New prediction model for the screw pull-out strength of polymer composite railway sleepers

Estimation of the screw pull-out strength of composite railway sleepers is a complex problem due to the wide variability of their material properties and the availability of different screw geometries used to fasten the rail track. This study investigated the effect of screw geometry (screw diameter and embedded length) and sleeper material (timber, synthetic composites, recycled plastics and particulate filled resins) on the pull-out strength and developed a new prediction model for polymer composite railway sleepers. The two-way analysis of variance showed that the material shear strength has a higher influence than the screw geometries on the pull-out strength. Increasing the thread embedded length has a significant effect on all tested sleeper types due to the increased thread engaging area while the screw major diameter affects timber rather than composites owing to timber's continuously high shear strength. Based on the failure behaviour and type of fibre reinforcement, an analytical model was developed to estimate the pull-out resistance and was also verified by the results from the available literature and reports. The proposed model is found over 50% more reliable than other existing theoretical models.