A “Toolbox” for Forensic Engineers

132 Forensic Materials Engineering: Case Studies
Figure 5.1 Section across the fractured brake lug from the motorcycle (fracture
at right-hand side).
weakened by gas porosity and shrinkage. A similar fault accounted for the
failure of a robot arm casting in aluminum alloy. The robot was designed for
use in a nuclear installation but the arm broke off the body while it was
undergoing proving tests. The seriously defective condition of the flange that
carried the arm was obvious from the state of this machined surface, so the
casting should have been rejected out of hand and certainly not built into a
robot destined to be used for handling radioactive materials. Unfortunately
the project was behind schedule, so the casting went straight into the machine
shop as soon as it was delivered and the machinist did not consider it part
of his duty to report such faults. This was a difficult casting to make and a
number of replacements were x-rayed and found to be similarly defective.
The problem was traced to gases released from the resin binder used to form
the sand molds in the foundry coupled with inadequate feeding of the heavier
sections of the body casting.
Another example, not involving a casting, concerned pivot eyes welded
to the ends of hydraulic cylinders forming part of a lifeboat davit on a
ship. Superficially, the welds appeared to be sound, but the first time they
were deployed in a proving test the eyes broke off and revealed there had
been practically no fusion between the eye and the base of the ram.
Figure 5.2 shows the ends of three of these rams; the two with the large
diameter eyes facing the camera had failed and caused extensive damage
to the installation.