1. Concepts of Deformation

8/7/2019 1. Concepts of Deformation

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CONCEPT OF DEFORMATION

A. R. Bhattacharya

Department of Geology

University of Lucknow

Lucknow


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- Deformation is the process that results in

a change in the shape or size of a body by

particle displacement within the body.

- It involves the processes by which the

particle motions are achieved, and are of

three types:

(1) Rigid-body rotation Change in orientation(2) Rigid-body translation -Change in position

(3) Distortion- Change in shape

- Deformation may be continuous (lines not

broken) or discontinuous (lines broken).- Strain: includes all aspects of shape

change measured as changes in line length,

changes in angular relationships between

lines, or as changes in volume.


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Attributes of Deformation

(1) Homogeneous Deformation : amount of

strain in all parts of a body is equal. The

criteria are:- Straight lines remain straight lines

- Parallel lines remain parallel lines

(2)Inhomogeneous/HeterogeneousDeformation:

strain in different parts of a body is

unequal. The criteria are:

- Straight lines do not remain straight lines

- Parallel lines do not remain parallel lines


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(3)Elastic Deformation: strain that is recoveredinstantaneously on removal of an applied stress, so that the

object returns to its original undeformed shape. For suchmaterials stress shows direct linear relationship with strain,

according to Hookes law.

(4) Plastic Deformation: involves permanent, non-recoverable, strain occurring without loss of cohesion. This

type of deformation results from rearrangement of chemicalbonds in crystal lattices and may affect the entire rock mass.

The material does not show rupture.

(5) Viscous Deformation

- is pervasive and permanent.

- Stress is proportional to strain rate.

- Although most rocks do not behave as viscous materials, some

of their properties may be approximately explained by

assuming viscous behaviour.


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Ductile Deformation & Plastic Deformation

Ductile Deformation of rocks essentially involves

deformation mechanisms

- Resulting in permanent strain

-Involving accommodation of large

amounts ofnonlocalized homogeneous strain

- Reflecting uniform flow of rock masses.

Plastic Deformation of rocks essentially involves

deformation mechanisms

- At high temperatures (near their melting

point)

- Under the influence of diffusion processes

that de form the crystal lattices (crystal-

plastic or intracrystalline deformation)


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Rheology- simply means the behaviour of a rock to flow,

and gives an idea of how materials involved in

deformation behave under stress.

- It involves the study of the relationships

between stress, strain, time and rate of strain

of a material.

- Rheology of rocks and materials depends

upon both the type of mat erials under

consideration as well as on the externalparameters such as pressure, temperature and

time.


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Deformation Regimes

Rocks and minerals in the crust deform

in their typical way depending upon theprevailing conditions, or regimes, of

deformation. The crustal rocks thus show two

major deformation regimes : Brittle

deformation and ductile deformation.

Brittle deformation implies failure of rocks in the elasticrange. The rocks thus deform in direct proportion to the

applied stress and after the stress is removed, the body

immediately rebounds back to its original configuration. A

brittle substance thus fails by fracture when its ultimate

strength is reached.

Ductile deformation involves permanent strain thatoccurs b y continuous deformation by plastic or viscous flow. It

reflects the capacity of rocks to accommodate large strains

homogeneously (affecting the whole rocks mass) or

heterogeneously (localized in ductile shear zones). Ductility is

the ability of a rock mass to flow without fracture.


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Deformation Paths

- In the elastic range, there is a simple relation betweenthe stress and the strain, and this relation is

independent of the strain history or the strain path.

- In the plastic range, however, the properties of a pre-

strained material depend not only on the current total

strain but on the previous strain history or the strain

path as well.- Therefore, in the case of rocks deformed under large

strains, especially in the plastic (or ductile) range, it is

often necessary to determine the strain path of the

material. Deformation path thus represents the stages

of progressive deformation affecting a rock mass.


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Strain paths or Deformation paths are of two

types.

(1) Coaxial strain path is one in which theprincipal axes of strain remain fixed

with respect to the material,

(2) Noncoaxial strain path is one in which theyrotate with progressive deformation.


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Strain Path Partitioning

- The progressive deformation or the strain

path of a rock may not remain uniform all

through its deformation history, and may

thus involve separation of strain into different

mechanisms or movement domains in a rock

mass or crystal.

- A rock showing this phenomenon is said to

show de formation (or strain-path)partitioning.

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1. Concepts of Deformation