A new detecting method of clamping force of bolted joint

The clamping force of a bolted joint is usually controlled by torque or angle at tightening. However the fatigue failure of bolted joints occures if the clamping force is loosened in service. Therefore the clamping force in service after tightening must be checked to avoid failure of bolted joints. In this study, a new method to detect the clamping force of bolted joints after tightening has been proposed, in which the clamping force is detected by pulling the protruding thread of the bolt through the nut on a bolted joint. The relationship between the pulling force and the displacement at the pulling point is measured to detect the apparent spring constant of the bolt. When the pulling force reaches the clamping force of bolted joint, the apparent spring constant varies markedly. Thus the clamping force of the bolted joint can be defined on the basis of the variation of the apparent spring constant. The basic theory and the experimental verifications are presented.