What Does Residual Stress Mean?
Residual stress is a process-induced stress, frozen in a molded part, that exists in a body in the absence of external loading or thermal gradients. In a structural material or component, residual stresses exist in the object without the application of services or other external loads. They affect a part similarly to externally applied stresses.
A residual stress is the key culprit of part shrinkage and warpage. When an external service load is applied, the part warps upon ejection, or later crack, if strong enough to overcome the part's structural integrity.
Corrosionpedia Explains Residual Stress
Residual stresses remain in a solid material after removal of the original cause of stress. Residual stress may be desirable or undesirable. For example, laser peening imparts deep beneficial compressive residual stresses into metal components, but in a designed structure, unintended residual stress may cause premature failure.
Residual stresses can result in visible component distortion. Manufacturing processes are the most common causes of residual stress, which influences materials, as follows:
- Fatigue behavior
- Breaking strength
- Corrosion resistance
There are few metalworking methods that do not produce new stresses. Thus, when designing mechanical parts, the role of residual stress is very important.
Residual stresses occur through a number of mechanisms, such as:
- Inelastic (or plastic) deformations
- Temperature gradients during the thermal cycle
- Structural changes (or phase transformation)
When undesired residual stress from prior metalworking operations is present, the amount of stress may be reduced through the use of several methods classified as thermal or mechanical (nonthermal) methods. All methods involve processing the part so the whole stress is relieved.
The thermal method involves uniformly changing the temperature of the entire part through heating or cooling. Heated parts is known as stress relief bake, and cooled parts is known as cryogenic stress relief.
Mechanical methods include shot peening and laser peening. Shot peening typically uses a metal or glass material, while laser peening uses high-intensity beams of light to induce a shock wave that propagates deep into the material.
Residual stresses are indirectly calculated by measuring the existing material strains, which are generally measured by mechanical or x-ray methods.