Dynamic Multiscale Deformation Behavior And Particle-breakage Properties Of Granular Materials Subjected To Impact Loading

Granular materials such as sand and powders are ubiquitous in nature and every-day life. They are the most widely used engineering materials except water. The static packing and flow properties have been extensively discussed. A number of interest-ing phenomena such as force chain, jamming transition and vibrational patterns have been reported and discussed. However, the microscopic deformation behavior and par-ticle breakage properties of granular materials under impact loading have not been fully investigated. This constraints the development of engineering sciences, such as civil en-gineering, power technology (metallurgy, pharmacology), and defense engineering, etc. In addition, some worldwide scientific problems such as impact cratering, earthquakes and rock avalanches are closely related to granular dynamics.Granular materials are multi-scale, complex systems, and can be divided into three scales:granular assembly at the macro scale, particles/grains at the meso scale, and crystalline grains and defects (e.g. micro cracks and voids) at the micro scale. In the present thesis, the multi-scale deformation and particle breakage properties of granular materials have been investigated systematically, from both the experimental and numer-ical points of view. In the simulation, a multi-scale model is constructed for granular materials based on the discrete element modeling, and is applied to impact loading. The multi-scale model can simulate the impact compression responses of real granular ma-terials such as yield stress and logarithmic linearity, when the particle stiffness, bond strength and local damping level are properly chosen. In addition, we discuss the effects of local damping coefficient and particle friction coefficient on the impact compression responses of modeling granular materials. The multi-scale model plays an important role in explaining the micro mechanisms of experimental results. In the experiment, the multi-scale deformation and particle breakage properties of various granular materials, i.e., quartz sands, SiC powders, glass and sugar particles, under quasi-static and impact loading are investigated. The stress-strain curves under a wide range of strain rates are measured with the MTS 810 and split Hopkinson pressure bar. The grain size distri-butions of granular materials before and after loading have been measured through a laser diffractometry equipment Mastersizer 2000. Then the evolution of particle break-age with such factor as stress, strain rate, particle size, and gradation is quantified with the Einav breakage index. To fully discuss the microscopic deformation and particle breakage process of granular materials, a mini SHPB is implemented with the ultrafast, in situ X-ray phase contrast imaging technique, and the dynamic compression of granu-lar materials is captured. Dynamic strain fields are then obtained with the X-ray digital image correlation method. At the micro scale, we investigate the compression breakage of glass particles under quasi-static loading. Effects of the particle size on the breakage strengths and fracture modes of particles are discussed with the high-speed photography and theoretical analysis. In addition, we investigate the impact compression fracture of single crystal silicon with simultaneous X-ray phase contrast imaging and X-ray diffrac-tion. Anisotropy in the deformation and breakage modes of single crystals is discussed. The shock compression failure of brittle solids under 1D strain loading is investigated and the micro mechanisms for failure waves in brittle solids under shock compression have been clarified.Multiscale measurements facilitate us a deep understanding of the microscopic de-formation mechanisms, including strain rate effects, particle size and gradation effects, particle breakage modes and shock-induced damage. Bulk-scale stress-strain curves il-lustrate that granular materials exhibit significant strain-rate effects, i.e. the dynamic stress is higher than the quasi-static one at the same strain level. The particle breakage analyses show that the breakage extent under quasi-static loading is higher than that under dynamic loading at the same stress level, which explains the strain-rate effects illustrated in the compression curves since particle breakage dominates deformation of granular materials after yield. Discrete element modeling reveals that single particle breakage produces more fragments under impact loading, which leads to a lower ener-gy-breakage efficiency, and thus a lower breakage extent. This is the intrinsic mecha-nism of strain rate effects of granular materials. Particle size and gradation effects are actually attributed to the competition between the strength and average coordination number of particles. Well-graded granular materials exhibit smaller breakage extent, and thus smaller deformation and energy absorption than uniformly-graded materials. For uniformly-graded granular materials, coarse materials usually display greater parti-cle breakage extent, compression deformation and energy absorption. Granular materi-als can be viewed as homogeneous at the macro scale. However, their mesoscopic strain fields show strong heterogeneity, and the statistical distribution of strain values exhibits a similar form to that of contact forces, i.e. they both exhibit power-law decay in the large-strain range. Therefore, the strain heterogeneity is actually attributed to the het-erogeneous force transmission. The particle breakage extent exhibits a non-monotonic increasing trend with increasing particle friction coefficients. The intrinsic mechanism is that contact force distributions change with increasing contact friction, which induces transition in the breakage modes. The breakage strengths of particles under quasi-static loading show strong scatter, but follow a Weibull distribution. The average breakage strengths of particles of different sizes satisfy a Weibullian scale law. The particle break-age modes under impact loading are quite different to that under quasi-static loading. Under impact loading, many wing cracks develop perpendicular to the main cracks. The random nucleation, growth and coalescence of initial defects lead to catastrophic failure of materials. Moreover, 1D strain shock compression can induce formation of failure waves and lead to comminution of materials. The micro mechanisms are com-pression induced tensile and shear deformation localizations. In conclusion, the results obtained in the present thesis provide a solid basis for constructing the multi-scale mod-el of granular materials, and shed light on micro mechanisms of hot spots in polymer bonded explosives.