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.