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.

To investigate the dynamic characteristics, energy dissipation patterns, and failure modes of granite with filled joints of varying thicknesses under impact loading, we utilized the Split Hopkinson Pressure Bar (SHPB) test setup for impact tests on both unfilled and filled granite samples. Additionally, a high-speed camera was used to capture the dynamic failure and crack propagation processes of the rock samples in real time. The results indicate that the thickness of the filling material significantly affects the stress–strain behavior of jointed rock masses, particularly in terms of characteristics of stress variation and post-peak morphology. In comparison to unfilled jointed rock samples, a distinct “stress bimodal” phenomenon is present, and the rebound of strain following the peak gradually decreases. The fracture patterns observed in the jointed rock samples are primarily characterized by tensile failure. Damage is notably more pronounced on the left side of the samples (near the incident bar), the lower side, and in the areas filled with gypsum. The most severe degree of damage occurs when the filling thickness is 7.56 mm. As the thickness of the filling increases, the dynamic compressive strength of the rock mass diminishes, and the peak strain first increases and then decreases. Concurrently, the energy reflection coefficient of the rock mass increases linearly, while the energy transmission coefficient declines linearly. Furthermore, the energy dissipation ratio first increases and then decreases. The test data reveal that the critical filling thickness influencing the dynamic properties, energy absorption characteristics, and damage degree of jointed rock samples falls within 4.91 mm to 7.56 mm.

To study the dynamic response characteristics and energy evolution of through-jointed granite under impact load, granite with different dip joints was selected as the research object. Using the theoretical model of damage mechanics for jointed rock masses with varying dip angles, a series of SHPB (split Hopkinson pressure bar) impact tests was conducted on granite samples under high strain rates. The dynamic mechanical properties and energy dissipation characteristics of the rock samples were obtained. The results indicate that: (1) based on the Druck-Prager criterion, the Weibull strength distribution criterion, and the theory of elastic waves, the stress-strain model of jointed rock mass with different dip angles is established. This model can effectively reflect the dynamic mechanical properties of granite as the joint dip angle changes and exhibits a strong dip effect. (2) As the dip angle of the joints increases, the energy reflection coefficient rises linearly, the energy transmission coefficient decreases linearly, and the peak stress of the rock samples gradually decreases. Under the same joint dip angle, as the impact load increases, the energy reflection coefficient first increases and then decreases, the energy transmission coefficient first decreases and then increases, and the energy absorption rate decreases with the increase of the joint dip angle. (3) When the impact load is the same, the fragmentation degree of intact rock samples and those with joint dip angles of 0° and 15° is greater, while the fragmentation degree of rock samples with joint dip angles of 30° and 45° is smaller. When the joint dip angle is the same, as the impact load increases and exceeds the maximum compressive strength of the rock samples, the failure mode gradually transitions from shear and tensile failure to crushing failure, and the degree of fragmentation also increases.