1656949294093_Concept of stress and strain.ppt

a pdf of concept of stress and strain

1. SRI JAI NARAIN MISRA PG COLLEGE

SESSION : 2021-2022
DEPARTMENT OF GEOLOGY
BSC (N.E.P) YEAR 1 SEMESTER 2
ASSIGNMENT TOPIC - CONCEPT OF STRESS AND STRAIN
ROLL NO - 2110404010609
SUBMITTED TO - MR. SARANJEET SINGH SIR
SUBMITTED BY – SHIKHA YADAV
FATHER NAME - MR. AMAR SINGH YADAV
DATE OF SUBMISSION - 5 JULY 2022

2. ACKNOWLEDGEMENT
I should need to
 express my remarkable thanks of gratefulness to my teacher MR.
SARANJEET SINGH SIR similarly as our essential who gave me the splendid
opportunity to do this sublime endeavor on the school project (CONCEPT OF STRESS
AND STRAIN) which in like manner helped me in finishing a lot of school project and
I came to consider such tremendous quantities of new things. I am very appreciative to
them.
Other than I may moreover need to thank my people and buddies who helped me a lot in
settling this endeavor inside the limited time span.


SHIKHA YADAV

3. Concept of stress and strain

4. Abstract
Structural analyses of specific features in naturally deformed rock consist of geometric
observations (e.g., shape).
Kinematic measurements (e.g., strain), and dynamic models (e.g., stress). Although
analytical definitions clearly distinguish strain and stress, common usage of the terms
tends to blur the conceptual difference. Strain and stress do not have a simple cause and
effect relationship. The fundamental difference between strain and stress is that strain
terms reflect descriptive interpretations of what movements produced a structure, while
stress terms reflect genetic interpretations of why the structure formed. This descriptive
versus genetic distinction has several implications. First, kinematic analysis is less
speculative and more directly related to observations than dynamic analysis. Second,
kinematic analysis is less computationally and analytically intensive than dynamic
analysis. Third, kinematic analysis is amenable to more intuitive, but shallower,
understanding than dynamic analysis. The most useful terminology communicates this
conceptual framework through clear and accurate use of terms for strain, stress, and
related concepts. A variety of examples illustrate the descriptive and genetic usage of
strain and stress terminology.

5. Introduction
The terminology of strain and stress is elemental to continuum mechanics.
In its simplest form, strain is defined as the change in length or rate of
length change of a line divided by its original length (i.e., we wish to
simultaneously consider incremental and finite strains); stress at a point is
the force per unit area operating on an infinitesimal surface at that point .
Alternative definitions exist that require explicit reference to the tensorial
character of these parameters. Our objective in this contribution The
systematic application of continuum mechanics has transformed is
conceptual clarity rather than analytical generality, so we prefer to use
simple definitions. Although geologists readily agree that strain
fundamentally differs from stress, the conceptual distinction often is lost
among the equations. Worse still, the usage of strain and stress
terminology in the context of observation and interpretation of natural
structures frequently fails to communicate a clear conceptual framework.

6. Stress and Strain

Stress in Earths CrustStress in Earths Crust
Enormous slabs of lithosphere
move unevenly over the planet’s
spherical surface, resulting in
earthquakes. This chapter deals
with two types of geological
activity that occur because of plate
tectonics: mountain building and
earthquakes. First, we will consider
what can happen to rocks when
they are exposed to stress

Causes and Types of StressCauses and Types of Stress

Stress
 is the force applied to an
Stress
 is the force applied to an
object. In geology, stress is the object. In geology, stress is the
force per unit area that is placed on force per unit area that is placed on
a rock. a rock.

7. Types of stress
A deeply buried rock is pushed
down by the weight of all the
material above it. Since the rock
cannot move, it cannot deform.
This is called
 confining stress.
Compression
 squeezes rocks
together, causing rocks to fold or
fracture (break) . Compression is
the most common stress at
convergent plate boundaries.
Rocks that are pulled apart are
under
 tension. Rocks under tension
lengthen or break apart. Tension is
the major type of stress at divergent
plate boundaries.
When forces are parallel but
moving in opposite directions, the
stress is called
 shear 

8. When stress causes a material to change shape, it has undergone
 strain or deformation. Deformed
rocks are common in geologically active areas. A rock’s response to stress depends on the rock
type, the surrounding temperature, and pressure conditions the rock is under, the length of time the
rock is under stress, and the type of stress . Rocks have three possible responses to increasing
stress .

elastic deformation: the rock returns to its original shape when the stress is removed.elastic deformation: the rock returns to its original shape when the stress is removed.

plastic deformation: the rock does not return to its original shape when the stress is removed.plastic deformation: the rock does not return to its original shape when the stress is removed.

fracture: the rock breaks.fracture: the rock breaks.

9. Geologic Structures

Sedimentary rocks are important Sedimentary rocks are important
for deciphering the geologic history for deciphering the geologic history
of a region because they follow of a region because they follow
certain rules.certain rules.

Sedimentary rocks are formed with Sedimentary rocks are formed with
the oldest layers on the bottom and the oldest layers on the bottom and
the youngest on top.the youngest on top.

Sediments are deposited Sediments are deposited
horizontally, so sedimentary rock horizontally, so sedimentary rock
layers are originally horizontal, as layers are originally horizontal, as
are some volcanic rocks, such as are some volcanic rocks, such as
ash falls.ash falls.

Sedimentary rock layers that are Sedimentary rock layers that are
not horizontal are deformed.not horizontal are deformed.
(a) In the Grand Canyon, the rock layers are exposed like a layer cake. Each layer is made of sediments that
were deposited in a particular environment – perhaps a lake bed, shallow offshore region, or a sand dune. (b)
In this geologic column of the Grand Canyon, the sedimentary rocks of the “Layered Paleozoic Rocks”
column (layers 1 through 11) are still horizontal. Grand Canyon Supergroup rocks (layers 12 through 15)
have been tilted. Vishnu Basement Rocks are not sedimentary (rocks 16 through 18). The oldest layers are
on the bottom and youngest are on the top.

10. Folds

Rocks deforming plastically under Rocks deforming plastically under
compressive stresses crumple into
 folds .
compressive stresses crumple into
 folds .
They do not return to their original shape. They do not return to their original shape.
If the rocks experience more stress, they If the rocks experience more stress, they
may undergo more folding or even may undergo more folding or even
fracture.fracture.
Snow accentuates the fold exposed in these rocks in Provo Canyon,
Utah

11. Types of folds
Three types of folds are seen.
Mononcline: A
 monocline is a simple bend in the rock layers so that they are no longer
horizontal .
At Colorado National Monument, the rocks in a monocline plunge
toward the ground.

12. Anticline : An
 anticline is a fold that arches upward. The rocks dip away from the
center of the fold . The oldest rocks are at the center of an anticline and the
youngest are draped over them
a) Schematic of an anticline. (b) An anticline exposed in a road cut in New Jersey.
Syncline: A
 syncline is a fold that bends downward. The youngest rocks
Syncline: A
 syncline is a fold that bends downward. The youngest rocks
are at the center and the oldest are at the outsideare at the center and the oldest are at the outside.
(a) Schematic of a syncline. (b) This syncline is in Rainbow Basin, California.
When rocks arch upward to form a circular
structure, that structure is called a dome. 
When rocks bend downward in a circular
structure, that structure is called a basin 

13. Faults
A rock under enough stress will fracture. If
there is no movement on either side of a
fracture, the fracture is called a
 joint.

If the blocks of rock on one or both sides If the blocks of rock on one or both sides
of a fracture move, the fracture is called of a fracture move, the fracture is called
a
 fault . Sudden motions along faults cause
a
 fault . Sudden motions along faults cause
rocks to break and move suddenly. The rocks to break and move suddenly. The
energy released is an earthquake.energy released is an earthquake.
Granite rocks in Joshua Tree National Park showing
horizontal and vertical jointing. These joints formed when
the confining stress was removed from the granite.
Faults are easy to recognize as they cut across bedded rocks.

14. Slip
 is the distance rocks move along a fault. Slip can be up or down the
fault plane. Slip is relative, because there is usually no way to know
whether both sides moved or only one. Faults lie at an angle to the
horizontal surface of the Earth. That angle is called the fault’s
 dip. The dip
defines which of two basic types a fault is. If the fault’s dip is inclined
relative to the horizontal, the fault is a
 dip-slip fault . There are two types
of dip-slip faults. In
 normal faults, the hanging wall drops down relative
to the footwall. In
 reverse faults, the footwall drops down relative to the
hanging wall.

15. Types of Fault
A
 thrust fault is a type of reverse fault in which
the fault plane angle is nearly horizontal.
Rocks can slip many miles along thrust fault.
Normal faults can be huge. They are
responsible for uplifting mountain ranges in
regions experiencing tensional stress.
 The Teton Range in Wyoming rose up along a
normal fault.
A
 strike-slip fault is a dip-slip fault in which
the dip of the fault plane is vertical. Strike-slip
faults result from shear stresses.
At Chief Mountain in Montana, the upper rocks at the Lewis
Overthrust are more than 1 billion years older than the lower rocks
 

lacing Imagine pone foot on either side of a strike-slip fault. One block moves
toward you. If that block moves toward your right foot, the fault is a right-lateral
strike-slip fault; if that block moves toward your left foot, the fault is a left-lateral
strike-slip fault.

16. Stress and Mountain Building
Two converging continental plates
smash upwards to create mountain
ranges. Stresses from
this
 uplift cause folds, reverse
faults, and thrust faults, which
allow the crust to rise upwards.
Subduction of oceanic lithosphere
at convergent plate boundaries also
builds mountain ranges.
When tensional stresses pull crust
apart, it breaks into blocks that
slide up and drop down along
normal faults. The result is
alternating mountains and valleys,
known as a basin-and-range .
The Andes Mountains are a chain of continental arc volcanoes that build up as
the Nazca Plate subducts beneath the South American Plate.
(a) The world’s highest mountain range, the Himalayas, is growing from the collision
between the Indian and the Eurasian plates. (b) The crumpling of the Indian and Eurasian
plates of continental crust creates the Himalayas.

17. Summary
Stress is the force applied to a rock and may cause deformation. The three main types of stress
are typical of the three types of plate boundaries: compression at convergent boundaries,
tension at divergent boundaries, and shear at transform boundaries.
Where rocks deform plastically, they tend to fold. Brittle deformation brings about fractures
and faults.
The two main types of faults are dip-slip (the fault plane is inclined to the horizontal) and
strike-slip (the fault plane is perpendicular to the horizontal).
The world’s largest mountains grow at convergent plate boundaries, primarily by thrust
faulting and folding.

18. Strain
As we’ve just learned, the earth’s crust is constantly subjected to forces that push, pull, or twist it. These forces
are called stress. In response to stress, the rocks of the earth undergo
 strain, also known as deformation.
Strain is any change in volume or shape.There are four general types of stress. One type of stress is uniform,
which means the force applies equally on all sides of a body of rock.
 The other three types of stress, tension, compression and shear, are non-uniform, or directed, stresses.All
rocks in the earth experience a uniform stress at all times. This uniform stress is called lithostatic pressure and
it comes from the weight of rock above a given point in the earth.
 Lithostatic pressure is also called hydrostatic pressure. (Included in lithostatic pressure are the weight of the
atmosphere and, if beneath an ocean or lake, the weight of the column of water above that point in the earth.
However, compared to the pressure caused by the weight of rocks above, the amount of pressure due to the
weight of water and air above a rock is negligible, except at the earth’s surface.
The only way for lithostatic pressure on a rock to change is for the rock’s depth within the earth to
change.Because lithostatic pressure is a uniform stress, a change in lithostatic pressure does not cause
fracturing and slippage along faults. Nevertheless, it may be the cause of certain types of earthquakes.
 In subducting tectonic plates, increased pressure of greater depth within earth may cause the minerals in the
plate to metamorphose spontaneously into a new set of denser minerals that are stable at the higher
pressure.This is thought to be likely cause of certain types of deep earthquakes in subduction zones, including
deepest earthquakes ever recorded.

19. Types of Strain
Rocks are also subjected to the three
types of directed (non-uniform) stress –
tension, compression, and shear.
Tension
 is a directed (non-uniform)
stress that pulls rock apart in opposite
directions. The tensional (also called
extensional) forces pull away from
each other.
Compression
 is a directed (non-
uniform) stress that pushes rocks
together. The compressional forces
push towards each other.
Shear
 is a directed (non-uniform) stress
that pushes one side of a body of rock
in one direction, and the opposite side
of the body of rock in the opposite
direction. The shear forces are pushing
in opposite ways.

20. In response to stress, rock may undergo three different types of strain – elastic strain, ductile
strain, or fracture.
Elastic strain
 is reversible. Rock that has undergone only elastic strain will go back to its
original shape if the stress is released.
Ductile strain
 is irreversible. A rock that has undergone ductile strain will remain deformed
even if the stress stops. Another term for ductile strain is plastic deformation.

Fracture
 is also called rupture. A rock that has ruptured has abruptly broken into distinct
pieces. If the pieces are offset—shifted in opposite directions from each other—the fracture is
a fault.

21. Ductile and Brittle Strain
Earth’s rocks are composed of a variety of minerals and exist in a variety of conditions. In
different situations, rocks may act either as ductile materials that are able to undergo an
extensive amount of ductile strain in response to stress, or as brittle materials, which will only
undergo a little or no ductile strain before they fracture. The factors that determine whether a
rock is ductile or brittle include:
Composition—Some minerals, such as quartz, tend to be brittle and are thus more likely to
break under stress. Other minerals, such as calcite, clay, and mica, tend to be ductile and can
undergo much plastic deformation. In addition, the presence of water in rock tends to make it
more ductile and less brittle.
Temperature—Rocks become softer (more ductile) at higher temperature. Rocks at mantle and
core temperatures are ductile and will not fracture under the stresses that occur deep within the
earth. The crust, and to some extent the lithosphere, are cold enough to fracture if the stress is
high enough.

22. Lithostatic pressure - The deeper in the earth a rock is, the higher the lithostatic pressure it is
subjected to. High lithostatic pressure reduces the possibility of fracture because the high
pressure closes fractures before they can form or spread. The high lithostatic pressures of the
earth’s sub-lithospheric mantle and solid inner core, along with the high temperatures, are why
there are no earthquakes deep in the earth.
Strain rate - The faster a rock is being strained, the greater its chance of fracturing. Even brittle
rocks and minerals, such as quartz, or a layer of cold basalt at the earth’s surface, can undergo
ductile deformation if the strain rate is slow enough.
Most earthquakes occur in the earth’s crust. A smaller number of earthquakes occur in the
uppermost mantle (to about 700 km deep) where subduction is taking place. Rocks in the
deeper parts of the earth do not undergo fracturing and do not produce earthquakes because
the temperatures and pressures there are high enough to make all strain ductile. No
earthquakes originate from below the the earth’s upper mantle.

23. Stress and Fault Types
The following correlations can be made between types of stress in the earth,
and the type of fault that is likely to result:

Tension leads to normal faults.
Compression leads to reverse or thrust faults.
Horizontal shear leads to strike-slip faults.
Correlations between type of stress and type of fault can have exceptions. For
example, zones of horizontal stress will likely have strike-slip faults as the
predominant fault type. However there may be active normal and thrust faults
in such zones as well, particularly where there are bends or gaps in the major
strike-slip faults.
To give another example, in a region of compression stress in the crust, where
sheets of rock are stacked on active thrust faults, strike-slip faults commonly
connect some of the thrust faults together.

24. BIBLIOGRAPHY
The content written in this assignment has been completed by using
some websites and book.
 www.wikipedia.com
 www.coursehero.com
Book by R.G.PARK – FOUNDATION OF STRUCTURAL
GEOLOGY.

25. THANK YOU