Quantification of stress relaxation in a timber fastening system: a railway focused study

Elastic fasteners have been widely adopted in timber sleeper tracks in certain high-demand areas across North American freight network due to their excellent potential to mitigate rail-rollover derailments by resisting steering moment and rotation of rail from the vertical axis through intense elastic force to securely hold-down the rail to the sleeper baseplate. However, these systems have led to at least 13 derailments reported since 2000 because of sleeper baseplate spike fatigue failures. Previous spike-failure investigations established that the loss of friction at the baseplate-sleeper interface caused by the wave-action of rail was the major mechanism that transfers additional loads to the spikes, and results in spike stresses exceeding the endurance limits. Previous studies also demonstrated the positives of plate hold-down load on controlling spike stress levels; with this load being historically applied via spring washers. Although the static performance of such hold-down systems has been evaluated in the literature, the long-term, time dependent behavior has not been quantified previously. This paper quantifies the effects of timber sleeper species, spring washer resiliency, and installation load on stress relaxation of these systems over 1,000 h in the laboratory under constant climate conditions. Experimental data demonstrate the significant impact of installation load magnitude on relaxation performance – load retention of 96% and 67% observed under 11.1 kN (2,500 lbs.) and 66.7 kN (15,000 lbs.) installation load, respectively. However, the insignificant effect of spring resiliency on the relaxation behavior was reflected through a 2% only change in load retention over a four-fold change in resiliency. A 15% increase in load retention was achieved by using Red Oak in place of Mixed Hardwood which established sleeper species as a critical parameter in such applications. An assessment of an extended experimentation period (i.e., 2,450 h) was carried out to better estimate the end point of relaxation. The experiments were conducted in an environmental chamber that does not represent the harsh conditions (i.e., loads, vibrations, temperature, humidity, or moisture) of the revenue-service tracks. However, results from this work can reasonably be useful to guide the selection of appropriate components along with recommended installation loads for hold-down applications to improve the overall safety of timber sleeper tracks that leverage elastic fasteners.