Real-time monitoring of clamping force of a bolted joint by use of automatic digital image correlation

Bolts are widely used in industry for joining members together. The behavior and service life of bolted joints depend on several factors including bolt material, dimensions, surface finish, surface coating and thread tolerances. However, the uppermost factor affecting the reliability and durability is the correctness of the clamping force exerted by the bolt. Thus it is very important to monitor the bolt clamping force to ensure a proper preload during assembly process. Each of the available monitoring techniques including torque control, torque-angle control, strain gauged bolt and ultrasound suffers one or more limitations which affect the reliability of measurement. In this paper, we propose an alternative method for direct tension measurement, which utilizes a washer with a white light speckle pattern on its face as an optical force transducer. By measuring the strain of the washer using automatic digital image correlation, the clamping force can be determined. This method, which enables real-time monitoring of clamping force during fastening, has a potential of being used for mass measurement of clamping force in a production environment. Experiments conducted on bolted structures with washers of different sizes have demonstrated the reliability and usefulness of this new approach.