Post-failure tests at uniaxial and triaxial compression tests

In order to be able to conduct more accurate calculations of a construction in rock, which takes into account how the strength changes during cracking, information is needed from e g material tests. In a suggested standard ISRM 1999 [1] the rock material is categorized as ductile (class I) and brittle (class II) material according to the figure below.


For ductile materials (class I) the complete result curve is obtained by conducting the test with axial deformation control. For brittle materials (class II), the axial deformation control implies that the specimen will burst more or less explosive and will be totally damaged and no information about the post failure behaviour will be obtained. The stored elastic deformation energy along the whole load chain in the equipment and in the specimen, exceeds many times the total energy (crack release energy) that is needed to entirely break the specimen. In order to be able to follow the successive cracking of the material, it is required that the load, and thus the stored energy, is controlled during the cracking. One way to control the load (see ISRM 1999 [1]) is to use the radial expansion as a crack indicator and a measure of the actual cracking, and to let the load be controlled such that an even increase of the radial expansion will be obtained, see figure below.



Load control in order to capture post-failure response for brittle materials (class II).


A great number of uniaxial and triaxial compression tests on brittle materials (class II) have been conducted for the Swedish Nuclear Fuel and Waste Management Corporation (SKB) see e g [2,3]. More reports are published on SKB.


[1] ISRM. Draft ISRM suggested method for the complete stress-strain curve for intact rock in uniaxial compression. Int J Rock Mech Min Sci 36(3):279-289, 1999.

[2]   Jacobsson, L. Forsmark site investigation, Borehole KFM02A, Uniaxial compression test of intact rock. SKB-P04-224, Svensk Kärnbränslehantering AB, 2004. (Published on SKB)

[3]   Jacobsson, L. Oskarshamn site investigation, Borehole KLX03A, Triaxial compression test of intact rock. SKB-P05-96, Svensk Kärnbränslehantering AB, 2005. (Published on SKB)


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Contact Persons

Lars Jacobsson

Phone: +46 10 516 56 19

Mathias Flansbjer

Phone: +46 10 516 52 25

RISE Research Institutes of Sweden, Phone 010-516 50 00, E-mail

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