Estimation of Yield Clamping Force Based on Rigid-Plastic Model

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Abstract Yield clamping force of a bolt in plastic region tightening depends on yield point load of the bolt under combined loads of axial tension and thread torque induced by the tightening process. A new estimation method using interaction curve for yield is proposed, which is applied to the yield clamping force and maximum additional tension under external force. This estimation method utilizes the general yield criterion based on the rigid-plastic solution for combined loads of tension and torsion. The yield of the bolt is expressed by the point of the intersection between the interaction curve and loading path of the bolt in tightening or external loading. Considering the coefficient of friction at the flank of the thread and the residual thread torque after tightening, yield clamping force and maximum additional tension are obtained respectively by the simple formulas. The yield clamping force and the maximum additional tension of the bolt are examined experimentally by a combined load testing machine which can apply combined tensile and torsional loads on the threaded portion of the bolt. Estimated values by the proposed method shows good agreement with the experimental results. The proposed estimation method is superior to the conventional one which takes into account only the local yield criterion. Finally, the target zones of initial clamping force controlled by elastic region tightening, yield point tightening and plastic region tightening are examined. Usefulness of the yield clamping force is demonstrated based on the rigid-plastic model.