Fire
Truck Fitting Failure
A cast aluminum
alloy "Y" fitting failed during fire
hose pressure testing and injured a firefighter. The
failed fire trick fitting was
submitted for metallurgical failure analysis.
The, as
received, knock-on/quick disconnect coupler and
failed "Y" are shown in Photograph A.
Photograph A: Overall view of cast
aluminum "Y" fitting that failed during
fire hose pressure
testing.
The casting in
question, after apparently 27 years of useful
service, failed in a sudden manner. The casting
surface did not contain any manufacturer markings, serial
numbers, casting lot numbers, etc. which would
identify the manufacturer and/or the year of
manufacture.
Testing included
stereomicroscopic examination, scanning electron
microscopy (SEM),
metallography,
microstructural
examination, chemical analysis and
hardness testing.
Considerable macro and micro-porosity was observed
on the primary fracture face. Photograph B
illustrates an area of dendrite structure and
porosity
surrounded by clear indications of
cleavage
fractures. However, observing shrinkage and the
dendrite structure on a casting fracture surface
does not render that casting
defective. A true and
very valid test of adequacy is the performance
history of the casting. As time passes and usage
increases, the probability of a significant
flaw
being present diminishes quickly to zero.
Additionally, no evidence of progressive fatigue
failure was observed on the casting fracture surface
or near any area of casting porosity. Fracture features observed
where fracture did occur were cleavage,
indicating a sudden failure of a
casting material lacking in significant ductility.
The lack of significant ductility is not uncommon to
cast metals.
Photograph B:
Scanning electron micrograph showing dendrite
casting porosity (rounded egg shaped structure). The
yellow arrows denote areas of cleavage fracture
occurring during the sudden overpressure failure.
It was
determined that the cast metal "Y" fitting failed
due to
an over-pressurization from an unknown source.
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