So let's look at plastic deformation.
And what is plastic deformation?
I mean, we had for elastic deformation
that the sample dimension is returned to normal
upon unloading.
So we could say that the sample dimensions do not
return to normal, or return-- sorry,
to original upon unloading.
That's not bad, upon unloading.
So sample dimensions do not return to original.
In fact, we could do the same thing
we did for elastic deformation, and we could say, well,
that has to mean that at the atomic level, atoms,
or molecules, if we're going to be broad here
about plastic deformation, independent
of the class of materials, so metal, ceramic, polymer
molecules move past one another, let's say,
to new resting positions.
OK?
And now, in fact, how that happens is a bigger question.
We'll cover this later.
If I model atoms as little circles,
here, in two dimensions, we're going
to learn how this actually occurs.
The mechanism is, for movement of these atoms,
we apply a shear stress to it.
And you'll see that what actually happens
is these atoms, they slide past each other in metal,
and that's how it occurs.
But we don't really need to worry about it too much just
yet.
Plastic deformation is going to involve somehow
atoms moving to new positions.
And what we could continue on with, here, is then,
what the stress strain behaviour would look like.
Before we wrap up plastic deformation,
though, let's just explore one aspect of this
that I think is potentially confusing sometimes.
And that's that plastic seems to refer to a material class.
And I like to break the world down into metals, ceramics,
and polymers.
And so the common, the lay term for polymers is plastic.
And you might think, well, plastic, then,
has to only refer to polymers.
But of course, that's not true.
Polymers are often called plastic because they are often
plastic themselves.
But plastic is referring to, in fact, the type of deformation,
not the material itself.
So in fact, plastic-- this is a little language lesson, here--
derives from the Greek plasticos, which means roughly
to shape or sculpt, with the implication
that it's done permanently.
You permanently change a shape.
In fact-- and this is probably a more generally exciting
or interesting topic-- but plastic surgery
is not called plastic surgery because you're
implanting polymers, although sometimes surgeons do.
In fact, that's quite important.
Whether it's for reconstructive purposes or cosmetic,
there are polymers that are implanted.
But it's called plastic surgery because you're changing
anatomical features, right?
And again, it could be reconstructive or cosmetic,
but we're changing some features permanently.
So that's the usage of plastic, here,
and that's the usage of plastic in plastic deformation,
as well.
It's a permanent change in shape.