A “Toolbox” for Forensic Engineers

328 Forensic Materials Engineering: Case Studies
Figure 10.7 Steering shaft failed in bending fatigue.
10.2 Wheel Detachments
On the road wheel detachments may result from both fatigue failure and
mechanical overload. When wheels are held on to the hub by means of nuts
with conical seats tightened onto studs in the hub or brake drum, the
sequence of failures can be readily deduced from examination of the individual
fractures. The first ones to fail will display the greatest area of fatigue
cracking and the later ones the least, often with severe erosion of the stud
on the last ones to break due to the wheel moving about as it became
increasingly loose. By identifying the first studs to fracture it is then a fairly
straightforward matter to investigate the cause. The fatigue cracking invariably
starts at the root of the first thread below the nut, as this is the smallest
cross-sectional area and subject to the greatest stress when the nut is tightened.
One of the reasons that wheel nuts are now tending to be replaced by
conical seating bolts with a long, reducing taper into the threads is that these
reduce the stress concentrations significantly compared with nuts that screw
onto a uniform thread profile. The root of the first thread in engagement is
the most highly stressed and is consequently where fatigue cracking is most
likely to initiate (and where a torsional overload fracture will occur if the nut
is grossly overtightened).
Wheel detachments have also been found to occur by the wheel center
itself disintegrating due to the joining up of fatigue fractures around an
individual bolt hole. Such detachments are usually attributed to incorrect
mounting of the wheel on the hub or failing to tighten the nuts.
Another common cause of wheel detachment as a result of fatigue is
when a stub axle carrying the wheel breaks away from the suspension, as
illustrated in Figure 10.7. Such failures usually occur when the vehicle is