Residual stresses are mechanical stresses that prevail in a body on which no external forces act . They can be caused by plastic deformation , inhomogeneous structure or thermal influences. Deformations (e.g. distortion during welding ) are closely related to the residual stresses .
Classification according to expansion:
- Residual stresses of the 1st type ( macroscopic , averaged over several grains)
- Residual stresses of the 2nd type ( averaged over a crystallite or a grain, as a deviation of the residual stress values of the first type)
- Residual stresses of the 3rd kind (within a grain, as a deviation of the residual stress values of the first and second kind)
Classification according to the passage of time:
- Temporary tensions occur temporarily (for example during rapid, inhomogeneous cooling or drying) and then disappear completely
- latent stresses arise from temporary stresses when the yield point of the material is exceeded; typical examples: residual stress in glass objects and in toughened safety glass . Latent stresses can be eliminated by annealing or stress relieving.
- permanent stresses arise in workpieces with an inhomogeneous coefficient of thermal expansion during cooling; one example is the internal stress of a glaze layer on ceramic. Permanent stresses cannot be removed by annealing.
Thermal (intrinsic) stresses arise from temperature influences.
Residual stresses can also be caused by diffusion processes if inhomogeneous storage or expulsion of foreign matter dissolved in the solid leads to changes in volume.
The causes of residual stresses can be thermally, physically or chemically induced (examples):
- Thermally induced residual stresses can arise when the edge and core of a workpiece cool down at different rates after being heated accordingly (e.g. in the case of cast workpieces). The faster cooling and shrinking of the areas close to the edge can lead to tensile stresses and a local exceedance of the yield point and thus to plastic deformation. Once the temperature has equalized between the edge and the core, internal compressive stresses develop in the edge area (internal stress of the first type).
- by phase transitions or formation of precipitates can cause local fabric tension come (residual stress 2. Art).
- Dislocations are surrounded by a stress field (internal stress 3rd type).
The diffusion of foreign matter into solid surfaces can lead to internal compressive stresses. The same can be observed with ion implantation .
Strong internal stresses can also be observed in thin layers .
Since residual stresses represent an intrinsic quantity, measurement in the classic sense is not possible. Rather, accompanying phenomena are measured, which can be converted into the underlying internal stress. A distinction is made between non-destructive and destructive processes. With the destructive methods (saw-cut method, borehole method, toroidal core method), material subject to internal stress is removed mechanically (usually with high-speed milling ) or by means of electrical discharge machining . The internal stress released in the process leads to a deformation of the surrounding material, which i. d. Usually measured with strain gauges . Using suitable correlations, these deformations can be converted into the underlying residual stress. Current research focuses on methods in which the strain is determined optically (e.g. digital image corrosion or holography ) and the material removal is substituted by a laser ablation system.
With the non-destructive method (e.g. X-ray systems, electron backscattering ), the distortion of the metal grid due to the prevailing voltage is determined. Here, high - energy X-rays are introduced into the workpiece to be examined. The reflection of the radiation is then expressed as a specific diffraction pattern , which enables direct conclusions to be drawn about the level of the underlying internal stresses. This method is initially limited to areas very close to the surface; for steel, the information depth is in the range of a few micrometers . However, residual stress depth profiles can also be determined by electrochemical removal of thin layers and suitable back-calculation of the stresses released. Higher energy processes ( neutron sources ) allow greater penetration depths.
Examples, effects, applications
Residual welding stresses cause the components to warp. Although attempts are made to counteract this, for example by making symmetrically positioned blind seams, the disadvantages that reduce strength are retained. Especially safety-relevant welded joints such as those in nuclear power plants or large gas pipes are tempered with low stress after welding.
During the manufacturing process, thin layers sometimes come under very high internal stresses.
Residual tensile stresses on the surface have a negative effect on the fatigue strength of a component. On the other hand, compressive stresses close to the surface cause an increase in the fatigue strength, since the surface cracks and microcracks are overpressed and cannot spread.
Metal surfaces are often shot-peened in order to increase the strength of surfaces under compressive stress . Welding seam transitions are treated more and more with the high-frequency hammering process HiFIT ( High Frequency Impact Treatment ). This has a beneficial effect on fatigue strength ( material fatigue ). For this purpose, glass surfaces can be chemically treated (chemically hardened or toughened glass) or (as in the case of toughened safety glass) they are blown from air nozzles when they are still soft in order to generate latent internal thermal stresses. In prestressed concrete , the steel component absorbs the tensile stresses in order to protect the concrete from it.
- http://www.uni-stuttgart.de/imtk/lehrstuhl1/Scripte/Pr-eds.pdf Residual stresses in thin layers
- https://www.stresstech.de/de/produkte/Bohrlochgerät/Bohrlochverfahren/ techn. Application of the borehole method with Electronic Speckle Pattern Interferometry (ESPI)