Relationship between strength and ductility? (Fu/Fy ratio vs. ductility?)

[jzhaan55]

Is there a relationship between strength and ductility for structural steel? In other words, does the Fu/Fy ratio have any connection to the ductility of a metal? If you take a metal rod with 20% elongation before fracture with a yeild strength of 33 KSI and a tensile strength of 45 KSI, how will these numbers theoretically be affected if the % elongation were, say, 35% instead? Any reference articles would be helfpul!

Thanks,
Jessica

 

[Metalguy]

Why do you care about ductility? For forming? Maybe you should be concerned about toughness?

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[mrfailure]

There could be a relationship between stress ratio and ductility if we were talking about true stress - i.e. using the actual cross-sectional area during testing when yield and tensile loads are achieved. But the reason there is no relationship is because we instead have to use engineering stress and calculate based on the original cross-sectional area of the test specimen before testing. This takes ductility out as a factor in stress ratio.

Aaron Tanzer

http://www.lehightesting.com

 

[jzhaan55]

I want the material to be more ductile so that it can be bent back and forth several times without cracking. Would this be a measure of fracture toughness instead? I'm lost any help would be great.

 

[jzhaan55]

Basically I have a bunch of steel sheets that will crack when bent back and fourth three times. The sheets come with mill certs and are rejected if they have a % elongation of below 28%. If they are below 28%, we file a non conformance report and must do an extra bend test to make sure that they will be usable. Their actual intended use in the field will consist of only one full cycle of bending (back and forth), but to be safe we make sure that they start cracking after three cycles. Is there a way that i can analyze previous mill certs that show yield, tensile strength, and % elongation to somehow lower this 28% requirement to say 25%. it is a pain to continually have to file nonconforming reports...

 

[Metalguy]

Why not do some testing to see if 25% is enough? Also, make sure the sheet rolling direction is 90 deg. to the bend.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[FennLane]

The problem is that you are looking at the effect of cyyclic stress-strain behaviour not the Monotonio beaviour that is measured by a tensile test.

I think that it is well understood that even a singe plastic cycle can have a signiifcant influence on material beahviour. - Bauschinger Effects etc.

If you bend it back and forth you will quickly use up any ductility and the work hardening exponent will also have an influence.

By plastically bending a material back and forth you will almost certainly damage the microstructure and change the baisc propeties and significantly effect both fatigue life and impact behaviour. The Fracture Transition temperature will almost certainly increase significantly.