3. Verification of fatigue in wire spring type rail fasteners subject to plastic deformation

There has been in increase in the use of wire spring fasteners on lines (Fig.1) in Japan over recent years. When in use, wire springs undergo plastic deformation, however it has not to date been possible to gain sufficient evaluation of their endurance using traditional assessment methods limited to adaptability in the elastic zone. Therefore a fatigue limit diagram was proposed which can be adapted to wire springs used in the plastic zone upon which an evaluation method could be designed.

Given that stress in the plastic zone is not easily estimated using a strain gauge and is therefore difficult to assess directly using a fatigue limit diagram showing stress. Consequently, using the known basic material properties of the wire spring, a proposal was made to replace the stress (Fig. 2 a)displayed in the traditional fatigue limit diagram, (Fig. 2 b)with strain. A fatigue test in the plastic zone was conducted on a real wire spring, and results agreed with results obtained using the proposed fatigue limit diagram, validating the method's applicability (Fig.3). In addition a limit line was proposed to show the drop in clamping force on the rail due to plastic deformation (Fig.3 green line). By adapting this fatigue limit diagram, it is possible to more appropriately evaluate the performance of wire spring type rail fasteners.