Forensic Engineering Investigation of a High-Voltage Transmission Line Anchor Shackle Failure
Keywords:high voltage transmission line, shackle hardware, Charpy tensile test, low-temperature fracture toughness, experimental design, statistical sample, forensic engineering
A forged alloy steel anchor shackle, one of a batch of more than 2,600 produced for the project, failed catastrophically in service on a newly erected 66-kilometer high-voltage transmission line in northern Canada. A failure analysis led to a hypothesis that forging laps had created the critical crack size to initiate propaga-tion under cold weather conditions. An extensive Charpy fracture toughness test program based on CAN/CSA C83.115-96 parameters was performed on 150 shackles, but the data did not support the initial hypothesis of temperature dependence. The forensic engineering team designed experimental tensile tests at ambient tem-peratures as low as -40°C to evaluate the propagation response of lap cracks in a statistically valid sampling of shackles. The trimmed forging flash area disguised laps from the manufacturing process, and subsequent galvanizing steps prevented detection by magnetic particle inspection. A focused recommendation for removal and replacement of the shackles was issued for those bearing major loads in the tower array.
For more information on applied quality theory in manufacturing practice, visit American Society for Quality at ASQ.org, and see https://asq.org/quality-resources/quality-glossary/.
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