A “Toolbox” for Forensic Engineers

324 Forensic Materials Engineering: Case Studies
for the vehicle leaving the road as the driver would have had no steering
control. If the braking system had been damaged by collision with the crash
barrier it would not have been possible to stop the vehicle before it reached
the bridge. There were no tire marks to suggest the brakes had been applied
prior to collision with the barrier.
A legal representative of the vehicle manufacturers questioned this witness
and asked if he had noticed any twisting of the splines of the steering
shaft. The witness replied that he had observed they were bent noticeably on
both sides of the fracture (see Figure 10.4); if the splines had been in this
condition before the accident they would have altered the position of the
steering wheel in the “straight ahead” position. When asked if it was likely
that this shaft had been twisted for months or weeks, the witness replied that
if it had, he would have expected someone driving the vehicle regularly to
have noticed it, unless the steering had been completely reset immediately
after the splines had been bent.
This witness was further questioned about the possibility that the shaft
might have been cracked before the accident. He replied that if the shaft had
been held together by the small area at the axis he would have expected it to
have failed suddenly on the road, even if the vehicle were unladen. He would
not expect such a small segment of metal to be able to withstand heavy
steering loads such as those experienced in maneuvering a fully laden vehicle.
Subsequent to his examination at the scene, he had taken the wheel and tire
to the forensic science laboratory for expert examination, along with the
steering box and broken sector shaft.
The forensic scientist was a highly qualified chemist with 20 years’ experience
in forensic science and an expert in tire examination. He had examined
the parts submitted to the laboratory. He stated his findings could not possibly
support the theory that premature failure or deflation of the tire caused
the accident. This witness had also examined the steering box and sector
shaft. The extract from his report read to the court was as follows:
Rocker shaft. The fracture surfaces of the two pieces were
examined and found to exhibit characteristics typical of
fatigue failure. It was apparent the components had been
operating for some time on the central quarter-inch of
metal, but after a cursory examination it is not possible
to state whether the final failure was spontaneous or the
result of impact.
A verdict of “accidental death” was recorded. The witnesses’ statements
all supported the view that the driver lost control because the steering arm
suffered a fatigue failure.