G. Harrigan, V. Goyal, P. Babuska, N. Trepal
Structural and forensic analysis concluded that the Arecibo Observatory M4N socket joint failure in August 2020 was due to a socket joint design with insufficient design criteria that did not explicitly consider socket constituent stress margins or time-dependent damage mechanisms. The socket attachment design was found to have an initially low structural margin with a finite service life, notably in the outer socket wires, which degraded primarily due to long-term zinc creep effects that were activated by longterm sustained loading and exacerbated by cyclic loading. Additionally, HAC and wire defects were found in a few outer wires that may also have contributed to initial outer wire failures. The design did not explicitly consider the time-dependent effects of creep and cyclic loading on design capability, account for a worstcase build condition traceable to in-service inspection of features (e.g., zinc creep/extrusion), specify an end-of-life capability requirement associated with service life degradation, or explicitly set service life inspection intervals with pass/fail inspection criteria. In-service inspections showed evidence of progressive zinc extrusion on several Arecibo sockets, which in hindsight were evidence of cumulative damage and effectively a missed opportunity to prevent cable failure. Open spelter sockets of this type are used throughout industry in stay cables. The following recommendations are proposed to prevent failures of similar socket joints: (1) Verification of positive stress margins in socket joint wires for all failure modes, (2) Periodic visual inspections with acceptance criteria for zinc extrusion that are tied to structural qualification, (3) Revisiting civil codes and industry standards based on lessons learned from this analysis.
Published on 08/12/21Accepted on 08/12/21Submitted on 08/12/21
Volume 21, 2021Licence: CC BY-NC-SA license
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