A “Toolbox” for Forensic Engineers

98 Forensic Materials Engineering: Case Studies
Figure 4.7 Manufacturing route revealed by metallography. A rolled screw
thread on alpha brass (A), and a machined brass screw thread on alpha + beta brass
(B). Cast structure of aluminum alloy (C), and hot extruded aluminum alloy (D).
sectioning should always be recorded. The orientation of the section may
also be of importance if the section is selected to show, or is suspected to
possess, directionality. Figure 4.6 is an illustration of a rivet that was sheared
off in a collision which demonstrates how easy it can be to misinterpret
microscopic evidence through selecting an inappropriate section. Figure 4.6A
and B are sections taken at 90 o through the sheared end, where Figure 4.6A
is the structure transverse to the direction of the shearing force and Figure
4.6B the longitudinal view of the same section. Without knowing the direction
of sectioning, Figure 4.6A gives no indication that the fracture was the
result of transverse shear forces whereas Figure 4.6B shows the direction of
shear and the grain distortion quite clearly.
Examination of metallographic sections can be invaluable to the forensic
engineer. As a tool, this technique can reveal important information such as
case depth of a case- or induction-hardened product, thickness and adhesion
of plated coatings and heat-affected zones (HAZ) of welds — all of which
may have a direct bearing on the cause of failure. By observation under
magnification, the metallographic section may also reveal information
regarding the method of manufacture (casting, hot forging, cold drawing,
etc.; see Figure 4.7), and thermal history (either intentionally during manufacture
or accidentally during service) of a component.
Other service effects such as work hardening, oxidation or corrosion and
crack characteristics can be revealed. A case that required identification of a