Discrete Element Modelling of Fractal Behavior of Particle Size Distribution and Breakage of Ballast under Monotonic Loading

Breakage of particles has a great influence on the particle size distribution (PSD) and the associated mechanical behavior of ballast under train loads. A discrete element method (DEM) simulation of triaxial testing under monotonic loading was carried out using FRM (fragment replacement method) breakable particles as ballast and a flexible shell model as membrane. The coupled model was validated by comparing the load-deformation responses with those measured in previous experiments and was then used to analyze the contact orientations and the distribution of particle breakage from a micromechanical perspective. The simulation results show that higher confining pressure and larger axial strain may increase the grain breakage (B-g) and the fractal dimension (D) of ballast. It was observed that most breakage was first-generation breakage, and that the proportions of the second- to fifth-generation breakage decreased successively. Moreover, as the axial strain or confining pressure increased, the percentage of small particle fragments increased in correspondence with the PSD curves that remained concave upwards, as the fractal dimension D of PSD increased. In addition, the evolution of D exhibited a linear correlation with grain breakage B-g. Contrarily, a quadratic curve relation between D and volumetric strain was exhibited under different axial strain stages. Therefore, D has the potential to be a key indicator to evaluate the degree of ballast crushing and PSD degradation, which may contribute to better decision making concerning track bed maintenance.

Automated summary

Particle breakage significantly affects the particle size distribution and mechanical behavior of ballast on railways. A simulation using the discrete element method was conducted to analyze the contact orientations and particle breakage from a micromechanical perspective. The results showed that increased confining pressure and axial strain led to higher grain breakage and fractal dimension of the ballast. The analysis also revealed that first-generation breakage accounted for the majority, while subsequent generations decreased in proportion. Additionally, there was a linear correlation between fractal dimension and grain breakage, while a quadratic curve relation was observed between fractal dimension and volumetric strain.