A “Toolbox” for Forensic Engineers

Establishing the Load Transfer Path 81
Figure 3.16 Unused bracket tested to destruction in the compression test. The
fracture is virtually identical to the accident bracket.
wheel being pushed inward. The defendant’s expert then obtained an unused
bracket from the manufacturer, fitted with an axle, and tested this in a
compression test machine. The clamps began to deform at 7.4 t, and the
brazed joint could be heard to crack at 8.2-t force and finally split open as
shown in Figure 3.16 at 9.1-t force, with the break virtually identical to that
of the accident bracket. There was no sign of any defect in the fracture surfaces
of the bronze weld. The cracking followed the interface between the bronze
and the steel.
Suspecting that the discoloration of the fracture surfaces of the plaintiff’s
bracket was postaccident and most probably resulted from exposure to moist
atmospheres during the 8-month storage period between the accident and
the expert’s examinations, part of this laboratory-produced fracture was
masked off and the hub was left in an outdoor store. The exposed area of
fracture quickly started to become discolored. This was because the fracture
followed a brittle intermetallic layer formed at the interface of the bronze
and the steel, which corrodes rapidly in moist air. The masked region was
hardly affected after 2 weeks but 3 months after the masking was removed
it was indistinguishable from the rest of the fracture interface. Hence, as
neither expert had seen the hub fracture until several months after the accident
and the machine had been stored in a farm barn for this period, there
was no way of deducing what the fracture would have looked like immediately
after the accident. The topography of the fracture displayed none of the
characteristics of metal fatigue, so the defendant’s expert continued to believe
it was an instantaneous fracture caused when the wheel hit the ground.
The plaintiff’s expert had obtained a replacement clamp and put it
through tensile tests to determine the force required to break the allegedly
unsuitable bronze welded joints on the lug. Instead of the interface failing as