Keywords: butt joints, interface, contact, bolt tightness, 3d finite element modelling.

Butt joints are used widely in bridge girders, and rails when simple or moment resistant connections are required between steel members. The butt joints are some times applied to accommodate thermal movement in rails and bridge girders. Those types of joints are not considered in this paper. The structural behaviour of the butt joints has a significant influence over the structural integrity. Ju et al [1] carried out a detailed 3D finite element analysis of bolted steel joints with a view to investigating the applicability of linear elastic fracture mechanics to such joints under loading that cause plastic deformation. Lazzarin et al [2] carried out a large number of experiments on bolted aluminium joints with a view to examining the effect of the geometry and environmental factors on the fatigue behaviour and failure mode. They concluded that the fatigue behaviour of the bolted aluminium joints was closely aligned to the behaviour of welded aluminium structures.

Typically in most structural connections, the main structural plate is thicker than the cover plates that cover the dry butt joint [3,4,5]. However, in rail joints, in particular where rails are joined to achieve electrical insulation (achieved by the insertion of electrical insulating material between the rails), the rail web is thinner than the cover plates (known as `joint bars'). The importance of the integrity of this type of joint may be visualised from the role they play in signalling and train location systems that are vital in the safety of railway operation. As the joint consists of insulating materials whose properties under service conditions (confinement, thermal loads and impacts) are not known and uses glue of highly nonlinear properties, it is very difficult to examine the structural integrity of the rail joint in its "as-designed" condition. Furthermore, there is very little work reported on rail joints although such joints significantly affect ride quality, increase dynamic/impact loading and deteriorate the rail surface with the continuous passage of wheel loading [6]. As rail joint is a complex problem, detailed analysis of various parts of the body constituting the rail joint is necessary. Therefore, simplification has been made in the analysis of glued insulated rail joint by breaking the complex problem into a number of simple problems. This paper describes one of the simple idealisations of the rail joint problem. A three-dimensional finite element model was employed to simulate and investigate the structural behaviour of the butt joint subjected to biaxial stress field generated by inplane flexure. Elasto-plastic material model was used for the main plates whilst all other components were assumed to remain elastic throughout the loading history. A two-step non-linear analysis was performed. In the first step, the bolts were pre-stressed with a pre-defined level of tension, which were allowed to relax a portion of the tensile strain due to the deformability of the underlying joint-bars and the structural plates. On attaining equilibrium, the assembly was subjected to in-plane bending that generated biaxial stress fields as the second step.

As glue is used between the plates and joint bars in the rail insulation connection, tie constraints were considered and applied on contact surfaces between the plates and joint bars in the simulations. Similarly glued connection between the surfaces of nuts and joint bars was also simulated by perfect tie constraints. The tie constraints were used between each of the nodes on the slave surface to the master surface.

The simulated results reveal that sufficient bolt tension must be maintained to draw the joint bars into a closer fit and prevent the loss of structural integrity under the bending of the joint. As the contact surfaces of the main plates were initially assigned to possess zero gap, the increase in pre-tension of the joint leads to the increase in the contact length or the decrease in the horizontal displacement under bending. The normal compressive stresses on the contact surface between the two main plates have not significantly changed with the increase in pre-tension of the joint due to the presence of glued joint bars covering the joint of interest. However, the loss of bolt tightness has increased the length of separation of the contact surface. Under bending, it was observed that the bolt shank stresses have exhibited insignificant changes (once again due to the tied contact between joint bars and main plates). The insignificance of the effect of bolt pre-tension to the integrity of the butt joints as illustrated in this example is due to the presence of glued cover plate covering the joint plane. Thus the industry practice of gluing and bolting appear to be conservative from this first phase of analysis.

1

S.H. Ju, C.Y. Fan, G.H. Wu, "Three-dimensional finite elements of steel bolted connections", J Engg Struct, 26(3), 403-413, 2004. doi:10.1016/j.engstruct.2003.11.001

2

P. Lazzarin, M. Quaresimin, "Scatter bands summarizing the fatigue strength of aluminium alloy bolted joints", Int J Fatigue, 19(5), 401-7, 1997. doi:10.1016/S0142-1123(96)00057-6

3

J.W. Fisher, J.H.A. Struik, "Guide to design criteria for bolted and riveted joints", New York, John Wiley & Sons, Inc., 1974.

4

B. Gorenc, R. Tinyou, A. Syam, "Steel designers' handbook", Sydney, University of New South Wales Press, 1996.

5

B.G. Johnston, F.J. Lin, T.V. Galambos, "Basic steel design", New Jersey, Prentice-Hall, Inc., 1980.

6

W.W. Hay, "Railroad engineering", New York, John Wiley & Sons, Inc., 1982.

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