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: Overall view of fractured clamp
 bolt on ceiling sprinkler system

A close-up view of the clamp bolt fracture surface is shown in Photograph B.

Photograph B: Overall end view of failed
 clamp bolt.

A close-up view, of the nut side, of the subject bolt fracture is shown in Photograph C.

Photograph C: Close-up view of failed clamp bolt.

A stereomicroscopic view, of the nut side, of the subject bolt fracture is shown in Photograph D.

Photograph D: Close-up view of nut side fracture
 surface on subject clamp bolt.

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: Low magnification (mag. 12X)
 scanning electron micrograph of nut side
 of subject fracture. Striated appearance of
fracture surface is denoted with black arrows.

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: Close-up view of striated
 bolt fracture surface.

Of particular importance is the pounded and abraded appearance of the fracture in Photograph F. The pounded and abraded appearance indicates that the metal fatigue on this fracture surface is not fresh, but has existed for some significant time period. As the water sprinkler system was exposed to variations in city water pressure, asperities (high points) on 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: Highly magnified (mag. 1000X) view
 of abraded and pounded region of fracture surface.

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 in Photograph H is the thread root in the subject failed clamping bolt.

Photograph H: Close-up view (mag. 400X) of
 pounding and abrasion on bolt head surface.

Thus, as a result of this investigation it was shown that the repair technician was indeed correct, the subject clamping bolt was cracked almost halfway through the bolt and thus failed at a very low applied 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 and/or over 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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