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In this paper, a model for fracture and fragmentation, considering the heterogeneity of material properties, is employed to investigate the mechanism of spallation in fiber-reinforced concrete with various fiber and aggregate contents under impact loading. Numerical simulations show that there is a marked difference in the failure patterns among concretes with various fiber and aggregate contents, and the fibers can remarkably improve the tensile strength of the concrete, effectively preventing the initiation and propagation of cracks and inhibiting the occurrence of spallation. Moreover, numerical simulations capture the whole process of the propagation of incident compressive stress waves in the fiber reinforced concrete and the reflection of stress wave on concrete surface and the spallation failure of fiber-reinforced concrete induced by the reflected tensile stress wave, which is obviously different from the failure pattern of fiber reinforced concrete under static loads. The results in this study can provide some valuable reference for studies on the tensile properties and failure modes of heterogeneous quasi-brittle materials.

The analytical solutions for the inception of strain localization in unsaturated elastic-plastic porous materials have been implemented into the constitutive driver for bounding surface plasticity model (Khalili et al., 2008). The solutions are based on the definition of effective stress proposed by Khalili and Khabbaz (1998), which is backed up by a substantial amount of experimental evidence. The strain localization behavior of unsaturated soils is further illustrated on the example of Bourke silt from Bourke region of New South Wales, Australia. To this end, the silt is subjected to three series of drained plane strain compression tests whereby the initial net mean stresses and suctions correspond to the range of initial overconsolidation ratios between 1.26 and 6.67. It is found that the axial strain at the inception of strain localization sharply decreases in a nonlinear fashion with the increasing overconsolidation ratio. BACKGROUND The onset of strain localization signifies the initiation of narrow zones, within which large strains evolve with continuing loading. These zones, which are also known as deformation bands, are typically orders of magnitude smaller than the associated macro-scale boundary value problems. The onset of strain localization is a failure precursor because it coincides with the inception of the emerging failure mechanism. So far the vast majority of research covering the strain localization has been devoted to dry soils while the research addressing this phenomenon in fully and partially saturated soils has been scarce. Schiava and Etse (2006) conducted a numerical investigation of strain localization in unsaturated soils at constant suction by using Terzaghi’s effective stress principle. Callari et al. (2010) performed a computational modeling of the initially saturated plane strain soil sample subjected to compression while maintaining the atmospheric pressure at the top and bottom of the sample. They used an average pore pressure, which appears in Lewis-Schefler effective stress. Ehlers et.