Particle shape effects on dynamic load transfer in granular media

Experimental and numerical studies have been conducted to investigate the effect of microstructural fabric parameters such as major axis orientation, contact normal and branch vector distributions on dynamic load transfer behavior in two dimensional granular material. The experimental method utilizes the combination of high speed photography and dynamic photoelasticity to study the local load transfer behavior in granular assemblies subjected to explosive loading. Numerical studies employed a computational scheme based on the discrete element method. Results indicate that the microstructural fabric has significant effect on the load transfer phenomenon, such as stress wave velocity, load pulse wavelength and contact load attenuation. An experimental investigation was also conducted to compare and contrast the dynamic load transfer process in single chain assemblies of two and three dimensional particles. The comparison shows that the average pulse velocity in disks is higher than in spheres and the pulse attenuation is lower in the disks. The results also show that there is a characteristic pulse which will propagate through a single chain of disks without significant dispersion. The characteristic pulse propagating in assemblies of spheres, however, shows dispersion. For both disks and spheres, when the input pulse is sufficiently long, it undergoes a ringing process which breaks the long pulse up into smaller signals of size approximately equal to the characteristic length. Stress wave propagation phenomenon in granular assemblies of concave-convex particles was then studied. In particular, attention was focused on the effect of contact profile and granular media porosity on inter-granular load transfer, wave velocity, and wavelength of a stress wave. The stress wave was generated by an explosive loading. The experimental results indicate that the stress wave travels faster in the single chain assemblies with higher contact stiffness, which changes with the variation of contact profile. Also, stress wave propagates faster in single chain assemblies with lower porosity. Contact profile does not appear to change wavelength. However, it is observed that porosity does affect wavelength. The experimental results also show that contact profile changes reflection mechanism within a particle therefore varying the contact load attenuation behavior. Finally an attempt was made to develop a mathematical relationship to predict velocity in single chain assembly of elliptical particles