Static and fatigue behaviour of composite railway sleepers

Static and fatigue behaviour of composite railway sleepers

Several composite sleepers have recently been developed as alternatives to hardwood timber. However, the mechanical properties of these alternative sleepers vary greatly with the bending modulus ranging from 1 to 37 GPa even though these sleepers have been developed as replacements for timber. This variation poses a significant challenge for the designers and track owners as they adopt these new technologies because of the lack of performance data. This thesis systematically evaluated the static and fatigue behaviour of timber and composite sleepers for their effective design and application in railway tracks.

In the first study, the behaviour of timber and its alternative sleepers supported by ballast was investigated by using a section of a railway track. The effect of varying bending and compression moduli was investigated. The Digital Image Correlation (DIC) technique was employed and validated with strain gauges to capture full bending profile and local decompression. The results showed that soft sleepers will exhibit a W-shaped profile while stiff sleepers show a U-shaped profile. The local decompression of soft sleepers accounts for 6% of total rail seat deflection on low modulus support and as high as 10% on stiff support, suggesting a significant difference in the behaviour of alternative sleepers on a simulated railway track.

The second study developed a new and simple test method called 'five-point bending' to induce the positive bending moment at the rail seat and the negative bending moment at the centre as experienced by railway sleepers in the track. Three different support types at the mid-span: steel, ethylene propylene diene monomer (EPDM) rubber, and neoprene were considered. The suitability of this method was evaluated by testing different sleeper materials and by validating using the Beam on Elastic Foundation (BOEF) design method. The results showed that the neoprene rubber as mid-span support would mimic the deflection profile and magnitude of bending moments experienced by the sleepers. The developed analytical equations were found to accurately predict the bending moments in any location of the sleeper.

The developed five-point bending test method was implemented in the third study to study the behaviour and failure mechanisms of composite sleepers under static load. The flexural failure loads of alternative sleepers were shown to be lower than that of timber sleepers, i.e., 85% for concrete, 56% for synthetic composites, and 42% for plastics. Moreover, all the sleepers showed distinct failure mechanisms, i.e., flexural crack for timber, longitudinal shear cracks for composites, and permanent deformation for plastics. Local decompression was also captured for foam-based sleepers due to the softness of the foam. The results of this study highlighted the significant difference in the static behaviour of alternative composite sleepers compared to that of timber.

Finally, the fatigue behaviour and degradation of timber alternative composite sleepers were investigated as the fourth study. Small- and full-scale samples were tested and correlated through the established fatigue degradation factors. The failure behaviour of small- and largescale sleepers was similar but the scaled-down specimens degraded 3.2 and 7.4 times faster than full-scale composites and plastic sleepers, respectively. Timber and composites lost 10% of their stiffness while the plastics exhibited a 6 mm permanent deformation after 1 million load cycles.

The results of this thesis enrich the understanding of the structural behaviour of timber-alternative sleepers, which have different mechanical properties. These new findings are very useful for their effective design, manufacture and implementation in the construction of new and interspersed railway tracks.

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