# With which parameter can you specify whether a metal is ductile or brittle

I have to do a durability approval of feather key (shaft-hub connection) and the formulas are dependent if the shaft respectively the hub is ductile or brittle.

But what is the decision about to say it is ductile or brittle.

• My norm for feather key isn’t saying anything how to distinguish if the metal is ductile or brittle.
• The norm for pressfits uses the parameter fracture elongation and reduction of area.
• And the literature is talking about brittle metal when the Maximum Stress Theory is valid $$(\sigma_{limit}/\tau_{limit}=1)$$ and ductile metal when the Shear Stress Hypothesis is valid $$(\sigma_{limit}/ \tau_{limit}=2)$$.
• $$\sigma_{limit}$$ is tensile strength
• $$\tau_{limit}$$ is the strength when plasticity begins

Okay, there are many possibilities… But which one is the right possibility? Or is there a connection between my findings that I’m not aware of?

• What is the material of shaft or hub ? What is the working temperature? Does any impact force happen during the working of the machine on the shaft or key connection ? And could you please specify the parameters ''fracture elongation and reduction of area" without the exact values difficult to say ? and at what range those parameters remain valid ? – Sam Farjamirad Oct 8 '18 at 15:09
• Sorry if this sounds dumb, but if the shaft is made of a brittle material you use the brittle formula, and if it's made of a ductile material you use the ductile formula. If you don't know what it is made of, then you can't do your "durability approval" at all. There are not "many possibilities" - only two! – alephzero Oct 8 '18 at 15:49
• I'm not aware of a hard definition for ductility, it is rather a qualitative description of the material ability to deform plastically before rupture. But as previous comments suggest, the ductility of a given material is also affected by strain rate (how fast you deform it) and temperature. – user190081 Oct 8 '18 at 16:37
• @ user190081: That’s exactly what I’m looking for. A hard definition of ductility. The reason why I’m asking this “dump” question is because different norms are using different definitions for ductility. And that’s a little surprising for me. But perhaps I have to say, that I calculated with the basis of “this” norm and therefore the ductility is defined with the parameter fracture elongation. – tueftla Oct 9 '18 at 6:26
• @ Sam Farjamirad: The idea of my question is to be more general. When working through a norm at a certain point it is needed to decide if your metal is ductile or brittle. And because different norms have a different definitions of ductility and brittleness there is no hard definition. And that, a hard definition, is exactly what I’m looking for. – tueftla Oct 9 '18 at 6:49