What is residual stress?
Stresses can the broadly classified into two parts.
- Load stresses
- Residual stresses
Load stresses are those stresses which remain within elastic limits and are induced due to external forces. When these forces are removed these stresses also become zero.
Load stresses can be easily calculated from the equations of strength of materials.
Residual stresses are those stresses which are independent of external forces. These stresses are generally a result of manufacturing process or assembly operations.
When the components with residual stresses are used in service then the external forces get superimposed on residual stresses.
Image source: Industrial metallurgist
Effect of Residual stress: Harmful or Beneficial
There are two conditions on whose basis the effect of residual stresses is determined to be either beneficial or harmful.
When residual stresses are added to the external forces than they are harmful.
When residual stresses are subtracted to external forces than they are beneficial.
Compressive Residual stresses are purposely induced on the surface of components which are subjected to fatigue loading. Compressive residual stresses increase endurance limit and retard fatigue crack growth.
Shot peening is used to induce compressive residual stresses in such parts.
Causes of residual stress
- Manufacturing process like forging, casting etc.
- Machining methods like milling, turning etc.
- Extrusion, rolling and cold working processes
- Chemical processes like corrosion, electroplating and oxidation
- Heat treatment process like quenching
- Assembly operations including some misalignment
Relief of residual stress
Residual stresses can be reduced by thermal and non-thermal methods. Each is discussed below.
It includes either heating or cooling of the stressed components uniformly and bringing them down slowly to the room temperatures. Stress relief bake and cryogenic stress relief are the examples of thermal methods.
Mechanical methods are also used to relief residual stresses. These methods generally replace residual stresses with beneficial compressive stresses. Non-thermal methods include shot peening and laser peening.