A “Toolbox” for Forensic Engineers

10 Forensic Materials Engineering: Case Studies
to allow design and production engineers a way of interacting and talking to
one another in a systematic fashion about product failures and the effects
that led to failure. If there is no deep appreciation of product defects and
their criticality, no management or statistical method will solve the problems
of product failure.
1.4 Product Defects
What are some of the common defects found in products? The first task is
to define more closely the meaning of the word “defect.” One working definition
might state that
a defect is a feature or characteristic of a product or
component which inhibits or prevents the correct functioning
or processing of that product or component
The term “feature” is used in a neutral way to denote any characteristic
of a product or component that does not normally appear in a specification
(such as an engineering drawing). Thus variations in surface texture often
appear on plastic moldings or cast metals as a result of the manufacturing
process. The definition of features, which are also defects, is useful because
it immediately allows us to examine those features that can affect product
function in more detail. There are at least two types of defects:
1. Those already present in the product before sale or use which lead
quickly to rejection or functional failure (in which case they are clearly
defects), and
2. Defects that develop over time in a product, and so can increase the
failure rate after normal use of the product
Such an approach exceeds the narrow legalistic view of defects. For example,
the U.K. Consumer Protection Act (1987) defines a defect as follows:
There is a defect in a product … if the safety of the
product is not such as persons are reasonably entitled
to expect….
The set of defects thus defined are of course safety-critical defects and,
while of overriding interest for much litigation, do not cover aesthetic defects
or defects that render a product inoperable.