A “Toolbox” for Forensic Engineers

104 Forensic Materials Engineering: Case Studies
precise as a chemical analysis but quite adequate for checking material specifications.
The SEM will provide fractographic and microstructural information,
where the EDAX will provide accurate quantitative information on local variations
in material composition. In addition to resolving chemical composition,
EDAX can be utilized to ascertain unusual microconstituents or contaminants
within the material, even down to a single inclusion particle. The advantages
of EDAX are that it is fast and easy, but sometimes requires followup work; for
example, it will reveal Fe and O, i.e., the elements, but will not readily distinguish
Fe 2O 3 or FeO phases. In addition, EDAX can be utilized for quantitative
analysis of thick and thin films, semiconductors and some polymers. In difficult
cases involving chemical attack, the data provided by microanalysis can be
crucial to successfully completing the failure investigation.
An investigation that readily demonstrates the use of the SEM coupled
with the power of EDAX involved the theft of a large quantity of copper
alloy. A factory, manufacturing extruded copper alloy tubing for steam
condensers, suffered a break-in and a large quantity of metal was stolen.
The metal in question was a rather unusual brass of 76Cu-22Zn-2Al, and
the factory was the only one in that area using aluminum bearing brasses
in its manufacturing process. At that time, commercial brasses used for
steam condensers and general manufacture of tubing were 70Cu-30Zn and
70Cu-29Zn-1Sn.
A suspect who worked in a nearby iron foundry was apprehended in
connection with the robbery. He claimed that he had spent the evening in
question drinking with his four buddies, and had never been near the brass
tube factory. However, swarf was dug out of the suspect’s boot, and compared
with a piece of swarf from the floor around a large lathe used to skim billets
prior to extrusion. The SEM comparison is shown as a montage in
Figure 4.12, where the top half was swarf dug out of the suspect’s boot and
the bottom half was swarf recovered from the crime scene. As a lathe tool
wears, it develops a serrated cutting edge that will produce characteristic
score marks rather like the marks on a bullet fired down the barrel of a
particular rifle. It can be seen that there is a perfect wear pattern match
between the two pieces of swarf that can be considered comparable to a
“fingerprint” in conventional detective work. Elemental x-ray analysis on the
SEM confirmed that the sample removed from the suspect’s boot was an
aluminum bearing brass. Therefore the combination of wear fingerprint and
alloy composition was more than sufficient evidence to place the suspect at
the scene of the crime. It later transpired that the suspect and his drinking
buddies were perpetrators of the robbery.
Examples of ESEM are given in Sections 6.6 and 9.5.