Investigation of Boundary Friction Effects in Polyaxial Tests
Development of boundary friction forces is a common problem in testing of materials, such as rock and concrete. This article reports on experimentations with different friction reducing pads, aiming at diminishing the side friction effects in polyaxial tests. This appears technically possible by using special Teflon®-grease pads, with optimum lubrication achieved at high pressures. Ineffective lubrication can cause an apparent increase of strength and Young´s modulus. The volumetric changes under polyaxial stress states appear relatively little affected by boundary friction, contrary to the partial volume changes of ?V2 and ?V3.