The yield strength is defined as the stress at which a predetermined amount of permanent deformation occurs. When subjected to stress, a material undergoes deformation, yield strength describes the maximum amount of stress required to elastically deform the material in a given loading arrangement like tension and compression. This means the stress beyond yield strength, the deformation is recoverable and any deformation that occurs due to the stress higher than the yield strength is permanent. Yield strength is measured in N/m²or pascals. Strain hardening, strain rate, the temperature of the metal and hydrostatic pressure will affect the value of yield strength.
In other words, for example, with little force, the aluminum deforms elastically firstly, when you release the stress, it can return to the original shape; then with more force, it deforms plastically, this is yield strength, when you release the stress, its shape is permanently changed; last, with a much higher force, it breaks, this is ultimate tensile strength. In CNC aluminum machining, we should have a basic understanding which condition the aluminum material can withstand to ensure the process go smoothly.