The mechanics of rigid irregular particles subject to uniaxial compression

Single-particle compression tests, in which an individual sand grain is vertically compressed between two rigid horizontal platens, are often used in particle-scale soil mechanics studies. They are useful index tests to examine the susceptibility of a given sand to particle breakage; they provide information for calibration of particulate discrete-element models that capture crushing; and they can give information on size-strength relationships. The test is conceptually simple, but the response of an irregular particle in these compression tests is not straightforward. During compression the particle can rotate. Both horizontal and vertical forces are induced at the particle-platen contacts, and so there may be frictional sliding at the contact points at the same time as, or prior to, compression of the bulk particle. Asperities can yield, changing the particle geometry. The variation in the response mechanism during compression leads to a load-deformation response that is not always easy to interpret. This paper describes two relatively simple analytical studies of an irregular particle in a particle compression test. The susceptibility of the particle to rotation under the applied compressive force is shown to depend on the particle geometry and the particle-platen friction. The rotation of the particle is shown to induce a kinematic degradation or reduction in the effective stiffness of the system, and the system stiffness depends on the particle size. Frictional sliding at the contact points will also cause a reduction in stiffness. These observations may have implications not only for the test itself, but also for the response of irregular particles participating in the strong force chains in stressed granular materials.