Resilience of ballasted railway tracks exposed to extreme temperature

At present, railway track buckling, caused by extreme heat, is a serious concern that can lead to the huge loss of lives and assets. With increasing exposures to high temperatures globally, a greater expansion in Continuous Welded Rails (CWRs) can induce higher risk of track buckling, especially when track defects exist. Note that track lateral stability is one of the most critical considerations for safe and reliable railway infrastructures. In ballasted railway tracks, ballast layer holds sleepers in place and provides lateral resistance and stiffness to the track. This doctoral thesis aims at investigating the buckling behaviour of ballasted railway tracks under extreme temperatures to clearly understand this vulnerability in order to improve the ballast track’s resilience to extreme temperatures. This doctoral thesis first presents 3D Finite Element Modelling (FEM) of traditional and interspersed railway tracks exposed to extreme temperatures, through realistic modelling. The new findings highlight the buckling phenomena and failure mechanism of interspersed railway tracks, which are usually adopted during railway transformations from timber to concrete sleepered tracks in real-life practices around the world. All possible phenomena observed in the field are captured. The novel in-depth insight unprecedentedly instigates vulnerability and resilience of traditional and interspersed railway track systems exposed to extreme weather conditions. Moreover, the coupling DEM-FEM modelling are developed to deeply investigate the effect of ballast degradation and vulnerability on track lateral stability. This shows that the inspection of ballast profile is essential even though ballast condition seems to be good according to a visual inspection, as the hidden degraded ballast in the bottom layer can still undermine the buckling strength unexpectedly, resulting in increasing vulnerability to track buckling. The obtained results can be used for stability and misalignment management of ballasted railway tracks. The key findings will enhance the development of inspection criteria for lateral resistance and support conditions, improve safety and reliability of rail network, and mitigate the risk of delays due to track buckling leading to unplanned maintenance. Lastly, this doctoral thesis also proposes a new type of resilient material to effectively increase track resilience and reduce the likelihood of track buckling

Large-Amplitude Vibrations of Spider Web Structures

Spider silk, as a natural material, shows exceptional performance in its properties. The combination of the superior properties of spider silk and the geometry of spider structures make the spider web very resilient. A spider web structure can be considered as a cable-like structure with inappreciable torsional, bending and shear rigidities. An investigation emphasising on natural frequencies and corresponding mode shapes with and without the consideration of geometric nonlinearity is presented in this paper. This study is the world’s first discovery of large amplitude free vibration behaviours of spider web structures. Large deformable finite element 3D models of spider web structures have been developed and validated. By using the energy method, the variational model of the spider web structures have been established to further extend the finite element model, consisting of the strain energy due to axial deformation, kinetic energy due to the spider web movement and the virtual work caused by the self-weight per unit unstretched length. The emphasis of this study is placed on the linear and geometric nonlinear behaviour of the spider web structures considering different structural patterns and material properties. To determine the large-amplitude free vibrational behaviours, a series of pretension load is applied to the first step in Abaqus to initiate the nonlinear strain-displacement relationships enabling a precursor to free vibrations. The parametric studies stemming from structural patterns (the number of radial and capture threads), elastic modulus, density, and inertia moment have been highlighted. The insight will help engineers and scientists to adapt the concept of spider webs, their geometric properties, and damage patterns for the design of any structural membranes, preventing any failure from dynamic resonances and nonlinear phenomena