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Multi-scale study of the degradation of railway ballast

Authors
  • Deiros Quintanilla, Ivan
Publication Date
May 02, 2018
Source
HAL-UPMC
Keywords
Language
English
License
Unknown
External links

Abstract

After some years of high-speed lines in France (HSL), ballast has proven not to be resistant enough. The performance of ballast, as a thin layer of coarse grains, strongly depends on the shape, size and mineralogical nature of the grains composing it. However, in HSL, grains wear faster than expected due to the traffic of trains at high speeds and the accumulation of maintenance operations (tamping). Ballast replacement has therefore been required much before than its originally expected lifespan.Under the dynamic stresses imposed by the circulation of trains and tamping operations, ballast is gradually worn by fragmentation of grains and attrition at the contacts. The direct consequence of this degradation is the evolution of grain size and shape: the grading curve is shifted towards small and fine particles and the grains progressively lose their angularity. Eventually, the cumulated wear will no longer allow ballast to perform properly: the shear resistance of the layer is reduced limiting both the anchorage of sleepers and the distribution of loads to the platform. In addition, the presence in excess of fine particles renders tamping ineffective (fast evolution of track defaults) and reduces the permeability of the track. Thus, in order to search for optimized solutions for prolonging ballast lifespan, it is crucial to first understand the origins and mechanisms leading to ballast degradation when it is subjected to complex loading, for building a predictive model of ballast wear.The degradation of contact interfaces generates fine particles. The associated mass flux, which depends on the loading conditions, has been classically predicted by Archard equation. The model assumes that the generated volume of wear is proportional to the normal force and the relative displacement between the surfaces. Therefore, it is crucial to quantify the forces at the contact scale and the relative displacements between ballast grains in sliding contact. Discrete elements simulations by NSCD are used as a tool for performing a change in scale from the track scale to the contact scale, giving information of ballast as a granular layer, from its global behaviour down to the contact forces and relative displacements between grains. Contacts with a higher potential of generating fine particles (according to Archard model) are then identified and reproduced experimentally by two-grain shearing tests. In parallel, the Micro-Deval standard attrition test is used as a link between numerical and experimental results to validate Archard model, and to study the evolution of grain morphology by scanning a sample of grains using X-ray tomography at different stages of the test. Both experimental campaigns show the weakness of sharp asperities, especially on edges and vertexes.A model in two phases is proposed, accounting for a first phase of fast and aggressive degradation due to the high stress at the contact interface and a more stable second phase with a lower wear rate. A critical stress is identified as a threshold between phases. This model is then applied at each individual contact on the numerical simulations, resulting in a first approach of the production curve of fine particles within the track.

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