A “Toolbox” for Forensic Engineers

224 Forensic Materials Engineering: Case Studies
Figure 7.8 (A) Main crack near buttress corner showing witness marks from
fluid spills and weld line visible on lower side of fracture. (B) Detail of fracture
surface.
is a line of smooth material on the inner side of the fracture. This could
represent unfused polymer present within the weld line already detected. Such
indeed proved to be the case when the sample was examined in the SEM
(Figure 7.9). Due to the much greater resolution of the method, vague or illdefined
features from the optical microscope could be seen with much greater
clarity. The picture already shows substantial detail in the fracture, with a
three-dimensional view unequaled by optical microscopy.
One hidden bonus of SEM, at least in this case, was the need to coat
with gold to give a conducting surface (essential with polymers or nonmetals)
to bleed away electrons from the main beam. When the sample was
reexamined optically, the contrast and definition was much improved, as
Figure 7.10 shows. The weld line surface was now shown to be running for
a large distance along the fracture surface. But more significantly, there
were numerous visible cold slugs of partly melted granules embedded in
the surface (shown by the upper arrows in the photograph). Their presence
seemed to confirm a problem of “cold molding” in the sample. This occurs
when the molding machine is started up ready for production. The barrel
heaters are still warming up, and the polymer granules are not fully melted
and homogenized before injection into the tool.
What about the mechanical strength of the material? It was important
to test the material directly, and compare new and failed polymer samples.
Tensile testing would provide basic mechanical properties, which could, for
example, be used for comparison with the specification provided by the
material supplier.