Laboratory Observations of Stiffness and Friction of Normal and Sliding Contacts

Contact laws are required for the numerical modeling of the mechanical behavior of granular media, and such models can be made considerably more realistic if the contact laws are 1) tied to the physical and mechanical properties of the constituent grains, and 2) developed independently of the numerical model. To produce realistic contact laws for geologic materials and lunar simulants, on-going work is developing the necessary laboratory techniques for grain-to-grain contact experiments, incorporating the resulting laws into discrete element models and validating the simulations. Our initial experimental efforts addressed the stiffness and friction of normal contacts, and sliding contacts are now being examined, with other modes of deformation to follow. This presentation focuses on our findings with regard to normal and sliding contacts subjected to both monotonically increasing and cyclic force. The experiments yield force-displacement measurements which are analyzed to determine stiffness as a function of loading conditions, and frictional loss (as quantified by analysis of the hysteresis loops observed under cyclic loading). The presentation gives an overview of the experimental and analytical methods that we have developed to date, the results for several materials are presented and their interpretation and implications with respect to numerical modeling are discussed. Of particular interest is the nature of the power law that governs normal contact stiffness. The sliding contact experiments quantify the shear stiffness and frictional loss as a function of the normal force on the contact and the amplitude of the cyclic shear force. Shear stiffness values at very small deformations (e.g., microns) are seen to be on the order of the normal stiffness at low normal forces. In some cases, however, stable sliding can occur, leading to significant reductions in shear stiffness with increasing amplitude of the shear force. Such stable sliding under cyclic loading demonstrates one potential mechanism for permanent strain accumulation (ratcheting) and relevant data are presented and discussed. The materials of interest include a gneiss that has been the subject of previous contact mechanics experiments, triaxial testing and numerical modeling, and the lunar simulant JSC-1A. The experimental work is on-going and results for additional materials (lunar simulants in particular) that are available at the time of the meeting will be presented.