Effect of Non-Parallel Underhead Contact of a Tightened Fastener on Clamp Load and Optically Measured Deformation Field

This experimental investigation examines the effect of non-parallel contact of tightened fasteners on the resulting clamp load and on the optically measured deformation field using Electronic Speckle Pattern Interferometry (ESPI). A simple model of a single-bolt joint is used. In the model, tapered plates with two different wedge angles are used in order to simulate non-parallel contact under the fastener head. For the same level of tightening torque, the resulting clamp load, deformation field, and strain field are measured for two levels of wedge angle, and compared with those of a perfectly parallel contact. The tapered plates that were used in this study provide 5° and 10° wedge angles. The possibility of using optics to control the tightening process of threaded fastener, by monitoring the deformation and strain fields in real-time during fastener tightening, promises to significantly enhance the reliability of bolted assemblies. This is due to that the optical control would bypass the friction uncertainties that frequently make the torque control methods unreliable for critical assemblies. As a first step towards this long-term goal of developing and implementing an optical control method, this study investigates the deformation field that results from a known tightening torque for various scenarios of non-parallel contact under the fastener head. The deformation and strain fields are optically measured using an optical sensor that uses Electronic Speckle Pattern Interferometry (ESPI), and provides a 3D displacement field with high precision. A Special software is used in order to convert the deformation to a strain field. The sensor is mounted on the joint surface in order to eliminate the effect of rigid body motion. Measured clamp loads, deformation, and strain fields are investigated for 5° and 10° wedge angles and are compared with the results of perfectly parallel contact under the fastener head.