A “Toolbox” for Forensic Engineers

220 Forensic Materials Engineering: Case Studies
Figure 7.2 (A) Failed and new radiator tanks compared. The upper tank failed
after only 500 miles in service. (B) Failure of the tank by slow leakage through
a brittle crack (arrows).
Why should the distortion be important? It clearly distinguishes the
failed box from a new box, so might be critical in determining the cause
or causes of the failure. What then could cause such distortion? A problem
common to all shaping processes is the problem of frozen-in stress or strain.
When material is cooled from its molten state into the final product, such
stresses and strains can be caused by overly fast removal of heat from the
material. In polymers, the problem is one of frozen-in strain, which provides
a driving force for crack growth, as well as causing distortion and
hence mismatch with mating parts. In this case, the distortion had probably
been revealed by exposure to the hot water of the cooling system, at a
temperature not too far from the glass transition point of the nylon of
about 79°C. The melting point of the material is about 265°C (as shown
by the DSC curve of the material in Figure 7.4). Although there was no
obvious connection between the distortion and the crack, it was an observation
to keep in mind as the investigation progressed.