Optical Monitoring of Bolt Tightening Using 3D Electronic Speckle Pattern Interferometry

An optical method is presented for the real-time monitoring of the bolt tightening process that provides the necessary clamping force in bolted assemblies. The method uses the 3D electronic speckle pattern interferometry (ESPI) technique to measure and monitor the joint deformation and strain fields in the joint around the tightened fastener, and establish a reliable correlation with the clamp load provided by the fastener tightening. Because the relationship between the clamp load and joint deformation is independent of the frictional variables of the fastener, the direct monitoring of the deformation and strain fields in the clamped joint would provide a more reliable measure of the clamp load level, as compared to the conventional reliance on the tightening torque to predict the resulting clamp load level. This technique is beneficial, especially for monitoring, with high precision, the proper tightening of critical bolted connections such as those used in many pressure vessels in the nuclear power industry. The validity of the optical technique is also ascertained by comparing the results to that obtained by using finite element analysis and other classical mechanics of materials methods.