A “Toolbox” for Forensic Engineers

design to ameliorate if not eliminate failures, they stand or fall on the abilities
of product engineers to recognize failure modes in a very wide variety of
different situations. This is where the specific skills of forensic investigation
become paramount. One analogy that is appropriate to the work of a forensic
investigator is that of the forensic pathologist, and the parallels between the
two activities are explored in Chapter 2, a chapter that also includes an
introduction to the properties of different materials. A very high proportion
of failures are caused by unexpected loading, or repeated loading, or by
loading in the presence of aggressive chemicals, or by unexpected levels of
stress at specific points in a product. Such mechanical failures demand some
background information about the nature of loads, their magnitude and how
they relate to the properties of the material of which a product is made. The
nature of loading patterns is discussed in Chapter 3, which deals with the
difference between load and stress for an elementary situation that involves
only tensile forces. The concept of loading path is developed. The approach
is vital to understanding more complex loading patterns, such as the type
that occurs in automobile accidents. A variety of different techniques (the
so-called “toolbox” of Chapter 4) is available to study the physical remains
after an accident. Some tools are very simple: a hand magnifier is often the
only tool that is needed. Even so, the high magnifications from a microscope
are often needed to reveal the failure mode of a specific product. What is
seen is not always perceived, however, which is the justification for the small
amount of background theory supplied in the earlier chapters. All these parts
of the book are provided with illustrative examples taken from our case notes,
many of which have some intrinsic interest for their relevance to the changing
pattern of failure. Thus Chapter 4 includes mention of the failure of a soldered
joint in a circuit board. Thermo-mechanical fatigue is an important generic
failure mode for all electronic equipment, use of which has grown exponentially
in the last decade. It is therefore a failure mode that will grow in
importance as equipment ages. Hopefully, designers will recognize the significance
of the failure mode and rectify their products accordingly.
The following chapters are grouped according to the kind of defects that
lead to failure. Chapter 5 deals with manufacturing faults in products. It ends
with another area of interest: the application of new technology to improve
manufacturing performance. Thus gas molding seems to be a process that
offers everything without any drawbacks, apart from a license fee for the use
of patented technology. The reality is somewhat different, as the case study
about heavy-duty chair arms shows. Chapters 6 and 7 discuss the frequent
failures encountered with materials used for fluid containment (Chapter 6)
and storage vessels (Chapter 7). While pipe and cylindrical storage vessels
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