0.2% Offset vs. 0.5% Extension Under Load Yield Strength

[George Zucas]

Hello everyone,

I have designed a part that should be able to withstand a shear stress of 43 MPa ( give or take). The part will be made of Nickel Aluminum Bronze (CuAl10Ni5Fe4). When I check yield strength values for this material (I'll multiply it by 0.6 to estimate its shear strength), they are in general given in 0.5% extension under load yield strength. I am not familiar with this, I always used 0.2% elongation yield strength. How should I proceed in this case? It is given as 290 MPa.

I've researched it a bit, it seems that 0.5% extension under load y ield strength is given when the other one is difficult to measure, but didn't find much else.

 

[256bits]

Both the 0.2% and the 0.5% are arbitrary points on the stress-strain curve. The proportional limit is difficult to measure, and also so is the elastic limit ( tedious repetitions ) so another criteria was chosen so that one could compare one material to another, by just picking it off the graph.

Plastics, by the way use a 2% offset, so you have to know your materials and design implications.

As for the 0.2% offset and 0.5% EUL, design criteria might change from one to the other, which should be slight.
That I am not too sure about, but the difference in most cases should be a small percentage from one to the other.

 

[George Zucas]

Thanks. If the differences are slight, then the design is most probably OK. If it was closer to the limit I'd be worried (43 MPa is not all that high). Though it would be nice to know the exact differences.