An Automated Apparatus for Three-Dimensional Monotonic and Cyclic Testing of Interfaces
A new computer-controlled apparatus has been developed to study the behavior of interfaces between two materials under three-dimensional monotonic and cyclic loading conditions. The interface can be subjected to a normal stress, sn, and two shear stresses, tx, and ty, acting simultaneously on the interface plane. The normal stress is applied by a pneumatic actuator that is operated by a motorized regulator. Shear stresses, which are perpendicular to each other, are applied by two stepper motors. Tests can be either displacement or load controlled.
The results of an experimental program that includes two series of tests to study the behavior of an interface between a dry coarse sand and a rough surface are presented. In the first series, two-dimensional monotonic and cyclic tests were carried out to verify the performance of the apparatus. The effects of surface roughness, initial relative density of sand, and the magnitude of normal stress on the shear stress-shear displacement and volume change behavior of the interface are presented. In the second series, the loading was three-dimensional and monotonic. First, the interface was sheared in one tangential direction up to a shear stress level less than the peak value. Then, the interface was sheared in a perpendicular direction, while the shear stress in the previous direction was maintained at a constant level. Shear stress and shear displacement increments experienced different paths, while the resultant shear stress-shear displacement curves remained the same irrespective of stress paths.