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Clamp
Bolt Failure
A clamp on an active, pressurized ceiling fire
sprinkler system was found to be leaking. The repair
technician merely "put his wrench on the nut," when
the clamp bolt separated. An overall view of the
fractured bolt is shown in Photograph A.
Photograph A
A close-up view of the clamp bolt
fracture surface is shown in Photograph B.
Photograph B
A close-up view, of the nut side,
of the subject bolt fracture is shown in Photograph
C.
Photograph C
A stereomicroscopic view, of the
nut side, of the subject bolt fracture is shown in
Photograph D.
Photograph D
The nut side fracture was cleaned
in an ultrasonic bath of alcohol. An
SEM low
magnification (12x) image of the nut side fracture
surface is shown in Photograph E.
Photograph E
A striated appearance was noted
from the 2 to 4 o'clock position, extending in about
40% of the 3 to 9 bolt diameter. Two black arrows
denote the striated region. A more highly magnified
(30x) view of the striated region (a region of
possible
metal fatigue) is shown in Photograph F.
Photograph F
Of particular importance is the
pounded and abraded appearance of the fracture in
Photograph F. This indicates that the
metal fatigue
is not fresh, but has existed for some significant
time period. As the water sprinkler system was
exposed to variations in city water pressure, the
fatigue cracked fracture faces moved against each
other, causing abrasion of the sharp fatigue
striation pattern. This "pounded" appearance
occurred before the final fracture of the
subject clamp bolt. The result is a "pounded and
abraded" fracture feature. A close-up view of the
"pounded and abraded" fracture surface is shown in
Photograph G at a magnification of 1000 times
(1000x).
Photograph G
The pounding and abrasion was
clearly evident at the edge of the bolt fracture. A
close-up view of the pounding and abrasion at the
bolt surface is shown in Photograph H at a
magnification of 400x. The area to the far right is
Photograph H is the thread root in the subject
failed clamping bolt.
Photograph H
Thus, as a result of the
investigation it was shown that the repair
technician was indeed correct, the subject clamping
bolt was cracked almost half way through the bolt
and thus failed at a very low torque. Thus, he was
not responsible for the ensuing extensive water
damage.
However, it was later determined
that the subject service company, for whom the technician
worked, had originally installed the sprinkler
system and had maintenance responsibility. They or
their employees would have been responsible for any
prior tightening of the subject clamp bolt.
Evidently a previous bolt tightening by an unknown
(and over-zealous) repair technician had over
tightened the subject bolt and had initiated a
fatigue crack which later resulted in the fracture
of the clamp bolt.
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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 --- 