Steel and Composite Structures

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Evaluating long-term relaxation of high strength bolts considering coating on slip faying surface

  • Nah, Hwan-Seon (Smart Energy Lab., Korea Electric Power Corporation Research Institute) ;
  • Lee, Hyeon-Ju (Smart Energy Lab., Korea Electric Power Corporation Research Institute) ;
  • Choi, Sung-Mo (The University of Seoul, Department of Architectural Engineering)
  • Received : 2013.09.26
  • Accepted : 2014.03.31
  • Published : 2014.06.25
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Abstract

The initial clamping forces of high strength bolts subjected to different faying surface conditions drop within 500 hours regardless of loading, any other external force or loosening of the nut. This study develops a mathematical model for relaxation confined to creep on a coated faying surface after initial clamping. The quantitative model for estimating relaxation was derived from a regression analysis for the relation between the creep strain of the coated surface and the elapsed time for 744 hours. This study establishes an expected model for estimating the relaxation of bolted joints with diverse coated surfaces. The candidate bolts are dacro-coated tension control bolts, ASTM A490 bolt, and plain tension control bolts. The test parameters were coating thickness, species of coating. As for 96, 128, 168, and $226{\mu}m$ thick inorganic zinc, when the coating thickness was increased, relaxation after the initial clamping rose to a much higher range from 10% to 18% due to creep of the coating. The amount of relaxation up to 7 days exceeded 85% of the entire relaxation. From this result, the equation for creep strain can be derived from a statistical regression analysis. Based on the acquired creep behavior, it is expected that the clamping force reflecting relaxation after the elapse of constant time can be calculated from the initial clamping force. The manufacturer's recommendation of inorganic zinc on faying surface as $75{\mu}m$, appears to be reasonable.

Keywords

Acknowledgement

Supported by : KETEP

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