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Fatigue Life of Double Notch Alloy Steel Breakaway Couplings

File Language:
English


Details

  • Creators:
    Hahin, Christopher
  • Corporate Creators:
    Illinois Department of Transportation. Bureau of Research
  • Subject/TRT Terms:
  • Publication/ Report Number:
    PRR Number 176
  • Resource Type:
    Tech Report
  • Geographical Coverage:
    United States
  • Edition:
    Final Report: December 2016 - June 2022
  • Corporate Publisher:
    Illinois. Dept. of Transportation. Bureau of Research
  • Abstract:
    Double notch alloy steel breakaway couplings have sustained numerous fatigue failures during their service life in Illinois. This report is an in-depth analysis of breakaway couplings that ruptured, resulting in collapse of a light pole falling across a 55-mph expressway. Pole was fortunately cleared away by maintenance before any collisions occurred. This report is an analysis of breakaway coupling stresses derived from peak wind gusts sustained over a 28-year period of service life. The fatigue properties of the couplings made of SAE 4140 steel were also related to their impact and fracture toughness, exposure to deicing salts, and how high wind speed events resulted in fatigue crack propagation. Two recorded wind gusts occurred during service that exceeded 70 mph causing cumulative fatigue damage to the couplings. Couplings were also hot dip galvanized, ostensibly to increase their durability, but were subject to liquid metal embrittlement due to higher tensile strength of air-cooled SAE 4140 steel. Presence of two notches further increases their susceptibility to fatigue failure. Couplings had low impact toughness ranging from 5-6 ft-lbs. (7-8 J) at -10° to +60°F (-23° to 16°C). Corrosion-fatigue cracks generated by wind stresses were correlated with propagation rates (da/dN) for structural steels exposed to saline water that formed from dew and deicing salts. Predicted critical crack growth was within 1% of actual crack size at the time of failure. Installation instructions requiring “snug-tightness”, followed by 1/3 turn-of-the-nut, induced a substantial mean stress in the couplings, adding to stresses derived from windstorms. The mean stress was estimated using SAE 4140 data and the Goodman fatigue failure equation. Recommendations are made to change to less susceptible alloys with improved impact toughness and corrosion resistance to deicing salts and salt fall in coastal locations. To prevent future problems, coupling manufacturers must provide specific guarantees to assure a realistic number of years of service life for locations where deicing salts are applied or couplings situated along coastal areas. If failures occur, manufacturer’s cost of replacements, including installation, would be pro-rated based on the guaranteed years of service life.
  • Format:
    PDF
  • Collection(s):
  • Main Document Checksum:
    urn:sha-512:ffbf00445b7b73eef57ce2feb1667a0a00454409b0020c75f32f2771f6e52740590cb66fc338a8c39de9da982af08ed47772c4e606c807090536753d8645f083
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  • File Type:
    Filetype[PDF - 2.06 MB ]
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