Bolt axial force measurement using Digital Image Correlation Method considering bending force on bolt

Abstract

As a simple and highly accurate axial force management method during manufacturing and maintenance of bolt-tightened structures, we examined a method to calculate the axial force from images of the bolt head. In this method, the axial force is calculated using the strain on the bolt head measured by digital image correlation (DIC) technique from images of the bolt head and the relationship between strain on the bolt head and axial force calculated by finite element analysis (FEA). In this report, we have proposed a method for calculating axial force considering bending force on a bolt. Specifically, we have proposed a method for calculating the axial force and bending force ratio using the least squares method, based on a 5th-order approximation curve consisting of two variables: the axial force and the bending force ratio, which is obtained by numerical calculation with the bending force as a parameter, and the strain distribution measured by DIC. In bolt tightening tests using M20 bolt and tapered washers to reproduce bending force, the bending force ratio and direction obtained using the proposed method were in good agreement with those obtained from the strain gauges attached to the bolt shank, confirming that the proposed method can be used to evaluate bending force on bolts. In addition, the axial force calculated with the proposed method was in good agreement with that calculated with the bolt gauge, and the average absolute difference between the two was 4.9 kN for all tests. For test results with a large bending force ratio, the deviation in the axial force calculated before and after considering the bending force was around 14%, and it was confirmed that the proposed method enables more accurate evaluation of axial force in bolted structures where bending force occurs.