ABSTRACT
Since our childhood we have learnt to distinguish between solid, liquid and gaseous materials. Among them, only the solid materials can loose their integrity by fragmentation. Breaking, say, a chocolate down into two pieces creates two chocolates that are still solid bodies and can be, again, broken. What, however, if we break a body into a million of pieces? Is the resulting agglomerate of particles still a solid? Can it undergo fracture? Or is it unbreakable? Is it a fluid? — In fact, the resulting granular body belongs to the most fascinating materials, as it can flow though not being a fluid. The flow addressed here is the so-called plastic flow. Soils and other granulates are the most genuine materials to exhibit plastic flow. This is, however, a property of solid materials. In fact, a soil is a solid material provided than an externally applied stress presses the individual grains against each other.
