Basics about Stress and its unit. Credits to Abhishek and Other fellow mates of Central University Tamilnadu

1.  PRESENTED BY
ANISH KUMAR S
Roll No : 22270060
M.Sc. Geology
Central University Tamilnadu

2. Contents
1. Stress : Introduction
2. Types of Stress : Tensile Stress
3. Types of Stress : Compressional Stress
4. Principle axes of Stress
5. Unit of Stress

3. ▶ what is Stress?
➡ Physically we can define Stress as Force,
applied per unit area.
Mathematically,
Stress = Force/ Area
So, Stress ∝mass
∝1/surface area
➡ As we are seeing the above equation , that
Pressure is also define by the same
parameters. i.e. , Force/Area
What is
difference
between
Stress &
Pressure ..???

4. Types of Stresses
- There are 2 types of Stress.
1. Normal Stress ( Axial stress)
▶ Tensile Stress
▶ Compressional Stress
2. Shear Stress ( Non- axial)

5. Tensile Stress
 When equal and opposite forces are applied on a body then the stress
due to this force is called tensile stress.
 It is a type of stress in which the applied stress or force tends to
stretch the material in the direction of the axis of the applied stress.
 In other words it is the stress caused by pulling the material.
 Stress is the force per unit area of a material, thus
 Tensile stress = force/ area.
 Tensile stress may also be known as normal stress or tension.
 Sign convention of the tensile stress is negative.

6. Compressional stress
 If two vectors are pointing towards each other or they are
coming to a common point its called as compressional
stress.
 The compressional stress is always positive(+ve).
 Rock become shorter and thicker.
 It is the stress component perpendicular to a given surface
i.e., such as fault plane.
 Compressive stresses can also result in folding of rocks.
Because of the large magnitude of lithostatic stress in tectonic
plates in tectonic setting deformation is always subjected to
net compressive stress.

7.  From the figure, stress experienced can be expressed in nine stress
vector and these vectors are expressed in matrix vector.
 If the cube is in equilibrium then 𝜎21= 𝜎12, 𝜎31=𝜎13, 𝜎32=𝜎23 thus there
are only six independent component to the stress tensor.
 If the stress on a plane becomes perpendicular
to the plane, the shear stress value will be zero. So in this
case non-zero components are the normal stress
vectors i.e. 𝜎11, 𝜎22, 𝜎33
 So from the above matrix 𝜎11, 𝜎22, 𝜎33 are known as principal stress
where 𝜎11 >𝜎22> 𝜎33. They are also known as maximum, intermediate,
minimum axes of an ellipsoid known as stress ellipsoid.

8.  Mean stress: The mean stress is simply the average of the three principal
stresses.
 Deviatoric Stress: All other stress components (with or without shear stress
components), except the mean normal stresses, are considered as DEVIATORIC
STRESS.
 Hydrostatic stress: If all three of the principal axes are of equal magnitude,
then the ellipsoid simplifies to a sphere, and each of the infinite number of
stress vectors are equal. This particular type of stress is termed Hydrostatic
stress.
 Uniaxial Stress is where only one of the principal axis is non-zero.
 Biaxial stress is where two of the principal axes are non-zero.
 Triaxial Stress is where all three of the principal axes are non-zero.

9. Stress is defined as “The force per unit area of the material”. It is a tensor quantity.
Denoted by Greek letter σ. Mathematically expressed as –
σ=F/A
Where,
•F is the restoring force measured in Newton or N.
•A is the area of cross-section measured in m2.
•σ is the stress measured using N/m2 or Pa.

11. Dimension of
stress:
➡ Dimension of Stress = [ Force]/ [ Area]
= [M L T^2] / [ L^2]
= [ M L^1 T^2 ]
➡ So, the dimension of Stress is [ M L^1 T^2 ]
.