Analysis of Fractal and Energy Consumption Characteristics of Concrete under Impact Loading

In order to study the compressive deformation and energy evolution characteristics of concrete under dynamic loading, impact compression tests with impact velocities of 5, 6, and 7 m/s were carried out on concrete samples with aggregate volume ratios of 0, 32%, 37%, and 42%, respectively, using a split Hopkinson pressure bar test apparatus. The broken concrete pieces after destruction were collected and arranged. The fractal characteristics of fragmentation distribution of concrete specimens with different aggregate rates under impact were discussed, and the roughness of the fragment surface was characterized by the fractal dimension of the broken fragment and the crack surface energy was calculated. In addition, the analytical equation of the fractal dimension of the broken fragment and the crack surface energy was established. The relationship between the specimen energy absorption and the crack surface energy was compared and analyzed. The results show that the concrete specimens are mainly tensile split failure modes under different impact speeds. The fractal dimension, absorption energy, and crack surface energy all increase with the increase in impact speed and decrease with the increase in the aggregate rate. When the aggregate rate is different, the effective utilization rate of the absorbed energy is the largest when the aggregate content is 37%. The surface energy of the crack can be used to estimate the concrete dynamic intensity.

Variable-order fracture mechanics and its application to dynamic fracture

This study presents the formulation, the numerical solution, and the validation of a theoretical framework based on the concept of variable-order mechanics and capable of modeling dynamic fracture in brittle and quasi-brittle solids. More specifically, the reformulation of the elastodynamic problem via variable and fractional-order operators enables a unique and extremely powerful approach to model nucleation and propagation of cracks in solids under dynamic loading. The resulting dynamic fracture formulation is fully evolutionary, hence enabling the analysis of complex crack patterns without requiring any a priori assumption on the damage location and the growth path, and without using any algorithm to numerically track the evolving crack surface. The evolutionary nature of the variable-order formalism also prevents the need for additional partial differential equations to predict the evolution of the damage field, hence suggesting a conspicuous reduction in complexity and computational cost. Remarkably, the variable-order formulation is naturally capable of capturing extremely detailed features characteristic of dynamic crack propagation such as crack surface roughening as well as single and multiple branching. The accuracy and robustness of the proposed variable-order formulation are validated by comparing the results of direct numerical simulations with experimental data of typical benchmark problems available in the literature.

Анализ разрушения перлитной рельсовой стали с внутренней макротрещиной

One of the most dangerous defects leading to transverse fractures of rails is internal transverse cracks in the rail head. In this work, a fractographic analysis of the cross-sectional surface of a rail with a transverse fatigue crack is carried out. The rail sample was taken out of work after many years of service. Microstructural analysis of the crack surface and the surrounding material shows a significant degradation of the physical and mechanical properties of the rail steel.

Структурно-фазовое состояние металла рельса с внутренней трещиной после длительной эксплуатации

During operation, under cyclic force action, various physical and mechanical processes occur over time in the rail metal: plastic deformation of the rolling surface, the formation of internal and surface cracks, a change in residual stresses, etc. As a result, the mechanical characteristics deteriorate and the performance of the rails decreases. In this work, a microstructural analysis of the cross-sectional surface of two rails with internal cracks - longitudinal and transverse. Rail samples were taken out of service after many years of service. Fractographic analysis of the crack surface and the surrounding material indicates a significant degradation of the physical and mechanical properties of rail steel.