Optimal head design for preloaded bolts

Abstract

Bolt head or general t-head design is investigated in the present paper. For a preloaded bolted connection, the filet under the head is one of three major failing points, the other points of failure are related to the thread. The design of a bolt is typically specified by the standard ISO design. The strength of a preloaded bolted connection is given by the stress concentration factor together with the stiffness ratio between bolt and the compressed parts, both factors are important. The smallest design modification can be made directly at the bolt head filet by going away from the circular design, the found improvement in strength is there in the order of $24\mathrm{\%}$ . Introducing a washer (with same modulus of elasticity as bolt and plates) gives more design freedom. Modifying the standard washer still with contact area perpendicular to the axial direction a reduction in the stress concentration factor of $44\mathrm{\%}$ is found and an improvement in strength of $225\mathrm{\%}$ . For the final washer design a contact area inclination is introduced. With increased inclination angle the strength is improved even further, with an inclination angle of $\pi /4$ the stress concentration is reduced by $65\mathrm{\%}$ and an improvement in strength of $833\mathrm{\%}$ , both significant improvements.