A “Toolbox” for Forensic Engineers

26 Forensic Materials Engineering: Case Studies
Number of Failures
Failures identifying
minor design fault or
need to adjust material
specification
Isolated failures attributable to abnormal
operating conditions or abuse
Time in Service
Approaching end of
useful lifetime; failures
due to wear, fatigue,
creep, corrosion, etc.
Figure 2.1 Incidence of failures during the lifetime of a manufactured product.
useful lifetime of the machine and promotes safety. For an aircraft there are
no layovers at 40,000 ft.
The metallurgist and the materials engineer may aptly be regarded as the
pathologists of the engineering failure. The medical pathologist is called upon
to ascertain the cause of death by applying knowledge of the way the human
body works and how disease and trauma affect cells, tissues and organs. Hence,
if the driver of a crashed vehicle is found to be dead after an accident it is vitally
important to establish whether death was due to some natural cause such as a
stroke or heart attack that led to loss of control or whether it was the result of
violent impact in the collision. A number of parties will be keen to know the
findings, not only those investigating the cause of the accident but also insurers
of the driver and owners of property damaged in the accident. A whole chain
of litigation may be initiated on the basis of the pathologist’s findings.
But what if the driver was not killed but claims the vehicle suddenly went
out of control as it was entering a curve and, when the wreckage is examined,
it is discovered that some vital part of the steering or braking system is
broken? It is just as important to establish whether this was broken in the
collision or was the cause of the driver’s loss of control. This calls for investigation
by someone able to relate the way the part failed to the circumstances
of the accident. The necessary skills include the ability to interpret fractures
and identify failure modes in terms of the mechanical properties and manufacturing
methods of metals, plastics and other materials used in engineered
products. Armed with such knowledge and experience, it becomes a fairly
straightforward matter to determine whether a particular component recovered
after an accident was damaged or broken by sudden, massive overstress
in an impact or whether it failed spontaneously as the result of gradual wear
and tear or some progressive weakening (“disease”) such as corrosion or
fatigue.