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Failure Analysis Case Histories
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Mechanical Fatigue of Aluminum Antenna
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ENVIRONMENT:
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Atmospheric - High Winds |
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EQUIPMENT:
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Antenna mounted on top of a tower |
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MATERIAL: |
Aluminum |
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FAILURE:
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Mechanical Fatigue -
Vortex Shedding |
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The upper portion of an antenna consists of a 2" schedule 40 type 6061-T6 aluminum pipe 22 feet long. The antenna is supported at the bottom only. The supports are mounted on top of a tower.
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The location of the failure was at a ½" drilled hole in one wall of the pipe,
Figure 1. The pipe snapped in two pieces, and the site of final breakage occurred at a tear directly opposite from the hole.
Microscopic examination revealed a ductile fracture. No evidence of corrosive attack was found,
Figure 2.
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Vortex Shedding is a stress related failure caused by the cyclic oscillation of a vertical cylinder by wind.
The point of failure is in close proximity to a "node", or a fixed point that does not move during oscillation.
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Corrective action may be performed by shot or bead
preening the pipe surface, particularly at locations on the pipe which will be fixed points after antenna assembly.
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