Behaviour of fibre composite sandwich structures: a case study on railway sleeper application

Behaviour of fibre composite sandwich structures: a case study on railway sleeper application

Timber is the most widely used material for railway sleepers; however, as a sleeper material it deteriorates with time and needs appropriate replacement. Hardwood timber for railway sleepers is becoming more expensive, less available and of inferior quality compared to the timber previously available. This problem is accentuated in railway turnouts where larger, longer, stronger and more expensive timber is required. Research has therefore focused on the possibility of fibre composites replacing timber as the many issues related to the currently used sleeper materials could be simulated using this material.

This study is the first to investigate the concept of glue-laminated composite sandwich beams for railway turnout sleepers. The building block of this innovative beam is a novel composite sandwich structure made up of glass fibre composite skins and modified phenolic core material that has been specifically developed for civil engineering applications. The beam is produced by gluing layers of composite sandwich structure together in different orientations, i.e. flatwise (horizontal) and edgewise (vertical). This experimental beam enabled the author to determine the more effective use of this composite material for structural beam applications. In this way, a detailed understanding was achieved of the behaviour of the constituent materials and composite sandwich structures to determine the suitability of this construction system for railway sleepers.

An experimental study of the flexural and shear behaviour of the individual sandwich structures in the flatwise and the edgewise positions was conducted. The sandwich structures in the edgewise position possessed better structural performance compared to the flatwise position due to the introduction of the vertical fibre composite skins. The sandwich structure with the same dimensions in the edgewise position displayed almost 20% and 70% higher failure load in bending and shear respectively, than the sandwich structures in the flatwise position suggesting more effective utilisation of the fibre composite material. This structure also exhibited ductile failure behaviour which is important in the civil engineering perspective.

The effects of the number and the orientation of sandwich laminations on the strength and failure behaviour of glue-laminated composite sandwich beams were also examined. The glued sandwich beams with edgewise laminations have at least 25% higher flexural strength and over 20% in shear strength, compared to the individual sandwich beams. Gluing the sandwich beams in the edgewise position could offer up to 25% increase in flexural strength, a similar bending stiffness, and almost double the shear strength over beams in the flatwise position.

Theoretical prediction and numerical simulations were performed to gain a better understanding of the structural behaviour of the composite sandwich structures. Simplified Fibre Model Analysis (FMA) provides a preliminary indication of the flexural behaviour, while the shear prediction equation gives a good estimation of the shear strength of the sandwich structures. The Strand7 finite element program predicted the behaviour up to failure load of the sandwich structures reasonably well. This confirms that the behaviour and failure modes of composite sandwich structures can be well predicted by simplified analysis procedures and by using the currently available finite element software packages provided a good understanding of the constituent materials and the individual sandwich lamination is known. These can be important tools for design engineers permitting the design and development of fibre composite sandwich structures with a higher degree of confidence.

A grillage beam analogy was implemented to investigate the behaviour of sleepers and to obtain critical design parameters in a typical railway turnout system. The effects of the elastic modulus of sleeper, support modulus, and spot replacement were studied. All these factors have significant influences on the behaviour of turnout sleepers. An elastic modulus of 4 GPa was found optimal for a fibre composite turnout sleeper from the consideration of sleeper/ballast pressures and the vertical deflection. It was established that the turnout sleeper has a maximum bending moment of 19 kN-m and a shear force of 158 kN under service conditions.

Finally, the behaviour of the full-scale glue-laminated composite sandwich beams in three different layouts was evaluated to determine their suitability as railway turnout sleepers. The glued sandwich beams with edgewise laminations presented appropriate strength and stiffness for replacement turnout timber sleeper. The mechanical properties of these glue-laminated sandwich beams are comparable with the existing timber turnout sleepers demonstrating that the innovative composite sandwich beam is a viable alternative sleeper material for railway turnouts.

From this study, it is concluded that the glue-laminated composite sandwich structures can be effectively used for replacement railway turnout sleepers. An enhanced understanding of the behaviour of fibre composite sandwich structures for potential civil engineering applications is an outcome of this investigation.