Lectures: Monday, April 12 and Wednesday, April 14
Text: pages 292-313
Monday, April 12
MondayÌs lecture followed to text pretty closely, so I will not go into
Definition The strain of a body is the change in size and shape that the
body experiences during deformation. Homogeous strain is when the changes
in size and shape are proportionately identical for each small part of the
body and for the body as a whole.
Deformation is compsoed of 1) translation, 2) rotation, 3)distortion (or
We went through a quantitative description of strain and defined the following:
strain tensor as the sum of infinitesimal strain and infinitesimal rotation.
The tensor is symmentric when the off-diagonal terms of the matrix are
equal, and antisymmentric when the off-diagonoal elements are equal in
magnitude but opposite in sign.
Finite strain tensor (non-infinitesimal)
Dilatation (volumetiric extension ev = sv -1), with volumetirc strestch sv
Equations for infinitesimal stress and strain parallel each other. This is
not true for finite stress and strain.
There is a linear relationship between stress and the strain rate.
Strain Ellipsoid - This is covered in the text. One note is that the axes
of the ellipsoid are actually stretch s
strain e = s - 1
where s = deformed radius l/ undeformed radius L
Wednesday, April 14
Crenulation foliations vs. S-C mylonites
Crenulation foliation formed by harmonic folds that develop in a prexisting
foliation, so the new foliation cuts across the old one. If the
crenulations are symmetic, both limbs defines the cleavage. If they are
asymmetric, then only one of the limbs defines the new foliation. (p266-7)
S-C mylonites are formed by only one deformation event and are always
Dissolution occurs at grain boundaries and can re-precipitate next to the
grain (stress shadow).
Original State from Strained State
Fossils are good indicators for inferring the amount of strain because they
can have a known symmetric or radial shape. Some examples are trilobites,
brachiopods, and ammonites.
Otherwise, it is not possible to find a direct relationship for stress and
strain. Genearlly, the stress diraction is parallel to the incremental
strain or strain rate. However, large stress will produce large deviations
from its orientation.
We looked at strain progression diagrams for simple and pure shear. In
pure shear, the orientation of the ellips remains the same. For simple
shear, the ellpse can rotate and over-rotae while shortening, so the end
product does not indicate what past strain has occurse.