A “Toolbox” for Forensic Engineers

326 Forensic Materials Engineering: Case Studies
ductile material in torsional shear would tend to follow a helical path at 45°
to the axis, not at 90˚ as this one does (see Figure 2.17B).
In contrast, a circular shaft broken by mechanical overload in torsional
shear would display a degree of ductile deformation (twisting) all around the
periphery before the fracture started (hence the bent splines). The fracture
surface would appear as a series of concentric rings, like smears, finishing as
an area of tensile fracture near the center as the two pieces finally came apart,
thus leaving the characteristic “pip” (see Figure 2.7). Hence, from these
observations alone, the fracture was caused by torsional overload and not
bending or any other form of fatigue.
In this particular investigation the independent metallurgist appointed
by the insurers of the manufacturing company carried out an exhaustive
examination of the failed shaft and associated components to ascertain
whether they departed in any way from their material and manufacturing
specifications. None did, but when the other components inside the steering
box were examined, clear evidence was found of a massive impact against
the steering arm which had forced several of the recirculating balls so hard
against the worm that the case-hardened raceways had been indented.
Hence the shaft did not fail by fatigue, and should not have been held
responsible for the loss of steering control at the inquest. It was almost
certainly broken as the vehicle ran along straddling the crash barrier before
it went over the bridge parapet. Sadly, this is typical of the kind of accident
that results when the driver falls asleep at the wheel. The vehicle drifts off its
path and there is no evidence in the tire marks on the road of any braking
or steering correction prior to the truck falling over the bridge parapet, which
there surely would have been if the driver had remained alert.
10.1.4 Further Examples of Steering Shaft Failures from Motor
Vehicle Accidents
Figure 10.6 shows the bottom of a splined steering column where it entered
the steering box in a small family car. It was, unfortunately, not an isolated
case. Several broke in this manner when the cars were getting old and the
majority of these vehicles were found to have tight swivel pins on the front
axle due to lack of greasing. This is a complex fracture caused by cyclic
torsional loads, where fatigue initiated at the outside of every one of the
individual splines. The cracks started by following a helical path across each
spline and when they entered the main cross section below the splines they
continued as individual cracks toward the axis, all maintaining an orientation
at 45° to the axis and producing the star-like appearance in the final fracture.
However, just before they met up at the axis torsional overstress on the last
remaining cross section produced a small pip of ductile fracture at 90° to the
axis. The stress on a circular shaft subjected to torsion varies as the cube of