Hi my name is David Schaad. I am a professor of
the practice and associate chair of the department of civil and environmental
engineering here at Duke University. I want to talk to you today about how
engineers save lives and enable human civilization. Every time you walk
into a large building, or a small building, or go across a
bridge, or drive in a car, or an airplane, the engineers have thought about how to
keep you safe. Even though it becomes second nature to you to just walk through these things. But sometimes
it doesn't work that way and sometimes engineering
failure occurs. Today I want to talk to you about stress
and structural engineering failure. When we think about structural engineering
failure, we know that all failures are not created equal. So
borrowing from my colleagues at the law school
when we think about the foundation that our country rests on with the
jurisprudence is that everyone is presumed innocent until proven guilty.
Well why is that? Well when we think about failures in the law
system, obviously if someone commits a crime they should
have a consequence, that's not a failure. And if someone's innocent
they should have no punishment, that's not a failure. The failures would occur
when someone who commits a crime doesn't have
a consequence. We would consider that a failure of
the system. Or, heaven forbid, someone who's innocent is punished.
Which would we rather have? Again not all
failures are equal. We would much rather have a guilty
person go free than an innocent person go to jail.
So in engineering terms the way we talk about that is a type 1 versus type 2 failure.
We would much rather have a type 2 failure
failure, meaning we would let a guilty person go free, than have a type 1 failure,
which means we would punish an innocent person. So how we
think about this in engineering design is, obviously if we
design a building and we think it's going to be great and we build it and it
is great that's not a failure. If we think it's going to absolutely
fall apart and if we were to actually build it and it would fall apart
that's not a failure because our calculations confirm what happened in reality.
Well what would happens if we design it
we think it's going to fail but then for some reason we would build it.
That would be the mistake. The bigger mistake would have been if we thought it
was going to be fine and then we built it and then it
collapsed. We would definitely not want that. That is a type
1 failure. If we think it's not going to stand up
but it actually would, what do we do? Well we over design the
building. It's going to cost a little bit more
maybe it takes a little bit more person power to put the building up,
maybe it takes a little bit longer, but we over design it to make sure that it's
protective of human health and the environment.
That is the type of failure again that we would rather have
we would rather over design a building than have a building
fail. So we use what are called factors of safety
to implement that and we protect against the eventuality of a
failure happening. So how does engineering failure
apply to fundamental engineering? So we think about
forces that act on structures, whether that's a plane wing, whether that's a
bone, whether that's your car,
and we can pull on them, we can push on them, we can twist them, we can bend them.
And those develop all sorts of different forces that act on an object.
And so when we think about forces, we think about forces that act
perpendicular to a surface or parallel to a surface.
So in three-dimensional space, if we have an object,
or what we call a force vector, that is not orthogonal to one of those planes
meaning it's not perpendicular to one of those planes, we can find the projection
onto those planes and figure out what is the resultant force then is acting
perpendicular and acting in shear. And we realize we
know that different materials function
differently when loaded with different forces.
When we think about force it's really not the force that creates the problem
it's the force per unit, area which is stress.
So think about a very heavy weight supported by a very thin cross section
it's that force per unit area. So we think about force mostly in the U.S.
about pounds per square inch. In other countries it may be newtons per
square meter, but you're thinking about an applied force per unit area.
So the smaller the cross section with the same amount of force it has to bear
more and more and more stress and that's where we get into the problems of how materials fail.
What's really interesting about this is that we can look at how materials
fail and we can understand what were the applied forces
that caused it to fail. So again
returning to our understanding of
normal forces and shear forces, we realize that
ductal materials fail in planes of maximum shear,
and brittle materials fail in planes of max normal. What does that mean? So what we expect to see
is wherever the forces are the greatest is we're going to see the failure planes.
So think about a broken bone, or a broken structure, or a broken computer connection.
All of those things we know by
doing a forensic analysis after the fact, when we see a failure plane
we can tell how that was loaded.
So when we think about failure modes
we think about that a chain is only as strong as its weakest link
and so what ends up happening is when you think about the different ways
things can fail, so think about if you've ever
broken a bone. Maybe you twisted it, maybe it bent
however you loaded that bone
caused it to fail in the ways that it's going to be
the weakest. So again if you think about all the links of a chain
the chain is only as strong as its weakest link. So wherever that stress
is going to be the cause failure we're going to see that occur. So.
Let's talk about a piece of chalk. If we have a piece of
chalk and we load it, we're going to bend this
or pull it apart - it breaks like this. But if we take it and we twist it
it breaks on a different plane. And so we can understand
how, by looking at the plane at which that material failed, we could understand
what were the forces, what were the stresses
that were being developed in the material when it failed.
And so then we think about, this as an aluminum rod
that we put in a torsion testing machine, which means we basically twist it which
means the plane that's being developed is maximum shear.
This is ductile material, remember I said ductile materials fail in plane of max shear
And so that's exactly what we see. If we had a brittle material you would see that that would fail
this chalk is brittle, it fails in planes of max normal
and that in this particular loading happens to be 45 degree angle
a 45 degree angle across the specimen, the object of the specimen. So again
engineers save lives and make human civilization possible.
Hoped you enjoyed this and hope you found it informative. Thanks very much.
you