A Novel Optical Method for Real-Time Control of Bolt Tightening

A digital speckle pattern interferometry (DSPI) system is developed for the real-time measurement and monitoring of the out-of-plane surface deformation around a preloaded bolt head or nut. The proposed system is specifically developed for the dynamic control of the bolt tightening process by continuously monitoring the out-of-plane joint surface deformation that will have been independently correlated to the bolt preload. Spatial phase shifting is employed to quantitatively determine the distribution of phase data by introducing a spatial carrier fringe pattern to the speckle interferogram. This is achieved by leading the object and reference beams through two separate apertures. The configuration is also suitable for collecting the real-time surface deformation during bolt tightening. The experimental DSPI system is set-up with optical components on a vibration-isolation table. A MATLAB software is developed for image acquisition and phase data calculations that yield the out-of-plane surface deformation caused by the bolt preload. The test fixture uses an M12 steel fastener and aluminum joint. For miniature screw applications, however, a plastic joint is used.