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?

Thanks in advance for your help.

  • $\begingroup$ 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 ? $\endgroup$ Oct 8, 2018 at 15:09
  • 1
    $\begingroup$ 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! $\endgroup$
    – alephzero
    Oct 8, 2018 at 15:49
  • $\begingroup$ 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. $\endgroup$
    – user190081
    Oct 8, 2018 at 16:37
  • $\begingroup$ @ 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. $\endgroup$
    – tueftla
    Oct 9, 2018 at 6:26
  • $\begingroup$ @ 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. $\endgroup$
    – tueftla
    Oct 9, 2018 at 6:49

1 Answer 1


The simplest way to tell if a metal is ductile or brittle is to look at a fracture surface (e.g. from a tensile or impact test). To have either a ductile or brittle metal defines the failure mechanism which you can tell by having either a ductile fracture or a brittle fracture.

Elongation and reduction of area as well as notch toughness (impact test) and hardness testing can give a quantitative value to ductility, by measuring the amount of energy the material absorbs and converts to mechanical deformation. These values are normally used to allow 'design ductility' which is less interested in the failure mechanism and a lot of the time only cares if a material can absorb enough energy not to fail as soon as it is over stressed (or for deliberate deformation). But there isn't a fixed limit for defining a material as ductile or brittle as it depends on the material and is usually used to define a design criteria that must be met.

However if you have an elongation greater than 20% you are probably dealing with a ductile metal, but this can depend on the alloy type/condition and is not always the case for all materials.

  • $\begingroup$ One must define a strain rate before determining if a material is ductile or not. Many carbon steels are ductile in a tensile test and brittle in an impact test. $\endgroup$ Nov 26, 2018 at 17:43

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