A “Toolbox” for Forensic Engineers

Failure of Storage Vessels 227
been working to weaken the wall near the buttress corner. The final factor
was the distortion in the box noted earlier. It was possible to calculate the
effect it could have produced from a tensile curve made during mechanical
testing, together with the observation of the widening of the crack in the
whole failed sample box. The analysis gave the surprisingly large value of
about 20 MN m –2 . Thus the net effect on the potential initiator was for a
total stress of about 59 MN m –2 . This value is now comparable with the
weakest sample tested, so it seemed reasonable to conclude that the combination
of defects initiated one or, more likely, a series of cracks that created
the final leak.
7.2.7 Aftermath
The results of the investigation were clear cut. The problem was caused by a
combination of defects in the material of the wall of the box. They in turn
had been mainly caused by faulty molding. One scenario by which they were
produced has already been suggested: the failure box was made during startup
of the injection molding machine, when molding conditions had not been
established. The failed box was therefore a single maverick. If this was the
case, then several conclusions could be drawn.
In the first place, the radiator manufacturers could be reassured that the
basic design was not at fault, and they could not expect to see widespread
failures on new cars. Secondly, the suppliers could be asked to re-examine
their quality policy. It is normal and good practice for molders to supply
each operator on the machines a diagram of likely defects to look for in
moldings as they are made. This is the first line of defense in quality, and
perhaps the most important, because every component is (or should be)
examined individually. The faulty box must have been mistakenly accepted
as a good box at this stage, and forwarded through to the manufacturer. The
molder should be asked to confirm that such quality checks were indeed in
place, so that the user might be assured of specification-compliant products.
The molder generally has responsibility to supply quality products, but it is
also true that development designs supplied in short batch runs are not always
examined as closely as is really necessary.
Several useful and practical suggestions to improve the situation were put
forward for consideration by the investigation. Inspection could be improved
at the audit stage in QA, by lightly dusting with chalk to show up any surface
defects (such as cold slugs), which could indicate whether there was a problem
in molding. The design could be improved by increasing the radius of the
buttress corner. Sharp radii are easy to ameliorate by simply polishing the
corners of the core which produce the corner in the tool. The effects are often
dramatic, especially when the material of construction is brittle (as in this
case).