The main difference between stress and strength is that stress is the resistance offered by the object to the deformation and strength is the maximum value of stress that the material can withstand.
Stress:
Stress is the resistance per unit area developed by an object against external forces to avoid deformation.
Mathematically the stress is given by,
Stress = `\frac{\text{Force}}{\text{Resisting Area}}`
The stress is denoted by the symbol ‘`\sigma`’ and measured in terms of N/m² or pascal.
Example:
As shown in the below figure, the child standing on the wooden block applies force equal to his weight on the block. To avoid deformation, the block also tries to develop an equal amount of resistance against the load. This resistance per unit crosssectional area of the block is known as stress.
Thus the stress developed in the block can be given by,
σ = `\frac{F}{\text{Cross sectional area}}`
Strength:
Strength is the maximum value of stress that material can bear before undergoing deformation or fracture.
Based on the type of loading, the strength is also classified as, ultimate tensile strength, compressive strength, shear strength, flexural strength, etc.
The value of strength is constant for the particular material and does not depends on the amount of load applied.
It is the intensive property of a material and measured in terms of N/m² or Pascal.
Difference between stress and strength:
Sr.No.  Stress  Strength 

1  Stress is the internal resistance per unit area developed by the object to resist the deformation.  Strength is the maximum ability of the material to withstand the load without causing failure. 
2  The value of stress does not remain constant for the material.  The value of strength is constant for the material. 
3  The different types of stresses are longitudinal stress, shear stress, volumetric stress.  The different types of strengths are tensile strength, yield strength, flexural strength, etc. 
4  Stress is not a property of a material.  Strength is one of the properties of a material. 
5  The stress changes with the crosssection area or with an applied force.  The magnitude of strength never changes by changing the applied load or crosssectional area. 
6  The stress on the material can be calculated by the use of mathematical relations.  The strength of the material is found experimentally. 
7  The life of the machine component can be increased by reducing the stress on it.  The life of the machine component can be increased by increasing its strength (By changing the material). 
8  The stress depends on the load applied to the object and the resisting crosssectional area.  The strength depends on the material structure, working temperature, material composition, etc. 
FAQs:

What is the ratio between stress and strength?
The ratio of strength to stress is the factor of safety for the loading condition.
Factor of safety = Strength / Stress 
What is the importance of stress and strength?
The stress shows the resistance developed by the material to resist the failure and strength shows the ability of a material to withstand load without failure.
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