Hello and welcome to our first lesson in our course on performance indices. Today we'll be
deriving what a performance index is and showing how it connects to material performance and our
material selection methodology. But before I get into the definition, we need to think about what
we mean by performance. When building something physical, materials come into play. But what
exactly do we mean by material performance? Well, those of you who have taken our intro to material
performance course will already be familiar with this definition, but materials performance
is how a material behaves, with a specific objective in mind, once in service in a product.
A materials properties, which are related to its atomic structure and its processing conditions,
affect how the material performs in a given product, depending our design requirements.
In our basic systematic material selection course, we introduced the Ashby selection methodology as
a way to logically go through and translate our design requirements into a material's functions,
constraints, objectives, and free design parameters that we can use to identify the top material
candidates for a given design. Specifically, when we talked about the objectives we're talking about
what we're trying to optimize in our design and we mentioned how this is related to the function of
the design, especially in the case for load-bearing components. But all the examples we showed only had
one objective, so let's look at another example. Say that I'm trying to design some rope that
I'm going to use for rock climbing. Well, we're going to have some minimum safety criteria for
this rope as it's holding the weight of a human being in the air while they're scaling a mountain.
We can do this with our Ashby charts. Simply create a bar chart with yield strength on the y-axis.
If strength is the property that we're trying to optimize- well- I'm going to be considering
materials up, here like Steels, titanium, even carbon fiber. But if I have to hike to where I'm going
to be rock climbing, I also need this rope to be lightweight. Not interested in carrying
30 kilograms of rope on my backpacking trip. So we simply add density to the x-axis of our chart.
But now the materials that optimized our first criteria, strength, are some of the heaviest.
And the lightest materials are Foams, which aren't very strong and they're not going to meet our
minimum criteria for safety. I just mentioned that this objective relates to the function, but I have
yet to include the fact that this rope will be acting as a tie-in tension while I'm rock climbing.
How does this come into play in my chart? Well this is where performance indices come in.
A performance index is a single material property or a group of material properties
that represent the performance of our design. Now this flow chart that I'm showing on the screen
right here is what we'll walk through in order to derive our performance indices in future lessons.
But let's take a moment to step through it now and see how the elements that we
identified during the translation step of our methodology influence our performance index.
We start with the function. For our case example of the rope, this is a tie in tension.
Next we move on to our objectives. Now while strength is important for our rope for rock
climbing, we don't need ultimate strength. We can set some minimum safe value. Therefore the
property we're trying to optimize is weight, specifically we're trying to minimize the mass.
Then we consider our constraints. We are setting some minimum acceptable strength
as a limit within our design, therefore this is a strength limited design and when coupled with
the objective, will ultimately influence the performance index we get at the end.
But how do we get this combination of properties? Well tune in to our next lesson were we'll go through
the derivation in detail and give you some additional resources about the theory behind
them. My name is Dr Kaitlin Tyler, thanks so much for joining me and I'll see you in the next one.