# What is the actual difference between strain and stress ?

The actual difference between strain and stress lies in their definitions and how they are measured. Stress is the internal force per unit area within a material that arises due to externally applied forces. It is measured in units of pressure, such as pascals (Pa) or pounds per square inch (psi). Strain, on the other hand, is a measure of the deformation of the material. It is the relative change in shape or size due to applied stress and is a dimensionless quantity, often expressed as a percentage or a ratio. While stress quantifies the internal forces, strain quantifies the resulting deformation.

True stress and true strain differ from engineering stress and strain in how they are calculated, especially at large deformations. True stress is the actual stress on a material considering the instantaneous cross-sectional area, while true strain is the natural logarithm of the ratio of the current length to the original length. True stress accounts for the changing area as a material deforms, making it more accurate at higher strains. True strain provides a continuous measure of deformation, which is more realistic for large deformations, compared to engineering strain which uses initial dimensions throughout the calculation.

In the context of material deformation, stress comes first. When an external force is applied to a material, it induces stress within the material. This stress, if it exceeds the material’s elastic limit, results in strain, or deformation. Thus, stress is the cause, and strain is the effect. Stress leads to the internal forces that produce the deformation observed as strain.

Stress is not greater than strain because they are different physical quantities with different units. Stress is a measure of force per unit area, while strain is a measure of deformation and is dimensionless. Comparing them directly as greater or lesser is not meaningful. Instead, stress and strain are related through material properties such as Young’s modulus, where stress is proportional to strain within the elastic limit of the material.

Strain cannot occur without stress. For a material to deform (strain), there must be an applied force causing internal stress. However, certain scenarios such as thermal expansion can cause strain without conventional mechanical stress. In such cases, stress arises from temperature changes causing material expansion or contraction. Even in these scenarios, the internal forces due to thermal effects can be considered a form of stress leading to strain.