Strain

Question 1

Define strain

Answer 1

A measure of the change in size and shape that a body experiences during deformation

Question 2

Define linear/longitudinal strain

Answer 2

The change in length parallel to a given direction (change in length / original length)

Question 3

Give four parameters that can be used to specify shear strain

Answer 3

Engineering strain (extension), stretch, quadratic elongation, logarithmic strain (natural strain)

Question 4

Define shear strain (can also be known as angular shear, ψ)

Answer 4

The change in angle between two lines

Question 5

When is angular shear positive and when is it negative?

Answer 5

Positive for an anticlockwise (sinistral) rotation and negative for a clockwise (dexral) rotation

Question 6

What is the difference between displacement, strain, and deformation?

Answer 6

Displacement is the change in position of a given particle, strain is the relative change in positions of particles within the feature, and deformation is displacement that may or may not involve strain

Question 7

Give the four basic types of displacement?

Answer 7

Rigid body translation, rigid body rotation, distortion, dilation

Question 8

How are Lagrangian and Eulerian specifications used differently in the deformation gradient matrix?

Answer 8

Lagrangian specification gives the deformed position using the original, Eulerian gives the original position using the deformed

Question 9

What type of strain cannot be accounted for used the deformation gradient matrix?

Answer 9

Rigid body translation

Question 10

What can the displacement gradient matrix be split into?

Answer 10

Symmetric (gives strain (distortion and dilation) and antisymmetric parts (gives rotation)

Question 11

What do 2nd rank tensors do?

Answer 11

Dictate the way that the values of the components in the strain matrix change and the orientation of the coordiante frame is changed.

Question 12

Define tensor

Answer 12

A mathetical quantity that describes the physical state of a material

Question 13

Wheh is strain not a single number?

Answer 13

At a single point

Question 14

How is strain in 2d visualised?

Answer 14

Strain ellipse (represents deformed shape of originally round object)

Question 15

What does the strain ellipse show?

Answer 15

How linear strain and angular shear vary with direction

Question 16

Where is there zero angular shear on a strain ellipse?

Answer 16

On the minor and major axes, where the extensions are at maximum and minimum values, these are the principal strains

Question 17

How is strain in 3d visualised?

Answer 17

Strain ellipsoid (represents deformed shape of an originally spherical object)

Question 18

What do the three semi-axes of the strain ellipsoid represent?

Answer 18

The three principal strains, these are orthongonal, parallel to them, the shear strain is zero

Question 19

Define plane strain in 3d

Answer 19

Where there is no stretching or shortning in one principal direction (Y direction), the strain can be regarded as 2d

Question 20

Define axially symmetric extension or compression in 3d

Answer 20

An irrotational deformation in which the length changes along two principle directions are equal

Question 21

What are Flinn diagrams used for?

Answer 21

Distinguishing the shapes of strain ellipsoids

Question 22

When is strain homogeneous?

Answer 22

If changes in size and shape for each small part of a body are proportionately identical to that for the body as a whole

Question 23

When is strain inhomogeneous?

Answer 23

Of changes in size and shape vary from place to place in the body

Question 24

Describe how the incremental strain ellipse is used

Answer 24

It splits up the deformation in a series of small strain increments because calculating strain says nothing about the deformation path

Question 25

Give four examples of common strain markers

Answer 25

Objects that were initally circular or elliptical or of known size or originally possessed bilateral symmetry

Question 26

Why are circular markers useful?

Answer 26

Their deformed shape is the actual shape of the strain ellipse

Question 27

What is the Fry method?

Answer 27

Estimating the strain of a rock that has grains that were initially equidimensional (requires them to be of similar size and not clustered)

Question 28

Give an example of how to use elliptical markers

Answer 28

Analysis of the shapes of clasts within deformed conglomerates

Question 29

How are elliptical strain markers analysed?

Answer 29

Rf /ϕ ´ method

Question 30

What is the Mohr construction used for?

Answer 30

Analyse strain using changes in line length and/or changes in angle

Question 31

When can it be useful to anlyse fabrics, such as cleavage, rather than markers?

Answer 31

When the marker has similar mechanical properties to their host rock

Question 32

Describe an L-tectonite rock

Answer 32

Where linear elements (prolate ellipsoid) dominate

Question 33

Describe an S-tectonite rock

Answer 33

Where planar elements dominate (oblate ellipsoid)

Question 34

Describe an LS-tectonite rock

Answer 34

Where there is both strong foliation and strong lineation

Question 35

What does the cleavage in deformed rocks represent?

Answer 35

XY plane (flattening plane)

Question 36

What does the formation of neutral points reflect?

Answer 36

Variations in the local orientation of the XY plane

Question 37

What does cleavage refraction show?

Answer 37

A difference in shear strain between layers and therefore a difference in the local orientation of the XY plane

Question 38

When is the strain ellipse shared by two layers?

Answer 38

When there is no slip at the interface between two layers

Question 39

What can strain ellipse maps be used to show?

Answer 39

Regional strain data