Static performance of particulate filled resin composite railway sleepers in the rail-seat region

Static performance of particulate filled resin composite railway sleepers in the rail-seat region

Polymeric railway sleepers have been increasingly developed as a new alternative to traditional timber sleepers due to their advantages of high strength-to-weight ratio and excellent durability. The material properties of this new technology can be engineered to behave like timber and the particulate filled resin (PFR) cored sleeper demonstrated promising performance. However, adopting this sleeper type is challenging considering the insufficient information to date on the development of sleepers made from PFR mixed with flexible fillers or fibre reinforcement. In addition, the limited understanding of the screw pull-out and lateral restraint behaviour has been indicated as another major challenge. Therefore, this research systematically evaluated the development of the PFR system and the rail-seat behaviour of composite sleepers to increase the confidence in using this new type of technology.

The first manuscript presented the investigation of crumb rubber and short fibre reinforcement introduced into the epoxy-based PFR core of composite railway sleepers to minimise cost and enhance mechanical properties. The physical and mechanical properties including the microstructure of the new polymer mixes were evaluated. The experimental results showed a high correlation between the increasing contents of new ingredients and the engineering properties of the PFR mixes. A simplified prediction equation was proposed to predict critical properties as a function of the compressive strength. Analytical Hierarchy Process was also implemented to determine the most suitable polymer mix for the development of a cost-effective and reliable performance composite railway sleeper.

The effect of material properties on the pull-out behaviour was investigated in the second manuscript. The pull-out strength and the hole microstructure of different sleeper technologies including timber, synthetic composite, recycled plastic and Particulate Filled Resin sleepers were investigated to determine the effect of material properties on their screw pull-out behaviour. The paired samples test revealed a high correlation between the pull-out strength and the shear strength of the materials. A simplified prediction model was developed to predict the pull-out resistance for railway sleepers based on the shear strength of the composite sleepers.

The effect of screw geometry was evaluated in the third manuscript. The influence of the diameter of the screw, embedded length of the screw and sleeper material properties was determined using the direct pull-out test. The results showed increasing the thread embedded length has a significant effect on the pull-out strength due to the increased thread engaging area while the major diameter is more likely to affect timber rather than composites owing to the strong load-bearing capacity of hardwood timber. Based on the significance of these parameters, an analytical model consisting of three prediction equations was developed to estimate the pull-out resistance and was also verified by the results from the available literature and reports. Each equation corresponds to a specific failure mode which can be determined by the type of fibre reinforcement adopted in the sleeper technology. Compared to other existing theoretical models, the proposed model is found over 50% more reliable.

The lateral restraint behaviour and failure modes were investigated in the fourth manuscript. This study investigated the lateral strength of composite sleepers at the standard required displacement of 5.1 mm and failure. The results showed screw and sleeper material yielding at a very early stage even before 5.1 mm for all tested samples. The grain/fibre shear-out failure was noticed in the orthotropic sleeper materials while the isotropic material mainly exhibited the bearing failure. Based on the isotropic hardening rule and Hill's criterion, finite element models were developed and well predicted the screw lateral behaviour. The stress distribution in the screw and the sleeper was also revealed and demonstrated a good correlation to the observed lateral failures.

This research provided a detailed understanding of how the critical parameters affect the rail-seat performance of polymer composite sleepers. Additionally, the results of this research offered useful analytical tools which may support researchers and design engineers to develop composite sleepers with desired rail-seat performance and improve the confidence in using this technology.

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