Whenever an external load is applied to a solid body, it deforms if the body is restrained from motion either fully or partially.
If the body is not restrained against the motion it may undergo displacements without a change in shape or size and these displacements are called rigid body displacements.

Stress is a resistance generated by the body to counteract the external forces applied to it. It has the same unit as pressure i.e., force/area.

Types of Stresses

Normal Stress
Shear Stress

Normal Stress can be further classified as

Axial Stress
Bearing Stress
Bending Stress

Axial Stress - Load is applied along the axis of a body i.e., normal to the section.
It can be either tensile or compressive in nature.

Bearing Stress - It is a compressive stress generated when one body is supported on another.

Bending Stress - Bending tension and compression produces normal stress.

Shear Stress

It is the stress that acts parallel to the applied force. Normal stress acts perpendicular to the plane causing either tension or compression while shear stress acts tangentially, that's why it is also known as tangential stress.

Source: Quora

It is of 2 types:

Direct Shear Stress
Indirect Shear Stress

Direct Shear Stress - It is that shear stress which tries to cut the section.
Indirect Shear Stress - It arises due to either tension/ compression or torsion.

Shear Stress on opposite faces of a stressed element is equal in magnitude and opposite in nature.

Source: Javelin-Tech

Stress tensor - Stress is not a vector quantity because its results cannot be obtained by parallelogram law of vector addition. It is a mathematical quantity called tensor.

It is represented by,
$\sigma =\left( \begin{matrix} {{\sigma }_{xx}} & {{\tau }_{xy}} & {{\tau }_{xz}} \\ {{\tau }_{yx}} & {{\sigma }_{yy}} & {{\tau }_{yz}} \\ {{\tau }_{zx}} & {{\tau }_{zy}} & {{\sigma }_{zz}} \\ \end{matrix} \right)$

Only 6 components are required to define the condition of stress at a given point in a 3D system because of the complimentary shear stress concept.
And in a 2D system, only 3 stress components are required to define the condition of stress at a given point.

Normal Strain

It is the deformation per unit length.

$\varepsilon =\frac{\delta }{L}$

$\delta=$ Change in length
$L=$ Original Length

Stress-Strain Curve for Mild Steel

Source: Quora

Failure Plane

Material	Tension Test	Torsion Test
Ductile	${{45}^{{}^\circ }}$	${{90}^{{}^\circ }}$
Brittle	${{90}^{{}^\circ }}$	${{45}^{{}^\circ }}$

Properties of Material

Elasticity: It is the property of a material by virtue of which, it returns to its original position after unloading.
If the material is unloaded before the elastic limit has reached, it will follow the original curve otherwise it will have a residual strain and the unloading curve will be different from original curve.
Plasticity: It is a property of a material which describes the deformation of a solid material undergoing inelastic strain beyond the strain at the elastic limit.
Creep: It is a property by virtue of which a material undergoes additional deformation with the passage of time under sustained loading within elastic limit.
Relaxation: It is the decrease in stress under constant strain.
Fatigue: It is a deterioration of a material caused by repeatedly applied loads results in progressive and localised structural damage.

Endurance limit is the stress level below which even large number of stress cycle cannot produce fatigue failure.
For structural stress, it is half of the .ultimate strength and due to corrosion, endurance limit further reduced by 50%
Resilience: It is a property by which a material absorbs energy when it is deformed elastically and release that energy upon unloading.

Source: NPTEL
The area under the load-deformation curve within elastic limit is called resilience.
Toughness: It is the ability of a material to absorb mechanical energy upto failure.

Source: NPTEL