Stress and strain are core parameters in material mechanics that characterize the relationship between force and deformation of materials. The stress-strain curve can visually reflect the mechanical properties of materials such as strength and ductility.

1. Stress: Stress refers to the internal resistance force generated within a material when it is subjected to external forces. It can be imagined as the "little guard" inside the material, always ready to resist the invasion of external forces.

The formula for calculating stress is: σ = F/A (F is the external force, A is the area of action), and the unit is Pascal.

2. Strain: Strain refers to the change in shape or size of a material when it is subjected to stress. Strain can be regarded as the material's "response" to stress. As a dimensionless quantity, strain eliminates the influence of material size differences through the degree of relative deformation.

In the axial tensile test, the formula for calculating strain is: ε = (L' - L) / L, where L' is the length after deformation.

3. Stress is defined as the resistance force generated per unit area within the material, while strain quantifies the degree of shape change of the material through ΔL/L (ΔL is the deformation amount, L is the original length). In the elastic deformation stage, the two follow the linear relationship of Hooke's Law. Once the elastic limit is exceeded, the material will enter the plastic deformation stage until it breaks.

4. Hooke's Law: Within the elastic range, stress and strain are directly proportional, which is the famous Hooke's Law. However, once the material exceeds the elastic limit and enters the plastic deformation stage, this linear relationship no longer holds.

In summary, stress and strain are important parameters for describing the relationship between force and deformation of materials and are widely applied in fields such as the communication industry, the automotive industry, industrial control and automation, the medical industry, and the home appliance industry.