Issue 45

F. Qui et alii, Frattura ed Integrità Strutturale, 45 (2018) 1-13; DOI: 10.3221/IGF-ESIS.45.01 2 I NTRODUCTION oncrete is one of the most popular materials in structural engineering. It has been extensively applied in buildings, bridges, mines, dams, power plants and transport facilities. In addition to normal static load, all these engineering structures may be subject to such dynamic loads as earthquakes, vehicle vibration, engineering blasting and debris flow. Heavy losses of life and property may occur if the concrete of these structures fails or collapses under these loads. Considering the high incidence of earthquakes and debris flows in China, it is very meaningful to examine the mechanical properties of concrete under dynamic load. There are obvious differences in macro strength and deformation features of concrete under dynamic load and that under static load. The dynamic load-induced response is a rather complex process, involving the evolution of microdefects in the material, the sensitive effect of material strain rate, and the impact of hydrostatic pressure correlation [1-5]. Under dynamic load, the change in concrete deformation and stress is often transmitted in the form of wave, featuring strong instantaneity. It is very difficult to observe the mesoscale destruction or explain the enhancement of mechanical parameters (e.g. concrete strength) through experimental research. By contrast, the deformation, stress change and damage morphology during the impact can be visually presented through numerical simulation, which can provide some guidance on the cost and effect of the experiment. The numerical simulation of dynamic response starts from the microstructure of concrete. In general, a numerical model should be established based on theoretical and experimental results. Then, the macro-mechanical properties of the material and the destruction process of concrete can be explored against the microstructure. On the mesoscale, concrete can be regarded as a composite of mortar, aggregate, and the interfacial transition zone (ITZ). Over the years, many micro-mechanical models have been developed, including but not limited to lattice model [6-7], stochastic particle model [8-9], random aggregate model [10-11], and random mechanical property model [12-13]. Based on meso-mechanical models, the numerical simulation can partially replace experimental research, provided that the models are rational and concrete parameters are precise enough. Nevertheless, the application of numerical simulation has been severely restricted by the lack of experimental data on the mechanical parameters of mortar, aggregate and the ITZ, and the low computing efficiency of 3D analytical models. Much research has been done on the dynamic features of concrete at home and abroad. For example, Liu Haifeng et al. [14- 19] investigated the mechanical properties and constitutive models of concrete under dynamic load, and simulated the dynamic features of concrete under the load using the Holmquist–Johnson–Cook (HJC) model. Du Xiuli et al. [20-21] subdivided the finite-element grids by characteristic unit scale method and projected the grids to the established random aggregate model. In this way, the random multi-scale mechanical model was created and applied to reveal the micro failure mechanism of concrete under dynamic load. Ren Wenyuan et al. [22] proposed to simulate the constituent materials of concrete in each phase with the micro finite-element model (FEM) based on X-ray computed tomography (XCT) images. Park et al. [23] conducted finite-element simulation of concrete and mortar at high strain rate, and analysed the bearing capacity, energy absorption and microstructure of the two materials under dynamic load. According to the aggregate grading curve of concrete, Wang Zongmin et al. [24] generated random aggregates by Monte Carlo method, prepared tensile test specimens with single-edge cracks from the aggregates, and simulated the whole process of the rupture failure of these specimens. Considering the random aggregate structure of concrete, Ma Huaifa et al. [25] put forward a 3D meso-mechanical numerical model that reflects the random aggregate distribution or random mechanical properties of material in each phase. Song Laizhong et al. [26] ensured the rationality of aggregate distribution through random placement of parameterized aggregates, thereby fulfilling the bulk mass, fully-grade, high-strength requirements of aggregate arrangement simulation. By means of light gas gun, Zhang Zhu et al. [27] tested the dynamic mechanical properties test of concrete at different loading speeds, and then performed a numerical simulation on ANSYS Autodyn. The simulation results were contrasted with the experimental data to explain the wave propagation during the destruction of the flyer and the target plate. There are only a few reports on how the size, distribution and volume content of aggregate on the dynamic mechanical properties of concrete. Taking concrete as two-phase heterogenous composite of aggregate and mortar, Liu Haifeng et al. [16-19] wrote a random distribution program of 2D spherical aggregate of concrete in ANSYS parametric design language (APDL), using Fuller’s grading curve and Walraven plane transformation formula, and employed the program to discuss the effects of aggregate particle size, distribution and volume content on the dynamic mechanical properties of concrete. Xu et al. [28] assumed concrete as a three-phase heterogeneous composite of mortar, aggregate and the ITZ, identified the regularity of aggregate grading and particle size distribution according to Fuller’s grading curve, and developed a random distribution program of 3D spherical aggregate of concrete in ANSYS APDL. Since the ITZ is too thin to simulate, the C