17-7ph CH900 spring designed to yield to size?

[PHPPAB]

The data I have for 17-7ph ch900 has a yield strength of 231 KSI and ultimate of 239 KSI.

An engineer designed a small leaf spring with this material that in tests plastically deforms (stretches).

This has to be impossible, right? With the yield and tensile right on top of each other, this is a very brittle material, correct? Maybe the parts we have were not heat treated properly to the ch900 spec.

Sanity check - if you're designing a spring, you shouldn't intend to stress it beyond the yield point, especially with a brittle material, correct?

 

[israelkk]

When a metal reaches the ultimate strength it doesn't break immediately it can be stretched under constant load until it actually break. This is called elongation. see

http://www.aksteel.com/pdf/markets_products/stainless/precipitation/17-7_PH_Data_Sheet.pdf

here it says that 17-7PH CH900 has 2% elongation.

You can actually design a spring for stresses beyond the yield and even beyond the ultimate. This is done by a preset process (scragging or set remove as it is sometimes called) for the spring but this is an advanced subject in the field of spring design.

http://israelkk.googlepages.com/home

 

[PHPPAB]

I understand what you are saying about creating a "prestressed" state in the component. This was a situation where the engineer probably didn't calculate the spring correctly and was surprised somewhat that there was plastic deformation. "uh, yeah, it supposed to do that. It's OKay!" My materials book says that, generally, materials with less than 5% elongation to failure are considered brittle. Between this brittle condition and the plastic deformation, I'm nervous about this design.

 

[desertfox]

hi PHPPAB

I agree with your thoughts you wouldn't normally design the spring to yield. Set removal or scragging is carried out on springs and yes the spring is designed to initially yield but can then work at a higher design stress than originally intended and it usually is done to improve fatigue life of a spring through favorable residual stresses as well as a means of adjusting the preload of the spring.
If the spring you refer to is a small single rectangular leaf spring subject only to a static loading then its unlikely that the designer intended the spring to yield in operation.

regards

desertfox