YIN Yajuan, REN Qingwen, SHEN Lei, HAN Yan

The cracks evolution during the cracking process of concrete materials shows randomness. The concrete materials are nonlinearity of themselves and the external conditions are complex. These results in largely difficult study on the dynamic evolution law of concrete cracks. To quantitatively study the evolution process of cracks and damage of concrete under loading, the meso-concrete mechanics model is established assuming that the concrete is a three-phase composite composed of aggregates, mortar, and the transition zone between them. A new calculation method of damage variable is proposed based on the crack fractal dimension, and the uniaxial tensile mechanical behavior of multiple groups of concrete specimens is studied in numerical simulation. The concrete damage crack distribution is characterized by fractal dimension. It shows that the results of meso-simulated crack pattern are consistent with the experimental results. The crack fractal dimension can better reflect the damage evolution process of concrete materials, and quantitatively describe the damage evolution characteristics of concrete materials. It finds the critical point of concrete materials from "uniform" damage to local failure. The stress-strain curve, the crack fractal dimension curve, and the damage evolution curve from meso-simulation results are all less affected by the element size. With the increase of aggregate gradation of concrete specimens, the aggregate particles and interface decrease, the overall energy consumption decreases, and the peak stress and crack fractal dimension decrease. In addition, it is suggested that the mutation point of the crack fractal dimension-strain curve after the stress peak point is the critical point of the concrete materials from the "uniform" damage distribution stage to local failure stage, which is the boundary point of damage and fracture. The corresponding damage value is about 0.8, which can be used as the damage threshold for the occurrence of concrete macroscopic crack. These findings in this paper deepen the understanding of the failure mechanism of concrete.

Keywords: concrete, mesoscopic, crack propagation, damage evolution, fractal dimension, mutation point