157
screening
Constraints are gates: Meet the constraint and you pass through
the gate; fail to meet it and you are out. Screening (see Figure 5.8)
does just that; it eliminates candidates that cannot do the job at all
because one or more of their attributes lie outside the limits set by
the constraints. As examples, the requirement that “the component
must function in boiling water” or that “the component must be
transparent” imposes obvious limits on the attributes of maximum
service temperature and optical transparency that successful candidates must meet. We refer to these as attribute limits.
ranking: Material indices
To rank the materials that survive the screening step, we need criteria of excellence. They are found in the material indices, introduced here, which measure how well a candidate that has passed
the screening step can do the job (Figure 5.8). Performance is sometimes limited by a single property, sometimes by a combination of
them. Thus the best materials for buoyancy are those with the lowest
density, ρ; those best for thermal insulation are the ones with the
smallest values of the thermal conductivity, λ—provided, of course,
that they also meet all other constraints imposed by the design.
Here maximizing or minimizing a single property maximizes performance. Often, though, it is not one but a group of properties
that are relevant. Thus the best materials for a light, stiff tie rod are
those with the greatest value of the specific stiffness, E/ρ, where E is
Young’s modulus. The best materials for a spring are those with the
greatest value of σ y E
2
, where σ y is the yield strength. The property
or property group that maximizes performance for a given design
is called its material index. There are many such indices, each associated with maximizing some aspect of performance. They provide
criteria of excellence that allow ranking of materials by their ability
to perform well in the given application. The appendix to this
chapter lists some of the more common ones. A further discussion
of material indices, including their derivations and applications, is
found in Ashby (2005) and Ashby, et al. (2007).
To summarize: Screening isolates candidates that are capable of
doing the job; ranking identifies those among them that can do the
job best.
Documentation
The outcome of the steps so far is a ranked shortlist of candidates
that meet the constraints and that maximize or minimize the
Using Charts to Select Translation, Screening, Ranking, and Documentation
screening
Constraints are gates: Meet the constraint and you pass through
the gate; fail to meet it and you are out. Screening (see Figure 5.8)
does just that; it eliminates candidates that cannot do the job at all
because one or more of their attributes lie outside the limits set by
the constraints. As examples, the requirement that “the component
must function in boiling water” or that “the component must be
transparent” imposes obvious limits on the attributes of maximum
service temperature and optical transparency that successful candidates must meet. We refer to these as attribute limits.
ranking: Material indices
To rank the materials that survive the screening step, we need criteria of excellence. They are found in the material indices, introduced here, which measure how well a candidate that has passed
the screening step can do the job (Figure 5.8). Performance is sometimes limited by a single property, sometimes by a combination of
them. Thus the best materials for buoyancy are those with the lowest
density, ρ; those best for thermal insulation are the ones with the
smallest values of the thermal conductivity, λ—provided, of course,
that they also meet all other constraints imposed by the design.
Here maximizing or minimizing a single property maximizes performance. Often, though, it is not one but a group of properties
that are relevant. Thus the best materials for a light, stiff tie rod are
those with the greatest value of the specific stiffness, E/ρ, where E is
Young’s modulus. The best materials for a spring are those with the
greatest value of σ y E
2
, where σ y is the yield strength. The property
or property group that maximizes performance for a given design
is called its material index. There are many such indices, each associated with maximizing some aspect of performance. They provide
criteria of excellence that allow ranking of materials by their ability
to perform well in the given application. The appendix to this
chapter lists some of the more common ones. A further discussion
of material indices, including their derivations and applications, is
found in Ashby (2005) and Ashby, et al. (2007).
To summarize: Screening isolates candidates that are capable of
doing the job; ranking identifies those among them that can do the
job best.
Documentation
The outcome of the steps so far is a ranked shortlist of candidates
that meet the constraints and that maximize or minimize the
Using Charts to Select Translation, Screening, Ranking, and Documentation