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The Case of the Escapee Cows and the Broken Chain Link

Pasture Gate Chain to Secure GateThe Problem:

The incident occurred near remote railroad tracks on New Year’s Eve.  The temperature was -12ºF.  There was no structural damage to the gate, vertical members of the gate, or chain post assigned to the chain link failure event.  During the night, several hundred cows exited the opened gate that was reportedly locked closed by the chain.  The cows travelled. single file through the snow and were all hit and killed by the locomotive of a freight train.

Cattle Push Against GateMy Assessment: 

The 2 subject chains were original one single length of chain almost 48” long until the failure incident, when the chain became separated into 2 pieces.  The single “broken link” was never found at the site and was unavailable for metallurgical laboratory assessments. An exemplar chain was obtained for comparison of mechanical properties with the subject chain.  The exemplar chain was categorized as a straight link coil chain, #2/0, zinc plated.  In the laboratory, measurements, visual examinations, mechanical tests and microscopy to 50X of several, failed exemplar links were performed.  

NACM Specifications contained common nomenclature and analysis techniques of the links in chains. The NACM Specs took exception to chain link performance for sudden applications of dynamic loads, which could exceed the working load limit.  

Failed Link in Chain To Secure GateThe Consequences: 

The subject chains were standard, galvanized and compliant with working load specifications.  The herd of cows would not be able to apply a sufficient dynamic load, by pushing on the gate or jumping on the subject chain, to fracture a single chain link and not cause noticeable damage to other chain links, chain post, or gate or all 3 of these items.

The Lessons Learned: 

The subject chain links should have exhibited damage if cows had exerted sufficient downward force or lateral force on the subject chain to break the subject failed link.  It was unlikely that the subject chain link broke as a result of sufficient, blunt force to a single link in the chain to cause the link to break.

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Welding Failures I Have Known, Interesting cautionary tales from Metals & Welding  expert, Dr Jesse A Grantham, PE