Design Realization/Detailed Design - Medical Device Design: Innovation from Concept to Market - page 167

Table 8.3: Example Material Search Table

Property Range of Values

< 1800 kg/m 3

Density

> 500 MN/m 2 Yield strength

Water absorption Low

Gamma irradiation No effect

Max operating temperature 40°C

Min operating Temperature − 20°C

it will state if it is used in the medical industry and sometimes even what for! For a fee you
can also link the materials database direct to ANSYS and SolidWorks (described earlier).

The second is not a search engine but a materials selection package developed by Cambridge
University (UK). CES (Cambridge Engineering Selector) enables you to enter design criteria
and then potential materials appear. More importantly, it enables you to put in the design
criteria in such a manner that not only do potential materials appear, but it is possible to select
the optimum material using merit indices (as described earlier). It is beyond the scope of this
text to teach you CES, but you will find most engineering departments will have access to its
full potential.

When using search engines you need to be structured. You can do random searches but this
normally ends with everyone using stainless steel as everyone simply ends up there. Instead
be very strategic. Develop some search criteria as illustrated in Table 8.3 . You will find that
you will have to undergo some rigorous design calculations first.

As with previous advice, a good PDS will have provided most of this information. However
each specific search will have more detail associated with it (such as yield strength) that
cannot be known at the PDS stage; the detail will only be made clear after some calculations
have been performed.

Once you perform your search you will have a range of materials to pick from. You need to
order these with some form of merit index. You are free to use the weighted selection criteria
demonstrated in previous chapters. Equally, Ashby (2004) proposes the use of merit indices.
If you come from an engineering background the merit indices are easy to understand; if you
do not then use the weighted selection criteria table. One thing you can all do is plot a graph.
Let us, for example, suppose that you really need a material that has low water absorption
properties but high yield strength. If you plot a graph of one property versus the other, and
then use points to represent the material, you may get a graph similar to that in Figure 8.7 .
Clearly the materials you really want lie in the bottom right-hand corner. Figure 8.8 illustrates
a typical Ashby merit indices graph - in this case Young's modulus versus density. Some
common medical device materials have been indicated. The shaded areas demonstrate where
typical material families reside on this graph. If you plot a line (such as the ones indicated)