Testing and introduction of new materials - Orthopaedic Biomaterials in Research and Practice - page 375

teams; however, the use of a team introduces additional interpersonal
dynamics problems that are beyond the scope of this chapter.) In this
case, the designer may decide, “I'm going to set the problem aside, go for
a 2-kilometer run, and when I come in, write down the first 10 things that
come into my head.” Such a procedure, with variants, has been adopted
by most creative persons and is sometimes referred to as “creative avoid-
ance” of the problem: undertaking other activities to distract the con-
scious mind and then using the products of subconscious deliberation,
without self-criticism or censoring.
The initial list is then reviewed for reasonableness and duplication
and perhaps the process is repeated or extended, until there is a sense
that all of the immediately possible options, in this case, design goals,
have been acquired. Often the review of an initial list “triggers” new
ideas not previously considered. The designer, in our example, has added
two specific goals and no general goals to those that might be inferred
from the general design objective and the other previously cited desires.
Note that this is an abbreviated example; step 2 of an actual design cycle
might produce dozens of specific and general goals.
The second substep is assigning a priority to each of the goals, to pro-
duce a set of refined goals. This is necessary since, in an actual design
case, the number of goals very rapidly grows to the point where it is
obvious, a priori, that all cannot be met. Thus, a ranking of importance
is necessary. In this case, the designer employed a common practice and
selected three priorities:
High (H): must be met for successful design
Medium (M): would like to meet during design cycle
Low (L): desirable to meet but may be sacrificed
Therefore, the material's modulus is identified as a much more impor-
tant attribute than its color, although the desire to satisfy the sales man-
ager is still considered as part of the later steps in the cycle. In a more
subtle distinction, it is recognized that in the intended application, the
endurance limit is a more important material attribute than the tensile
strength, although both are important. Additionally, some goals may be
restated for clarity.
Step 3: Set specifications Setting specific and general goals and then
refining the list produces considerable clarification of the problem in
hand but does not provide the details necessary for later steps in the
design cycle. To achieve this, it is necessary to translate the refined goals
of step 2 into measurable quantities.
As an example, the specific goal “modulus less than half that of
Ti6A14V” may be translated to a specification “modulus less than 50
GPa.” The general goal “minimum cost” might be translated to “cost of
less than $10 per kilogram.”
In some cases, when the goals are specific, such as “The modulus
must equal that of live cortical bone,” there is also a need to apply a
margin of error or variability to the specification. It is typical to select