|
Fire
Truck Fitting Failure
A cast aluminum
alloy "Y" fitting, that failed during
hose pressure testing and injured a firefighter, 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 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 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
testing, chemical analysis and hardness testing.
Considerable macro and micro porosity was observed
on the primary fracture face. Photograph B
illustrates 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. 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.
|
Dr.
R. Craig Jerner, Ph.D., PE specializes in accident
investigation and metallurgical failure analysis,
with over 30 years experience as a metallurgical
consultant and accident investigator. He has
testified as a metallurgical expert in over 250
depositions and more than 70 court appearances. If you or someone you know should need the services of Dr. Jerner and J.E.I. Metallurgical, please visit our web site at the buttons below, or e-mail Dr. Jerner --- 