Residual Stress

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What are residual stresses

residual stress

Residual stresses or locked-in stresses can be defined as those stresses existing within a body in the absence of external loading or thermal gradients. In other words residual stresses in a structural material or component are those stresses which exsist in the object without the application of any service or other external loads.

Factors that cause residual stresses

Residual stresses can be present in any mechanical structure because of many causes.
Residual stresses may be due to the technological process used to make the component. Manufacturing processes are the most common causes of residul stress. Virtually all manufacturing and fabricating processes such as casting, welding, machining, molding, heat treatment, plastic deformation during bending, rolling or forging introduce residual stresses into the manufactured object. Residual stress could be caused by localized yielding of the material, because of a sharp notch or from certain surface treatments like shot peening or surface hardening.
Among the factors that are known to cause residual stresses are the development of deformation gradients in various sections of the piece by the development of thermal gradients, volumetric changes arising during solidification or from solid state transformations, and from differences in the coefficient of thermal expansion in pieces made from different materials.

Thermal residual stresses are primarily due to differential expansion when a metal is heated or cooled.  The two factors that control this are thermal treatment (heating or cooling) and restraint.  Both the thermal treatment and restraint of the component must be present to generate residual stresses.

When any object is formed through cold working, there is the possibility for the development of residual stresses.

A good common example of mechanically applied residual stresses is a bicycle wheel.  A bicycle wheel is a very light and strong because of the way in which the components are stressed.  The wire spokes are radial aligned and tightening the spokes creates tensile radial stresses.  The spokes pull the rim inward, creating circumferential compression stresses in the rim.  Conversely, the spokes pull the tubular hub outward.  If the thin spokes were not under a proper tensile preload load the thin wire spokes could not adequately support the load of the rider.

What residual stresses can cause

Residual stresses can be sufficient to cause a metal part to suddenly split into two or more pieces after it has been resting on a table or floor without external load being applied.
Residual stresses can result in visible distortion of a component. 
Residual stresses relaxation can deform a peace when it is in machining.

Role of residual stresses

Residual stresses have the same role in a structure’s strength as common mechanical stresses. However, while stress due to external loads can be calculated with a degree of accuracy, residual stresses are difficult to foresee. It is, therefore, very important to have a reliable method able to measure them directly with minimum damage to the surface.
Residual stresses can play a significant role in explaining or preventing failure of a component at times.  One example of residual stresses preventing failure is the shot peening of component to induce surface compressive stresses that improve the fatigue life of the component.  Unfortunately, there are also processes or processing errors that can induce excessive tensile residual stresses in locations that might promote failure of a component.
It must be kept in mind that the internal stresses are balanced in a component.  Tensile residual stresses are counter balanced by compressive residual stresses. Residual stresses are three-dimensional. 

Residual stress measurement methods

x-ray diffraction
Ultrasonic methods
Magnetic methods
Electronic Speckle Pattern Interferometry
Hole drilling and strain gage technique
Core Hole drilling and strain gage technique
Photoelastic technology for residual stress measurement in glass
Barkhausen noise effect for ferromagnetic materials

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