RESIDUAL STRESS

Residual stresses can arise in materials in almost every step of processing. The origins of residual stresses in a component may be classified as:

ü  Mechanical

ü  Thermal

ü  Chemical

Mechanically Generated

These are the residual stresses which occur due to plastic deformation as a result of manufacturing process. They may develop during processing or treatment and the operation that may develop theses stresses, which are ofcource undesirable, are rod or wire drawing, welding and machining processes as turning, milling and grinding. Whereas shot peening, autofrettage of pressure vessels, toughening of glass or cold expansion of holes may introduce compressive residual stresses in the components.

The stresses which are undesirable for the component are due to the reason that they contribute towards fatigue failure, quench cracking and stress-corrosion cracking. Such stresses are tensile stresses developed in the surface of the component. On the other hand compressive residual stresses contribute towards fatigue strength and resistance to stress-corrosion cracking, and increase the bending strength of brittle ceramics and glass.

During grinding process the undesirable tensile residual stresses are generated as a result of a conventional or highly abrasive grinding wheel whereas if a soft gentle grinding wheel is used beneficial compressive residual stresses will be generated as shown in the figure below.

Therefore it can be concluded that if a body is in equilibrium its tensile residual stresses are balanced by its compressive residual stresses. Also if the residual stresses are being operated in the plane of applied load and are on opposite in sense, then these residual stresses are termed to be as beneficial.

Figure 1.1. Characteristic residual stress distributions in hardened steel for three different grinding operations [1]

THERMALLY GENERATED

The residual stresses may be developed in a material during manufacturing and processing which results in differences on coefficient of thermal expansion between different constituents resulting in a thermal mismatch. These are he micro-stresses as they changes occurs at atomic scale.   Macro stresses results during the forming processes or non uniform heating or cooling as in casting, welding, heat treatment etc. [2]. If the stresses generated as a result of thermal imbalance exceed the yield strength, plastic deformation will occur.

CHEMICALLY GENERATED

The chemically generated stresses can develop due to volume changes associated with chemical reactions, precipitation, or phase transformation. Chemical surface treatments and coatings can lead to the generation of substantial residual stress gradients in the surface layers of the component. Nitriding produces compressive stress in the diffusion region because of expansion of the lattice and precipitation of nitrides, and carburizing causes a similar effect. The magnitude of residual stresses generated in coatings can be very high –compressive stresses of the order of 6-8GPa or higher have been measured at the interface of some thermal barrier coatings (TBCs).

[1]E.B. Evans, “Residual Stress in Processing” Encyclopedia of Materials Science and Engineering Vol. 6, 4183-88 (1986) 

[2] G. Totten, M. Howes & T. Inoue, “Handbook of Residual Stress and Deformation of Steel” 2002