A “Toolbox” for Forensic Engineers

68 Forensic Materials Engineering: Case Studies
Figure 3.5 Pieces of front wheel recovered after the accident.
manufacturing weakness in the steering tube, namely, the reduction in
wall thickness where the internal and external thread roots coincided.
The lawyers were not too confident that they could succeed in such an
action, recognizing that the defendant manufacturers would argue that this
particular steering tube was no different from thousands of others fitted to
their machines and such tubes had no history of spontaneous failures such
as alleged. Unfortunately the engineer died before they had gone beyond the
initial stage of litigation, so they decided to seek a second opinion. All the
earlier reports were made available to a second expert, who immediately
recognized that the two specialist laboratories had each been sent only one
damaged item from the machine. Each laboratory had reported on the one
part as instructed, without having an opportunity to see any of the other
components from the motorcycle. The engineer had done nothing with the
rest of the machine except take two or three general photographs. Neither
he nor the laboratories had considered the load transfer path.
The second expert was provided with the laboratory reports but immediately
recognized that the chain of damage started at the front wheel and
finished at the broken steering tube. He first considered the rider’s statements
and then followed the load transfer path, starting from the tree instead of
the fracture in the steering tube.
Although the rider claimed he remembered seeing sparks coming from
the road below the front wheel immediately before he lost control, no signs
of abrasion could be found anywhere on the pieces of rim from the front
wheel. A magnesium alloy casting exhibits little ductility, so it had shattered
into pieces rather than deforming as a steel wheel would have done. All but
a few tiny pieces had been recovered and none of them showed any evidence