The objective of the paper is to examine bolt loosening behavior in a bolted joint under transverse loading both experimentally and numerically. Bolt loosening tests were carried out using Junker’s type loosening machine. In the Junker’s loosening tests, repeated transverse displacements (displacement range S = ±0.35mm were applied to a bolted joint with a M10 hexagon bolt and nut, and then a reduction in the axial bolt force was measured. In the FEM calculations, bolt loosening behavior was examined for a model of Junker’s type loosening machine. In addition, the effects of the flatness and the surface roughness at the bearing surfaces in a clamped part on the reduction in the axial bolt force were examined in the FEM calculations. As a results, it was found that the nut rotation occurred when a glider plate moved toward to the center of the joint from the right and the left dead points. In addition, bolt loosening mechanism was elucidated using the change in the contact stress distribution at the engaged screw threads. The reduction in axial bolt force was found to be the largest for the first 1 cycle loading and then it decreased gradually as the loading cycles increased. An amount of the reduction in the axial bolt force was predicted in the FEM calculation taking account of the flatness (inclined of clamped part) and the surface roughness (contact surface ratio) in 10 loading cycles. The contact surface ratio (surface roughness) was found to significantly affect the reduction in axial bolt force. The predicted results were in a fairly good agreement with the experimental results. Furthermore, the effects of the grip length and the nominal bolt diameter were examined on the bolt loosening behavior experimentally.

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