Residual stresses are elastic deformation and the corresponding stress in a solid in the absence of mechanical action from outside.
Residual stress arises in a material during its heat treatment, transition from a liquid state to a solid state, during mechanical processing, welding, etc. Residual stresses are always present in plastic, metal, and glass. The cause of the residual stresses is the heterogeneity of the deformed state of the solid due to various changes in different places of its length or volume.
The occurrence of residual stresses during the transition of a material from a liquid to a solid state is explained by the fact that solidification begins in the surface layers and is accompanied by shrinkage. Advance hardening of the outer layer leads to the appearance of internal tensile stresses in it.
Residual stresses can be made for structural purposes (self-expanding space antennas [1] ) or be harmful. Harmful residual stresses are a latent defect. A set of measures is being taken to reduce them.
Content
Classification
Residual stresses, depending on their size, are divided into:
- Residual stresses of the first kind in sizes comparable to the dimensions of the whole body;
- Residual stress of the 2nd kind or microscopic, in sizes comparable to the grain size of the metal. Microscopic stresses are studied by x-ray methods;
- Residual voltage of the 3rd kind or submicroscopic distortions, in sizes comparable to the sizes of the atomic crystal lattice.
Harmful residual stresses (often tensile) lead to the destruction of the product, the appearance of cracks in it, accelerate the formation of corrosion. Useful stresses, most often compressive, increase the elasticity of the product, endurance, and increase corrosion resistance.
Residual stresses during welding
In the process of welding a structure when the metal is cooling, stresses arise in it caused by unequal heating of the base and deposited metals, shrinkage of the metal after welding, structural changes in the metal due to heating and rapid cooling, changes in the solubility of gases in the weld when it is cooled. Internal stresses contribute to both the deformation or destruction of the welded product. To eliminate residual stresses, structural and technological measures are carried out.
Constructive measures:
- As the base metal, a metal is selected that does not form quenching structures when cooling in air. The metal of the electrodes must have plastic properties which are not lower than the plastic properties of the base metal.
- During welding, the concentration of welds and their intersection must not be allowed.
- When welding, seams in the form of closed contours should be avoided. This increases planar tension.
- Avoid welding scarves, linings leading to an increase in planar stresses.
- When welding, it is necessary to give preference to butt welds, which are less rigid. In them, the concentration of power stresses is much less than in fillet welds.
- In the design process of welded structures, it is necessary to provide for the possibility of manufacturing individual welded units, which could then be combined into a common structure. This generally reduces planar tension.
Technological events:
- Preliminary and concurrent heating of products during cooking;
- After welding, hot metal is forged;
- Tempering after welding reduces residual stresses by 85-90%;
- Rolling rolling welds.
Usage
Residual voltage is used to make springs, self-expanding antennas, metal roulettes and other products. At the same time spend metal vacation . Residual stresses usually occur during hardening as a result of heat treatment.
Literature
- Bashkatov A.V. Stresses and deformations during welding. Voronezh. Ed. VSTU. 1999.
- Glikman L. A., Methods for the determination of residual stresses, “Tr. Leningra. engineer-econ. in-that ”, 1960, no. thirty;
- Birger I. A. Residual stresses, M., 1963;
- Ugolev B. N. Internal stresses in wood during its drying, M. — L., 1959;
- Vasiliev D. M., Dobrodeeva N. M., “Solid State Physics”, 1962, v. 4, No. 1, p. 140–47.
- P.Ya. Bokin. The mechanical properties of silicate glasses, 29. Nauka, L., 1970.
- Cheredov V.N. Defects in synthetic fluorite crystals. SPb: Science. - 1993. - 112 c.
- Hosford, William F. 2005. "Residual Stresses." In Mechanical Behavior of Materials, 308–321. Cambridge University Press. ISBN 978-0-521-84670-7
- Cary, Howard B. and Scott C. Helzer (2005). Modern Welding Technology. Upper Saddle River, New Jersey: Pearson Education. ISBN 0-13-113029-3 .
- Shajer, Gary S. 2013. Practical Residual Stress Measurement Methods. Wiley. ISBN 978-1-118-34237-4
Links
Notes
- ↑ Construction in Space Archived June 15, 2009.