Need help with school work again.. This time with strength testing
 

The filthy indian TA gave us a long list of equations for this crap, but none of them have anything to do with break strength under tension. I have ultimate tensile strength for steel equals BHN time 500, but that's not the break strength. Anyway, here's the question.

At what load would a .275" diameter by 6" long steel specimen break in tension if the hardness of the specimen was 321 BHN?

This crap is due in less than an hour and this is the only problem I've had trouble on.

Re: Need help with school work again.. This time with strength testing
 

Get a Tutor with great big tits.... :6

Re: Need help with school work again.. This time with strength testing
 

It seems the breaking strength is the ultimate tensile strength according to this description.

Re: Need help with school work again.. This time with strength testing
 

Ultimate tensile stress is not breaking stress. Once you reach the ultimate stress, the stress will continue to decrease due to necking (thus cross sectional area reducing) until you reach your failure stress.

Re: Need help with school work again.. This time with strength testing
 

Yes.

I have turned this report in, and answered the question sort of half-assed. I got to class early and the general concensus was just to plug ultimate tensile strength into the equation UTS = 500(BHN), then use the answer to derive P in the equation UTS = P(true load)/A(cross sectional area at break). That still only gives load at ultimate tensile strength, but that was how everyone else in class did it, and none of them could find another way either.

Re: Need help with school work again.. This time with strength testing
 

Well were you supposed to use engineering stress or true stress?

Re: Need help with school work again.. This time with strength testing
 

Does not specify, but I take it that engineering stress is the same thing as nominal stress? If so, that the opposite of what I solved for.

Re: Need help with school work again.. This time with strength testing
 

thats the best lol

Re: Need help with school work again.. This time with strength testing
 

wrong...

Yield strength: The stress at which material strain changes from elastic deformation to plastic deformation, causing it to deform permanently.
Ultimate strength: The maximum stress a material can withstand.
Breaking strength: The stress coordinate on the stress-strain curve at the point of rupture.

Re: Need help with school work again.. This time with strength testing
 

Mat, does this remind you of college?

Re: Need help with school work again.. This time with strength testing
 

Ya a little. I remember doing the uts tests in the lab. Man that thing would jump!

Re: Need help with school work again.. This time with strength testing
 

Nothing I said was wrong, you just added info ::) Have you ever looked at a stress strain curve? From your post, it looks like you have no idea what you're talking about.

Re: Need help with school work again.. This time with strength testing
 

Engineering stress is not the same thing as true stress. Engineering stress is taken by the force applied divided by the initial area, there the true stress is the force applied divided by the area at a certain instant.

You can relate the two by the equation:

True Stress = Engineering Stress * (1 + engineering strain) since during plastic deformation the volume is conserved.


Jago, you're right. D-man didn't pay attention to engineering stress-strain curves in school.

Re: Need help with school work again.. This time with strength testing
 

Yes I have looked at a stress strain curve. I'm a welding engineering technologist and have taken 3 years of metallurgy/ engineering materials. Just trying to correct the terminology used. What you described was yeild strength not ultimate tensile.

Re: Need help with school work again.. This time with strength testing
 

Read what I said again. I made NO mention of yield strength. I stated the difference between ultimate stress and breaking stress.

Re: Need help with school work again.. This time with strength testing
 

You said that when you reach the ultimate tensile strength the material will start to plastically deform (necking as you described it) and the stress will decrease until failure. That point that it reaches where it starts "necking" is called the yeild strength but you called it UTS.

Now that I'm reading this thread more closely I've come to the conclusion that we are both arguing the same point but saying it differently.

Re: Need help with school work again.. This time with strength testing
 

No, necking does NOT occur at the yield strength. The specimen starts to deform plastically, but necking doesn't occur until AFTER the ultimate stress has been reached. From the yield point up until the ultimate stress it will undergo strain hardening, which happens before necking. You might want to review what you think you know.

Re: Need help with school work again.. This time with strength testing
 

From here.

http://en.wikipedia.org/wiki/Ultimate_strength


After a metal has been loaded to its yield strength it begins to "neck" as the cross-sectional area of the specimen decreases due to plastic flow. When necking becomes substantial, it may cause a reversal of the engineering stress-strain curve, where decreasing stress correlates to increasing strain because of geometric effects. This is because the engineering stress and engineering strain are calculated assuming the original cross-sectional area before necking. If the graph is plotted in terms of true stress and true strain the curve will always slope upwards and never reverse, as true stress is corrected for the decrease in cross-sectional area. Necking is not observed for materials loaded in compression. The peak stress on the engineering stress-strain curve is known the ultimate tensile strength. After a period of necking, the material will rupture and the stored elastic energy is released as noise and heat. The stress on the material at the time of rupture is known as the breaking stress.

Ductile metals do not have a well defined yield point. The yield strength is typically defined by the "0.2% offset strain". The yield strength at 0.2% offset is determined by finding the intersection of the stress-strain curve with a line parallel to the initial slope of the curve and which intercepts the abscissa at 0.002. A stress-strain curve typical of aluminum along with the 0.2% offset line is shown in the figure below.

P.S. What do you think the plastic deformation is?? It may not be as noticeable but it does start "necking".

Re: Need help with school work again.. This time with strength testing
 

You believe everything you read on wikipedia? That article uses the term necking incorrectly, I should correct that.

Between the yield point and ultimate stress, the specimen will stretch, but this is NOT necking. The cross sectional area is decreasing but necking doesn't occur until the ultimate stress is reached.

http://en.wikipedia.org/wiki/Necking_%28engineering%29

Re: Need help with school work again.. This time with strength testing
 

Ok well I'm going to quit arguing because its not getting anyone anywhere.

When you first posted I thought you were describing the uts as yeild but now that I read it more closely it makes sense. That's why I posted what each were. UTS, yeild, and breaking.

I'll have to dig out my old books and look up the necking thing. I've been out of school for a while now and don't really practice my metallurgy too much. I'm more into the robotics now.

Re: Need help with school work again.. This time with strength testing
 

I gotta say, what jago is saying is what my lab instructor taught us about this. It'll plastically deform between before yield, but only with uniform stretch and thin. Necking, or the narrowing of a specific point on the specimen, doesn't occur until after yield strength.

Re: Need help with school work again.. This time with strength testing
 

Yes that's what I was saying as well. Necking does occur after yeild when it starts to plastically deform but I didn't say if it was after or before UTS. I thought it was before but meh whatever. Must have been wrong.

Totally agree 100%

Re: Need help with school work again.. This time with strength testing
 

You can plactically deform without necking.

Re: Need help with school work again.. This time with strength testing
 

http://www.shodor.org/~jingersoll/we...rial/img21.png
http://www.shodor.org/~jingersoll/we...rial/img21.png

Faggots.

The question is simple. It will break at a lower stress than the ultimate because of necking. Just plug in the ultimate.

Ultimate Strength = Force/Area

Re: Need help with school work again.. This time with strength testing
 

^^ This graph shows it all.

Yield stress is given typically by the .2% rule, which means it is the stress that corresponds with the strain + .2% after that critical point. THAT is the yield stress. Volume is conserved in a specimen as until the you reach the materials Tensile Strength when necking occurs.

Re: Need help with school work again.. This time with strength testing
 

If I had a scanner I'd scan the pages of my Mechanics of Materials book. Seriously one of the best text books I've ever used. I'll probably never sell it.

These kind of questions are pretty easy. You either use Yield or Ultimate. You design for yield times a safety factor just about always. If you want to find if something fails, you set it to the ultimate. The actual length in this problem means nothing. You'd only need to know that if you were doing an elongation problem as this point in your semester.