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Left eyelet broke off of a Meridian aluminum-frame tandem used for about 25,000 km, about half with little or no rack load, the rest hostel tours with a pack weight around 15 kgf.
The eyelet is a separate block of metal welded to the dropout on three sides. Dark marks indicate oxidation from long-term failure, with light areas indicating the final failure.
Note the absence of dark marks near the unwelded edge: the failure appears to have started away from the unwelded edge, suggesting the lack of a fourth weld did not cause or contribute to the failure.
Further inspection showed the rack sat at an angle to the droptout. With the mounting bolt tight, it would have applied a "biased" load to the eyelet.
The rack is a Bruce Gordon tubular steel rack, which is fairly stiff. The rear vertical tube was dented about 100 mm from the bottom, and a straight-edge on the rack showed about a 3 mm displacement or "J" bend at the eye for the mounting bolt. The right side was parallel the eyelet.
Likely, the rack was damaged in handling or when the bicycle fell over without panniers. It seems plausible the dropout was damaged in the initial bending event and/or was damaged by long-term cyclic loading with a bias trying to bend it.
Note the rack section at the eyelet is more than 50% the dropout thickness, and steel is about 3X the stiffness of aluminum, so the eyelet and/or the weld would tend to flex most.
The eyelet that broke off was added at the buyer's request, so an eyelet failure would not disable the bicycle. The broken eyelet was discovered three days before the start of a 3-week tour, but due to a second unused eyelet was still ridable.
See also FAIL-143.html.