Mechanics of Fractures - Fracture Propagation
Failure Stress: Unconfined or Uniaxial Compressive Strength (UCS) Test
The UCS test provides a measure of the minimum strength a rock material can have under compression.
The failure modes of rock materials underuniaxial compression can provide useful information for safe and economic
design of various engineering structures.
UCS Test Data
Brittle crystalline rock materials contain initial damage, micro-cracks, etc., and under uniaxial or triaxial
compression undergo a complex sequentialprocess of crack closure, crack initiation, stable crack growth, and
unstable cracking before eventual failure.
Direct Shear Stress Test
During the direct shear test, the evolution of permeability is monitored as a function of effective normal
stress, shear displacement, and wear products.
The time-dependent brittle deformation of rocks or of a fracture interface caused by a constant applied stress is quantified
during the creep test.
Creep Test Data
The three stages of the creep curve have been conventionally described as: (1) primary or decelerating creep, (2) secondary or
steady-state creep, and (3) tertiary or accelerating creep.
Pre Heated Stress Tests
Heating induces chemical and poro-thermal-mechanical changes on the matrix of a rock sample. In a Pre-Heated Stress test,
a sample is first heated (up to 600°C) and then tested at normal conditions in order to evaluate the contributions of the
following to fracturing: thermal cracking, anisotropy,thermal expansion, initial porosity and grain size.
Pre Refrigerated Cool Stress Tests
"Cooling induced stresses cause a complete rotation of the stress field such that stress parallel to the secondary
cracks becomes the in-plane major principal stress and may exceed the in situ stresses of the geothermal
reservoir"(A.Ghassemi ,2012**). The Pre Refrigerated Cool Stress test is used to thermally crack the rock matrix
perpendicular to the main fracture. The secondary cracks can be particularly important in reservoir development in enhancing the
heat exchange area or increasing fluid loss.
**Ghassemi, A. Geotech Geol Eng (2012) 30: 647. https://doi.org/10.1007/s10706-012-9508-3