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.