Flow stress corresponds to the plastic deformation of solids. When a solid material is subjected to external forces, it undergoes a change in shape without a significant change in volume. This is known as plastic deformation. Flow stress is the stress required to initiate and sustain this plastic deformation in a material.

Explanation:
1. What is flow stress?
Flow stress, also known as yield stress, is the stress at which a material begins to deform plastically. It is the point at which the material transitions from elastic deformation to plastic deformation. In other words, it is the stress required to initiate the permanent change in shape of a material.

2. Plastic deformation of solids
When a solid material is subjected to external forces, the atoms or molecules within the material start to move and rearrange themselves. This rearrangement of atomic or molecular structure leads to the permanent deformation of the material. Plastic deformation can occur in various forms such as bending, stretching, or compression.

3. Relationship between flow stress and plastic deformation
Flow stress is directly related to the plastic deformation of solids. It represents the resistance that a material offers to the external forces applied to it. The higher the flow stress, the more force is required to cause plastic deformation in the material. Conversely, a lower flow stress indicates that the material is more easily deformed.

4. Importance of flow stress in engineering
Understanding the flow stress of a material is crucial in engineering applications. It helps in determining the maximum load a material can withstand before it starts to deform plastically. This information is essential for designing structures and components that can withstand the expected loads and stresses during their service life.

5. Determining flow stress
Flow stress can be determined through experimental tests such as tensile testing, compression testing, or torsion testing. These tests involve applying controlled forces to a test specimen and measuring the resulting deformation. The stress at which a significant plastic deformation occurs is considered the flow stress of the material.

In conclusion, flow stress corresponds to the plastic deformation of solids. It represents the stress required to initiate and sustain the permanent change in shape of a material. Understanding flow stress is essential in engineering to design structures and components that can withstand the expected loads and stresses.