Establishment of design criteria for tightening bolted joints
Hariri, Bassam T (1990) Establishment of design criteria for tightening bolted joints. Master of Engineering thesis, Dublin City University.
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Joints are extremely important parts of any structure. Bolted joint joints also make disassembly and reassembly possible. The information on tightening bolted joints is important to the design engineer, in order to ensure the reliability of the joint and a minimum clamp load which is necessary to provide adequate in-srevice durability. Therefore designer are drawn more and more towards tightening processes which minimise clamp load variation in order to obtained the benefits of more predictable service. The design of bolted joints and techniques used to tighten them has received increased attention by the Automotive, Aerospace and Petro-Cheroical industries.
In this research theoretical and experimental investigations have been undertaken to determine the response of fasteners to combined torque and axial load. The principal aims of these experimental investigations are to establish the following: (i) The mode in which the initially applied torque in the bolt is reduced due to subsequent application of axial stress. (ii) The mode of reduction in initially applied axial load due to the subsequent application of torque, (iii) The mechanism in which the torsional and tensile strain energy is dissipated in a bolt. (iv) Any variation in the yield stress of the bolt as the axial load is applied after initial tightening torque.
The present work has been carried out by using bolts having thread rolled after heat treatment. The fasteners selected for testing were M12X1.75 MM I.S.O., Grade 10.9. An electronic hand torque wrench was used to tighten the fasteners. No lubrication was required during these tests. The effects of axial load on the tightened fastener were examined by static concentric loading. The uniaxial tensile load was applied by a hydraulic cylinder.
The results obtained in the experimental evaluations show that the the material of the bolt is elastic-work hardening. It also shows that the reduction of the initial torque due to the application of axial load and the reduction of the initial axial load due to the application of torque possibly follow the von Mises yield criteria. This will dictate that the material is yielding only at the outside diameter during combined loading. The experimental results also show that no significant reduction in the axial tensile load takes place for all magnitudes of the applied torque upto the yield torque.
The reduction in the initially applied torque or axial load due to subsequently applied axial load or torque respectively may also take place in such a manner so that the plastic zone at the outer surface progresses towards the inner core. It is most likely that such off loading takes place in mixed modes. Experimental tests and analysis confirm that the yield load of a bolt in tension is not affected appreciably when the bolt is subjected to any torque.
It is recommended that in a bolted joint the bolt should be
subjected to as low a clamp load as possible with relatively high level of initial torque. This would allow higher external load to be applied to a joint with less chance of reducing the tightening torque substantially.
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