Axial Force Measurement Using Digital Image Correlation

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). We examined this method through two comparisons. The first was to compare the strain on the bolt head measured by the DIC method with the strain measured by strain gauges. The second was to compare the axial force calculated by this method with the axial force obtained from strain gauges on the bolt neck. The strain measured using the DIC method on M10 bolts had a mean absolute error of 15 μstrain compared to the strain measured by strain gauges. We confirmed that the DIC method can be used to measure the strain on the bolt head generated by bolt tightening with a high degree of accuracy. In all tightening tests in which M10 bolts were applied to axial forces of 16 kN, close to the proof load of 18 kN in strength category 4.8, the tightening coefficient was 1.11 based on the axial force calculated using this method. The coefficient of tightening was also 1.33 when considering geometric tolerances and variations in the coefficient of friction. We confirmed that the axial force management method based on the axial force calculation method using images of bolt heads can be used to manage axial forces with the same or higher accuracy than the conventional general torque management method with a tightening coefficient of 1.4 to 3.0.