Chapter 4 Flashcards
One type affects the entire volume of a rock, the outside as well as the inside
body forces
Body forces define threedimensional fields. The most important type of body force in structural geology
gravity
The other type of force acts on surfaces only and is referred to as
surface force
Surface forces originate when one body pushes or pulls another body. The force that acts across the contact area between the two bodies
is a
surface force
A stress vector oriented perpendicular to a surface is called
normal stress on the surface
while a stress vector that acts parallel to a surface is referred to as
shear stress
An obvious requirement for expressing
stress as an ellipse (ellipsoid) is that there is not a combination of positive and negative tractions. The ellipse
is called
stress ellipse and the ellipsoid id the stress ellipsoid
its orientation tell us everything about the state of stress at a given point in a rock, or in a rock volume in which stress is homogeneous
stress ellipsoid
The axes are called the principal
stresses and are the poles to the principal planes of stress. These are the only planes where the shear stress
zero
It is useful to put the nine components of stress into a matrix (second-order tensor) known as
stress sensor or stress matrix
is a real number, reflecting temperature, mass, density, speed or any other physical magnitude that has no direction
Scalar
has both magnitude (length) and direction, such as force, traction (stress vector)
or velocity
Vector
In the latter case, the elements of
the tensor are functions of position and the tensor forms what is called a ________ This simply means that the tensor is defined at every point within a region of space (or space-time),
Tensor field
Any stress tensor can be split into two symmetric matrices
first part represents the mean stress and the second is called the deviatoric stress.
the difference between the mean
stress and the total stress. represents the anisotropic component of the total stress
The deviatoric stress
The difference between the maximum and minimum principal stresses (s1 s3) is the diameter of the circle.
The differential stress
is generally limited to media with no or very low shear resistance (fluids).
Pressure (p)
is used when dealing with media with a minimum of shear resistance (rocks).
Stress (σ)
A stress vector oriented perpendicular to a surface is called the
normal stress
A stress vector that acts parallel to a surface is referred to as the
shear stress
are the principal stress vectors
eigenvectors
the matrix which are the principal stresses
eigenvalues
There is no shear stress “anywhere” and there is no off-diagonal stress in the total stress tensor.
hydrostatic stress
n a fluid. and represents an isotropic state of stress. In the lithosphere, the mean stress is closely related to __________ (where the pressure is due to overlying rock, not fluid).
lithostatic pressure
A practical graphical way of presenting and dealing with stress is based on a diagram referred to as the
Mohr diagram
