Cyclic normal load–displacement behaviour of clay-coated sand grain contacts

A micromechanical experimental study is presented in this paper investigating the normal contact response of quartz sand grains under cyclic loading. Sand grains with clean surfaces, termed uncoated, and grains coated with commercially available kaolinite powder were studied. The grain contacts were subjected to ten cycles of normal loading within a range from 0 to 5 N. It was observed that uncoated sand grains showed dominantly elastic behaviour, but coated particles had a gradual transition into elastic behaviour after significant plastic deformations in the initial cycles. The normal contact stiffness of coated particles initially increased notably with the number of cycles and reached a maximum value, but uncoated particles showed no variation of normal stiffness against the loading cycles. Similar trends were observed for the stiffness ratio against the loading cycles. Total work input required to reach a given maximum normal load along with elastic and plastic fractions of work input were calculated for coated particles. For uncoated particles, these values were negligibly low, and it was impractical to find elastic and plastic fractions precisely. For coated particles, with a lower preloading at 2 N, the normal load–displacement curve reaching 5 N deviated after a threshold value of 2 N was reached – that is after reaching the previous maximum load the grains contact had experienced. This effect was more pronounced with the increase of the cycles during preloading at 2 N. However, a preloading at 10 N – that is the grains were subjected to greater previous load prior to the normal load–displacement test at 5 N, caused a decrease in plastic deformations and increase in stiffness ratio in the subsequent cycle at 5 N. Uncoated grain contacts did not demonstrate notable variations under the preloading conditions.