Micromechanics of failure in brittle geomaterials. Final technical report (for 7/1/1994 - 8/31/2000) Page: 5 of 6
This report is part of the collection entitled: Office of Scientific & Technical Information Technical Reports and was provided to Digital Library by the UNT Libraries Government Documents Department.
The following text was automatically extracted from the image on this page using optical character recognition software:
Micromechanics of compressivelailure: observation and model
Quantitative microstructural study and theoretical modeling elucidate the micromechanics of
failure. Menindez, Zhu and Wong  characterized the evolution of damage in Berea
sandstone using both optical and scanning electron microscopy. In the brittle faulting regime,
shear localization did not develop until the post-failure stage after the peak stress had been
attained. The microcrack density data show that very little intragranular cracking occurred before
the peak stress was attained, even though appreciable acoustic emission activity was observed.
The inference is that dilatancy and acoustic emission activity in the prefailure stage are due
primarily to intergranular cracking, probably related to the shear rupture of lithified and
cemented grain Contacts. Near the peak stress, intragranular cracking initiates from grain
contacts, and this type of Hertzian fracture first develops in isolated clusters, and their
subsequent coalescence results in shear localization in the post-failure stage. The very high
density of intragranular microcracking and pronounced stress-induced anisotropy in the post-
failure samples are a consequence of shear localization and compactive processes operative
inside the shear band. In contrast, Hertzian fracture was a primary cause for shear-enhanced
compaction and strain hardening throughout the cataclastic flow regime. Grain crushing and pore
collapse seem to be most intense in weakly cemented regions.
Motivated by the microstructural observations, Wong and Wu  investigated the effect of
cementation on the micromechanics of compressive failure using the finite element technique.
The development of stress-induced cracking is governed by the fracture mechanics at grain
contacts. The Hertzian model for grain contact assumes two spheres in elastic contact, and
theoretical solutions for the stress fields are available when either only normal load is applied or
when tangential load causes full slip conditions. However, the contact grains may be under
normal and tangential loadings simultaneously and the contacts may undergo partial slip. Shah
and Wong  formulated a superposition technique by which the stress fields and fracture
mechanics parameters for this more realistic scenario can be evaluated. Preliminary theoretical
results for a model of uniform spheres obtained by Shah and Wang  are in qualitative
agreement with experimental observations.
We also investigated the micromechanics of failure and spatial evolution of damage in the
Darley Dale sandstone of intermediate porosity (-13%). Our observations indicate significant-
differences with the micromechanical behavior of a weakly cemented porous sandstone, and the
microstructural observations underscore the variability of micromechanics of failure [Wu, Baud
and Wong, 2000].
The Brittle-Ductile Transition in Porous Carbonate Rocks
We have completed a study on the mechanics of the brittle-ductile transition in the Solnhofen
limestone. For the first time, a fairly complete set of data on porosity change and failure mode in
this limestone with porosity of-3% have been acquired. Our mechanical data show that the
failure modes are associated with complex interplay of dilatancy, pore collapse and crystal
plasticity processes, and several micromechanical models have successfully been employed to
consistently interpret the phenomena [Baud, Schubnel and Wong, 2000]. This study has
established the framework for future study on the failure and porosity change in porous
carbonate rocks in general.
Here’s what’s next.
This report can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Report.
Wong, Teng-fong. Micromechanics of failure in brittle geomaterials. Final technical report (for 7/1/1994 - 8/31/2000), report, December 1, 2000; United States. (https://digital.library.unt.edu/ark:/67531/metadc733923/m1/5/: accessed May 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.