The FPMs (UMons-Belgium) device for investigating the mechanical behavior of materials subjected to true triaxial compression

A polyaxial system was developed in order to study the influence of high confining true triaxial stress conditions on the mechanical behavior of rock materials. This device, which has three rigid pistons, uses 30-mm-sided cubic samples. The device is fully automated in order to perform tests at controlled stress or strain rates. Different stress paths were implemented in order to perform classical compression tests or time-dependent tests. This permits an accurate qualification of the mechanical behavior of the tested material. An original methodology was also developed to build and model the limiting envelopes on the selected octahedral plane. For approximately twenty years, various rock materials have been tested: limestones, sandstones, shales and others. Selected results gathered on these materials are presented in the paper, mainly in terms of hardening-softening and associated compaction-dilatancy. The evolution of the limiting envelopes on the octahedral plane is also discussed.