Modulus of Rigidity

Ratio of shear stress to shear strain is called as 'Modulus of Rigidity'.

Volumetric Strain

Change in volume with respect to original volume when material is subjected to forces in all three directions is called ‘Volumetric Strain’.  

Poisson's Ratio

When a material is subjected to some force, its dimension changes in the direction of force applied. Due to the resulting effect of the longitudinal strain, a lateral strain (strain in the perpendicular direction of force applied) is also produced in the material.
This happens because of the constant mass of material i.e material quantity. Lateral dimension changes to sustain the constant amount of material.
The ratio of Lateral strain to Longitudinal strain is always constant for a particular material which is called 'Poisson's Ratio.'  It is denoted by '𝒱'.

Shear Strain

The amount of deformation of material in the direction of shear force with respect to the dimension perpendicular to the shear force is called 'Shear Stress.'

Shear Stress

When the tangential forces are applied on the body, the stress produced in the body is called 'Shear Stress'. 
And the forces that produce shear stress is called 'Shear Forces'.
       

Shear Stress tends to shear the material.

Normal Stress

Component of forces acting normally on per unit area of the material is called 'Normal Stress'.

Normal Stress (σn)  =   Normal component of force/Area
                                  =   Fn/A

Stress in a Material

The internal; resistance offered per unit area by a material towards the external forces applied is called ‘Stress’. Internal resistance offered is equal to external forces applied on body.

Stress is denoted by 'σ '.

Stress ( σ )   =       P/A

                                                                 

Where ‘P’ denotes the force applied & ‘A’ denotes the cross section area subjected the force P.

Factors Affecting Strength of Materials

a) Type of loading:-
Type of loading has very significant effect on strength of materials.
  e.g

for a striaght bar of same material and same size (i.e same length, width and thickness), the load (W) bearing capacity will be higher in case (i) then that in case (ii)

b) Temperature:-
Temperature of material inversely affects its strength. Increase in temperature reduces the inter-molecular forces. Therefore the load bearing capacity of material reduces as the temperature is increased.

Strength of Material

When some external forces are applied to a material, an internal resistance is shown by that material towards the external forces applied in order to prevent itself from cracks, breakages or permanent deformations.
In short it can be stated as 'The maximum resistance that is shown by a material towards the external forces applied is called Strength of Material.'