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Mechanical Fatigue of Aluminum Antenna


Atmospheric - High Winds


Antenna mounted on top of a tower
MATERIAL: Aluminum


Mechanical Fatigue - Vortex Shedding

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.

The location of the failure was at a " 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.

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.

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.

Figure 1. Fractured Aluminum Pipe (0.55X Original Magnification) Figure 2. Fractured Surface of Pipe, no evidence of corrosion.

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