Uniform and non-uniform deformation in metals

Uniform and non-uniform deformation in metals


So we’ll start with a
general stress strain curve for a metal. Now as you probably know,
it looks sort of like this, a little bit linear
elastic, deviates from that, curves its UTS,
then it’s fracture. So what I wanted to do is spend
a moment discussing the type of deformation that occurs. So of course we know
in the linear region it’s pretty safe to
say it’s elastic. I mean, technically
a few dislocations could move in here. But safe to say, it’s elastic. Then, although I’m exaggerating
here so this is not to scale, there’s going to be some
non-linear, elastic behavior. And this is all so-called
uniform deformation. This is all uniform, up to now. That is, the deformation
is taking place over the entire reduced section
of your tensile specimen. There’s our tensile specimen. It’s uniform across the entire
length of the reduced section here. And really we would focus on
the gauge length within that– some region where we define
l0 clip on a string gauge. Then after the nonlinear
elastic region, we’re going to have some
plastic deformation. Plastic deformation. In fact, the plastic
deformation has also uniform. In fact, it’s uniform if you
go up to the onset of necking, which you know occurs at the
UTS, or the ultimate tensile strength, at the peak of the
engineering stress strain curve. It’s still uniform. Necking is the onset
or the beginning of non-uniform
plastic deformation. And so what that looks like
is somewhere within the gauge length, you get
this little region where an instability is set up,
and you get plastic deformation that is not spread
across the entire length, but instead is occurring in
one region, one local region. It’s localized plastic
deformation, localized. That’s what necking
is all about. So this is uniform up
to the point of necking. The deformation is
uniform up to necking. And then for a metal, all of
the deformation that occurs after that is non-uniform. It’s all localized there.

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