C haPter 5 Material Property Charts and their Uses
164
of judgment. The most important property is given the largest w;
the second most important, the second largest, and so on. The W i ’s
are calculated from Equation 5.5 and 5.6 and summed. The best
choice is the material with the largest value of the sum
W
W
i
i
= ι
(5.7)
Sounds simple, but there are problems, the most obvious of which
is the subjectivity in assigning the weights.
systematic Methods: Penalty Functions and
exchange Constants
Real-life materials selection almost always requires that a compromise be reached between conflicting objectives. Three crop up all
the time. They are:
■ Minimizing mass. A common target in designing things
that move or have to move, or oscillate or that must respond
quickly to a limited force (think of aerospace and ground
transport systems).
■ Minimizing volume. Because less material is used and because
space is increasingly precious (think of the drive for everthinner, smaller mobile phones, portable computers, MP3
players … and the need to pack more and more functionality
into a fixed volume).
■ Minimizing cost. Profitability depends on the difference
between cost and value; the most obvious way to increase this
difference is to reduce cost.
To this we must now add a fourth objective:
■ Minimizing environmental impact. The damage to our surroundings caused by product manufacture and use.
There are, of course, other objectives specific to particular applications. Some are just one of the four previously described, expressed
in different words. The objective of maximizing power-to-weight ratio,
for example, translates into minimizing mass for a given power
output. Some can be quantified in engineering terms, such as maximizing reliability (although this can translate into achieving a given
reliability at a minimum cost) and others cannot, such as maximizing consumer appeal, an amalgam of performance, styling, image,
and marketing.
164
of judgment. The most important property is given the largest w;
the second most important, the second largest, and so on. The W i ’s
are calculated from Equation 5.5 and 5.6 and summed. The best
choice is the material with the largest value of the sum
W
W
i
i
= ι
(5.7)
Sounds simple, but there are problems, the most obvious of which
is the subjectivity in assigning the weights.
systematic Methods: Penalty Functions and
exchange Constants
Real-life materials selection almost always requires that a compromise be reached between conflicting objectives. Three crop up all
the time. They are:
■ Minimizing mass. A common target in designing things
that move or have to move, or oscillate or that must respond
quickly to a limited force (think of aerospace and ground
transport systems).
■ Minimizing volume. Because less material is used and because
space is increasingly precious (think of the drive for everthinner, smaller mobile phones, portable computers, MP3
players … and the need to pack more and more functionality
into a fixed volume).
■ Minimizing cost. Profitability depends on the difference
between cost and value; the most obvious way to increase this
difference is to reduce cost.
To this we must now add a fourth objective:
■ Minimizing environmental impact. The damage to our surroundings caused by product manufacture and use.
There are, of course, other objectives specific to particular applications. Some are just one of the four previously described, expressed
in different words. The objective of maximizing power-to-weight ratio,
for example, translates into minimizing mass for a given power
output. Some can be quantified in engineering terms, such as maximizing reliability (although this can translate into achieving a given
reliability at a minimum cost) and others cannot, such as maximizing consumer appeal, an amalgam of performance, styling, image,
and marketing.