Yuki KANEDA

Analytical study on the evaluation of axial stress on corroded high-strength bolts to propose a practical estimation equation

Eiji IWASAKI

In old structures, there is a problem with high-strength bolts joints, which due to their shape and the environment they are in, there is concern about corrosion occurring in the joint which decreases the tightening axial force of the bolt and the frictional joint strength. However, a clear evaluation index for the residual axial force of the bolts against this corrosion has not yet been determined. Therefore, the purpose of this study is to evaluate the residual axial force of high-strength bolts with corrosive wall thinning to propose a practical estimation equation for the residual axial force useful for maintenance.
The residual axial force of the high-strength bolt was evaluated by using FEM. First, the validity of the analysis method that reproduced the corrosion of high-strength bolts, and examined the effects of the shape and dimensional tolerance of each material used in the joint on the decrease in axial force was verified. Then, the effectiveness of the estimation in the analysis model was confirmed by comparing it with the past experimental results. Finally, the estimation equation was proposed.
Even if the threaded part of the high-strength bolt is simply modeled using the effective cross-sectional area, the analysis result is not affected, but the chamfered part of the washer causes a decrease in axial force. In the estimation of the remaining axial force, the effectiveness of the evaluation based on the remaining width across flats of the nut and bolt head was clarified. Furthermore, the remaining axial force can be estimated by measuring the remaining width across flats near the washer. In addition, since the analysis results and the past experimental results were similar, the analysis model of this study is effective for estimating the residual axial force. Finally, regarding the corrosion of nuts, bolt heads and washers, the estimation equation of the residual axial force was proposed by polynomial approximation.