Numerical analysis of the rail fastening spring clip type SKL-1

Dalibor Bartos, B.Sc.C.E., Stjepan Lakusic, Ph.D.C.E., Tomislav Vinski, student University Zagreb, Faculty of Civil Engineering, Department of Transportation

Kaciceva 26, 10000 Zagreb dbartos@grad.hr, laki@grad.hr

Introduction. The spring clip is an element of the rail track accessories intended for elastic fastening of the rail to the base. The most important requirement expected from the spring clip is to be in the constant contact with the rail in any condition which might occur during the exploitation. Spring clips type SKL-1, which are the subject of the research described in this paper are used for both railway and tramway tracks. The application of the elastic fastening is especially important for tracks mounted on concrete slippers or concrete base, which is the case for tramway tracks in Zagreb, where this clip is used as the part of the fastening system type DEPP, Fig. 1. During supervision of works for construction and reconstruction of tram tracks in Zagreb town, it was noted that these SKL 1 clips were broken. Breaking of the clip in some cases happened even during the assembly (mainly due to faults in production) and also during the exploitation phase. That caused first experimental testing, followed by numerical analysis, to discover reasons for breakage. For the numerical analysis a three-dimensional static model of the clip was prepared, with the aim to obtain better view of the spatial stresses on the clip, both during the assembly phase and in exploitation. By modelling the clip and by numerical analysis not only can one see the spatial stress and deformation, but also locate the critical places where the critical stress appears and the loading of the clip can be checked. Numerical analysis. As said earlier, this paper is focused on the DEPP fastening system (Fig. 1), and the clip Vossloh SKL-1.

Figure 1

The clip was modelled in a 3D space and the clip geometry was very realistically described, as can be seen on the Fig. 1, detail A. 3D model was produced by modelling the clip in the ACAD, after which the 3D model was read-in to the ABAQUS version 6.4 (student edition) as a 3D solid body. ABAQUS is a program which performs calculations by using the Finite Element Method (FEM). The student version of the program was used, which caused some difficulties during the calculations. Namely, with the student version of ABAQUS we were able to perform the calculations for up to 1000 nodes. The calculations for the clip assumed that the material is isotropic, linearly elastic and that the load is transferred symmetrically to both halves of the clip, so only one half of the clip needed to be analysed in

calculations (Fig. 2, Fig. 4). Material of the clip is steel for springs according to the DIN 17221 standard, with elastic module of E = 205 GPa, poisson coefficient ν = 0.3, and tensile strength of σm= 1450 N/mm2. The stated mechanical characteristics (σm) were obtained by laboratory experiments and tests, [3]. The loading of the clip was done in two phases. The first phase represented tightening of the clip, and the tightening force was FI = 13.0 kN, which, under assumption of the symmetrical distribution means FI' = 6.5 kN for one side of the clip, (Fig. 2).

FI= 6.5 kN

I phase II phase

FII= 5.0 kN

Figure 2

During the phase II the clip was loaded with force of FII=10 kN, which for one side of the clip means FII' = 5.0 kN (Fig. 2). This force represents the limit force at which the so called second contact is established according to the working diagram for the SKL-1 clip, shown on Fig. 4. The conditions for leaning of the clip in both loading phases were simulated by defining the border conditions. The Fig. 2 shows schematic representation of the border conditions and loading of the clip for phases I and II.

Detail A

Levelling layer

SYSTEM "DEPP"

2

1

Vossloh clip SKL-1

Rail pad PSteel baseplateRail pad P

Discretisation of the model to finite elements was done with the finite element f the type C3D4 (Fig. 4).

Figure 3 This element type represents spatial tetrahedron with four nodes, as can be seen on the Fig. 4. Each node has 6 degrees of freedom 3 rotations and 3 translations. This model consists of 3163 tetrahedral elements with 958 nodes. Reason for selection of this element lies in the fact that with the student version of ABAQUS we were able to perform the calculations for up to 1000 nodes. For numerical calculations the main tensile loading was set σm= 1450 N/mm2 for both phase I and phase II of the clip loading.

Figure 4 Results. The calculation results show equivalent stress ''σe'' according to the Huber-Mises-Hencky theory [1], maximal principal stress ''σ1'' and minimal principal stress ''σ2'' and deformation of the clip ''u3''. During tightening of the clip, (phase I) certain pre-stress is applied by tightening the bolt, and that stress stays permanently present in the clip during the whole period of exploitation. The border tensile stress σm= 1450 N/mm2 was given. Main stresses σe, σ1, σ2 (N/mm2) are shown on the Figure 5. The Fig. 6 shows the deformation u3 (mm). Caused by forces in the track, (phase II) the clip is loaded and stresses obtained for this phase are superimposed on the stresses from the phase I.

Figure 5

Figure 6

Main stresses σe, σ1, σ2 (N/mm2) are shown on Fig. 7 while the deformations are shown on Fig. 6. The Fig. 8 shows the cross-section through the critical section, where the highest tensile stresses appear.

Figure 7

Figure 8 Conclusions. By numerical analysis of the SKL 1 clip the location of maximal stresses was established, as can be seen on the Fig. 7. If a cross-section is made at the critical section (Fig. 8) it can be seen that a large overload of tensile strength happens at that section, which in cases where larger load is applied (curves in the track, lateral impact from the vehicle wheels and similar) can lead to breaks in the clip. The results of numerical analysis correlate rather well with the experimental results. This is emphasised by the fact that the breaks in the clips during the experiments were happening at the exact place where the numerical analysis predicted critical stresses. References. [1] Androic B. and others, Steel constructions 1, IGH, Zagreb, 1994. [2] ABAQUS, Superior Finite Elements Analysis Solutions, web page: www.abaqus.com [3] Report on experimental research for tightening clips SKL-1, Faculty of Civil Engineering, Report no. 2712-593/90, Zagreb, 1991.

σe [N/mm2] σ1 [N/mm2] σ2 [N/mm2]

σe [N/mm2] σ1 [N/mm2] σ2 [N/mm2]

face 3face 4

face 2

face 1

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0 1 2 3 4 5 6 7 8 9 10 11 12 13

displacement [mm]

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e [k

N]

I. phase II. phase