Thick-walled cylinder collapse strength
The so-called TWC-test (Thick-Walled Cylinder test) is carried out in a pressure vessel under hydrostatic loading.
The plug used for this test has a particular geometry: As usual in compressive strength testing, plugs are applied with a length to diameter ratio of 2:1. In addition to this, however, a central hole of 1/5 to 1/3 of the plug’s diameter is drilled and a thick wall cylinder remains. The most common diameters for TWC-tests are between 1.5" to 60 mm with a central hole of 0.5". At Gesteinslabor we use plugs of 60 mm diameter.
The sample is placed into a sleeve to avoid the intrusion of the confining pressure fluid into the sample. The inner hole remains atmospheric and the confining pressure is increased hydrostatically. At some point, the sample cannot withstand the pressure and fails, usually starting with failure of the central drill hole wall. The failure will be visible as a pressure drop and the failure stress point is recorded. Moreover, the confining pressure over time and the volume of pressure medium over time will be reported.
Several (advanced) variations of the TWC test exist which can be used to (1) identify the pressure when sanding starts in the central hole, (2) do real 3D strength measurements and (3) measure the pressure necessary for fracturing hydraulically the cylinder wall from the central hole.
For (1) the central hole is rinsed to remove the loose sand from the inner wall of the cylinder to identify exactly the confining pressure at which collapse of the bore hole starts. For (2) besides the confining pressure an axial load and a fluid pressure inside the central hole are applied and varied to model several 3D stress stages. Those can be used to apply more sophisticated strength criteria than the Mohr-Coulomb criterion, e.g. the Drucker-Prager yield criterion. For (3) a fluid pressure is applied to the central hole higher than the confining pressure. This internal fluid pressure is increased until the cylinder cracks and fails from the inside.