Deviatoric stress
δσij is 2nd order tensor represented by a 3x3 matrix of form:
| σ11-σm | σ12 | σ13 |
| σ21 | σ22-σm | σ23 |
| σ31 | σ32 | σ33-σm |
with six independent stress parameters (σ12 = σ21, σ13 = σ31, σ23 = σ32) and
σm = (σ11 + σ22 + σ33)/3
In a triaxial test it is possible to arrange the stress components in a way that the diagonal stress components σ11, σ22 and σ33 are oriented parallel to the applied Cartesian coordinate system. Consequently, these three stress components become the principal stresses σ1, σ2 and σ3 and the off-diagonal components equal zero. Therefore, in a laboratory situation the deviatoric stress tensor [δσij] can be simplified to:
| σ1-σm | 0 | 0 |
| 0 | σ2-σm | 0 |
| 0 | 0 | σ3-σm |
According to Engelder (1994) deviatoric stress is very often confused with other types of stresses such as differential stress and effective stress. Engelder (1994) recommends not to use the term “deviatoric stress” unless for fault slip problems.
Differential stress
ΔS is the difference between the maximum principal stress S1 and the minimum principal stress S3.
ΔS = S1 - S3
is also the diameter of a Mohr circle.
Effective stress
σij (with i=j and the main principle stresses σ1 ≥ σ2 ≥ σ3) is the difference between the applied external stress Sij and the pore pressure Pp. In its simple form (Terzaghi 1923):
σij = Sij - δijPp
it denies the importance of pore volume and its compressibility by disregarding the Biot coefficient α. This equaition is applied generally in soil mechanics where full efficacy of pore pressure (i.e. α = 1) can be assumed.
In case of solid rock things appear more complicated and pore pressure efficacy never is at 100%. Consequently, the Biot coefficient has to be considered and the exact form of the equation according to Nur & Byerlee 1971 is:
σij = Sij - δijαPp.
Entry pressure
See "Threshold pressure"
News
With the recent acquisition of the state-of-the-art DIL 502 Expedis® Select thermal dilatometer from Netzsch, we are expanding our capabilities for the precise analysis of the thermal properties of rocks and mineral materials. We are now able to determine the coefficient of thermal expansion in accordance with DIN EN 14581:2004, ASTM D4535-13e1. (2013) or using a measurement program specifically tailored to the customer’s needs (for more information, see Thermal Dilatometer).
The project, which was funded by the Free State of Thuringia, was co-financed by the European Union through the European Regional Development Fund (EFRE).
This year’s EAGE GET Conference and Exhibition is taking place in Germany:
Gesteinslabor Dr. Eberhard Jahns will again be taking part as an exhibitor this year. From 2 to 6 November 2026, you can find us at Stand 31 at the HCC in Hannover.
We would be delighted to welcome you to our stand.
Gesteinslabor Dr. Eberhard Jahns is project and research partner within the HENRI (Hydrogen Energy Reservoir) project.
Our main focus is on determining the impact of H2 on the cap rock. Besides other parameters, we measure the capillary threshold pressure using various gases and gas mixtures of hydrogen and methane.
Results were presented at the third HENRI conference in Bratislava in September 2025
We are pleased to be part of the Dutch subsurface research for geothermal heat SCAN.
Further information can be found on the official homepage:
https://scanaardwarmte.nl/english/
