Residual stress

Residual stresses are mechanical tensions that prevail in a body on which no external forces attack and is in thermal equilibrium. They may be caused by plastic deformation inside an object. The residual stresses are also closely related deformations ( eg distortion during welding).

Classification

Classification by expansion:

• Residual stresses first type (macroscopic, averaged over several grains)
• Residual stresses 2nd type ( averaged over a crystallite or grain, as the deviation of the residual stress values ​​of the first kind )
• Stresses third type (within a grain as a deviation of the residual stress values ​​of first and second type )

Classification according to the course of time:

• Temporary stresses occur intermittently on (for example, in the rapid inhomogeneous cooling or drying) and then disappear completely
• Latent tensions caused by temporary stress when doing the yield strength of the material is exceeded; typical examples: residual stress in glass articles and toughened safety glass. Latent tensions can be eliminated by annealing or stress relief annealing.
• Permanent tensions arise in workpieces with inhomogeneous thermal expansion coefficients during cooling; an example is the residual stress of a glaze layer on a ceramic. Permanent voltages can not be eliminated by annealing.

Thermal (internal ) stresses caused by temperature influences.

Residual stresses can also be caused by diffusion processes when inhomogeneous incorporation or expulsion of dissolved impurities in the solid state leads to volume changes.

Causes

The causes of residual stresses can be thermally, physically or chemically induced be (examples):

• Thermally induced residual stresses can arise that the edge and the core of a workpiece after appropriate heating cool at different rates (eg cast workpieces ). Due to the rapid cooling and contraction of the near-edge portions it can get there to tensile stresses and to a local excess of the yield strength and thus to plastic deformation. After the temperature equilibrium between the surface and core it, then to the formation of residual compressive stresses in the edge region (residual stress first kind ).
• By phase transformation or formation of precipitates can lead to local structural stresses occur (residual stress type 2 ).
• Dislocations are surrounded by a field of tension (residual stress type 3 ).

By diffusing impurities into solid surfaces can occur there compressive stresses. The same can be observed in the ion implantation.

Strong residual stresses are also observed in thin layers.

Measurement

The measurement of residual stresses can be carried out non-destructive or destructive. When destructive measurement ( Sägeschnittverfahren, hole-drilling method, ring core method) the elastic recovery is detected and evaluated due to the disturbed equilibrium. Non-destructive method (eg Radiographic residual stress measurement, EBSD ) is determined, the stress of the metal grid as a result of the ruling power. This process is initially limited only to very shallow areas, for steel the information depth is in the range of several micrometers. By deformation- free removal of thin layers and suitable back-calculation of induced voltages but it also residual stress depth profiles can be determined.

Energy Richer method (residual stress measurement using neutron ) allow greater penetration depths.

Effects

Tensile residual stresses at the surface have a negative effect on the fatigue strength of a component. In contrast, near-surface compressive stresses cause especially in glass, a strong increase in strength since this purchase and micro-cracks can not propagate to the surface. In order to bring surfaces under compressive stress, metal surfaces are often shot peened. Weld transitions are treated more and more with the high frequency peening methods HiFIT (High Frequency Impact Treatment ). This has a favorable effect on the fatigue strength ( fatigue ). Glass surfaces can be treated chemically or this, they are ( as in the case of toughened safety glass ) blown during solidification in the still soft from air nozzles to produce latent thermal residual stresses.