Testing and introduction of new materials - Orthopaedic Biomaterials in Research and Practice - page 373

Last cycle

1
Define
objectives

2
Set goals

4
Develop
concepts

5
Select
approach

3
Set
specifications

6
Complete
design

Gather
information

7
Evaluate
design

Not ok

Review

Ok

Next cycle

FIGUre 15.1 (See color insert.) the design cycle.

writing is an important step, as it focuses the attention of the designer or
design team on the design process.
Let us suppose that a materials design cycle is to be undertaken at
the request of an engineering design group that is working on a novel
total hip replacement (THR) prosthesis. The group reports that none of
the materials in their handbooks provide the appropriate combination of
stiffness, strength, and fatigue life that they require.
Thus, the design objective may be stated as

The objective is the design of a new material suitable for use in fabri-
cation of THR prostheses that combines optimum stiffness (modulus)
with greater strength and a higher endurance limit than those presently
available.

Step 2: Define goals Design is not a simple process that leads to a unique
output. Thus, objectives must be refined to limit the number of choices at
each step and to guide the design cycle. This is achieved by selecting goals
whose attainment either (1) is necessary to attain the desired objective
or (2) represents generally recognized “good” attributes of engineering
design. The first type is called specific goals, while the second is called
general goals. An initial list of specific and general goals that might follow
from the previously stated objective is given in Table 15.1.*
The initial list of objectives arises from a discussion with the “cus-
tomer,” in this case the device design group, and represents the true
starting point of the design cycle. It embodies the customer's concept

* These are sometimes referred to as “must have” and “should have,” respectively. The
design process often turns up a third category: “like to have.” If all of the first two types
can be met, then this latter type is a bonus!