A “Toolbox” for Forensic Engineers

204 Forensic Materials Engineering: Case Studies
loads. EDAX analysis showed traces of sulfur on the eroded part of the
surface, supporting the acid attack hypothesis. However, the traces of sulfur
disappeared further into the surface, suggesting that the crack grew mainly
by simple strain of the pipe system later in the failure. So what loads led to
crack growth?
6.6.3 Loading of the Return Pipe
The return pipe was not clamped to the engine manifold, but rather lay freely
against it, as shown by the abrasion zone of Figure 6.22. Ties are normally
available to prevent contact, but the present pipe examined appeared to have
lost its ties some time ago. Flexible fuel pipes can move freely against rigid
parts of the vehicle chassis, movement occurring when the engine is turned
on and thereafter with driving. So what did the major and lesser striations
represent in terms of the history of the vehicle? It was possible to suggest that
the major crack arrest lines (Figure 6.24) represented start-up of the engine
from cold, while the minor striations represented engine acceleration/deceleration
during driving. Diesel fuel would thus have been leaking under low
pressures (about 4–8 psi) from the return pipe with the engine running and
the junction partially severed. The fuel lubricated points of abrasion, and
reduced the load on the failing junction while the vehicle was moving. However,
when stopped overnight, the diesel would have evaporated away, leaving
a dry junction by the time the vehicle was started up again the next morning.
Friction at such junctions would have been greatest during start-up. Such
leakage increased with time as the hinged crack grew, with final separation
just before the major spillage onto the road near Dundee.
6.6.4 Conclusions
The most likely explanation of the accident was therefore as follows. A small
leak of battery acid occurred during routine maintenance of the recovery
vehicle, with acid falling onto the nylon junction immediately below the
battery. Chemical erosion of the edge led to stress corrosion cracking of the
junction (Figure 6.27). One crack grew radially across the section, major
growth occurring at engine start-up. The seven such major crack arrest lines
represent seven engine start-ups from cold. Since garage vehicles are generally
kept running through the day, each crack arrest front probably represents
growth in 1 day, so the total sequence lasted about a week. Diesel fuel would
have leaked slowly down the chassis and onto the nearby road, and would
have been visible to a vigilant driver. Regulations in the U.K. stipulate daily
checking of commericial vehicles for potential hazards, for example. Diesel
fuel leaves a very slippery deposit on the driving surface, when the lighter