the material has aged, which means that after a few months the interstitials have diffused back to the imperfections in the lattice and the yield points starts to become visible again
    "Reality is that which, when you stop believing in it, doesn't go away." (Philip K. Dick)
    Reply With Quote  
     

  14. #13 Hello 
    New Member
    Join Date
    Oct 2009
    Posts
    1
    Chemical engineering is the branch of engineering that deals with the application of physical science (e.g. chemistry and physics), and life sciences (e.g. biology, microbiology and biochemistry) with mathematics, to the process of converting raw materials or chemicals into more useful or valuable forms also for a faster medication delivery get to Online Pharmacy get all the products
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •