The bending of spring wire - material-technical structural changes and stress states during and after bending
The bending of spring wire is a complex process that causes profound structural changes in the spring wire and includes material-mechanical processes at the atomic, microscopic and macroscopic level.
In order to understand the effects of this process on the spring material, it is necessary to know the changes in the crystal lattice, the stress states in the wire and the structural changes through plastic deformation.
Bending spring wire significantly influences not only the mechanical properties such as strength and ductility, but also the spring behavior and long-term durability.
Ductility is the ability of the wire spring material to deform plastically under load without breaking. To be more precise, ductility is the ability of the material to stretch plastically before breaking without being damaged or the toughness against breaking. The deformation takes place after the elastic deformation and ensures that the spring material can absorb or absorb energy.
1. Material mechanical processes when bending spring wire
When bending spring wire, a combination of elastic and plastic deformations as well as microstructural changes at the atomic level occur.
These changes particularly affect the movement of
dislocations, the deformation of grains and the creation of
residual stresses in the spring material.
1.1 Elastic and plastic deformation The deformation of the wire begins in the elastic range and moves into the plastic range as the load increases.
Elastic deformation: In this stage, the crystal lattice deforms reversibly.
The atoms are shifted in their lattice positions, but return to their original position after the load is removed. This corresponds to a linear stretch, which is described by Hooke's law. The elastic deformation remains within the yield point of the material.
Plastic deformation: When the yield point Rm is reached, plastic deformation occurs, in which the material undergoes a permanent change in shape.
This results from the movement of dislocations such as atomic lattice defects in the crystal lattice. The dislocations slide through the material and cause a non-reversible shift in the atomic layers. At this stage, the material loses its elastic properties and permanent deformation and deformation remain.
1.2 Stress states during bending During bending, both tensile stresses and compressive stresses arise in the wire cross-section.
These stresses are not evenly distributed, but are highly position-dependent.
Tensile stresses: The outside of the bent wire experiences tensile stresses. Here the material is stretched and the atoms are pulled apart. This zone is prone to cracking because the atomic bonds are under strong stretching and can break more easily.
Compressive stresses: The inside of the wire is under compressive stresses, which compress the material. The atoms are pressed closer together, which is less likely to cause cracking, but can locally lead to plastic compression or wrinkling if the deformation becomes too strong.
Shear stresses: Inside the wire, especially near the neutral fiber, shear stresses occur. These are stresses that arise from the dislocation of atomic layers along the shear planes. Shear stresses play a crucial role in plastic deformation and the movement of dislocations.
1.3 Stress distribution across the cross-section The stress distribution in the cross-section of the wire is not linear and varies depending on the position within the wire:
Outside: Here the tensile stresses are highest, which leads to the greatest elongation. The material is stretched to the limit of its ductility and the risk of plastic failure or cracking is particularly high.
Inside: The compressive stresses on the inside compress the material, which causes less damage but in extreme cases can cause plastic instabilities such as buckling.
Neutral fiber: There are neither tensile nor compressive stresses in the neutral fiber. It represents the area in which the length of the material remains constant. Shear stresses are often concentrated here, which are responsible for the dislocation movements and the onset of plastic deformation.
2. Material structure changes during bending
Bending spring wire leads to significant changes in the material structure, which affect the strength, ductility and durability of the material. These changes primarily affect the dislocation density, the grain structure and the residual stresses that remain in the material.
2.1 Dislocation movements and work hardening One of the central material mechanical processes during bending is the movement of dislocations. Dislocations are linear lattice defects in the crystal that slide through the material during plastic deformation.
Dislocation formation: During bending, new dislocations are formed that move through the crystal lattice.
This movement is responsible for the plastic deformation of the material. The more dislocations move through the material, the more the material is strain hardened.
Strain hardening: As plastic deformation increases, the dislocation density and the number of obstacles to dislocation movement increase. This leads to an increase in the strength and hardness of the material, but at the expense of ductility. The material becomes less deformable and more prone to brittle failure.
Dislocation congestion: Dislocation congestion can occur at grain boundaries or other defects, leading to the formation of local stress fields.
These increased local stresses can increase the risk of microcracks or other material defects that can lead to failure under cyclic loading (e.g. in springs).
2.2 Grain structure changes and texture development During plastic deformation, the grain structure in the material changes, which is crucial for the subsequent mechanical behavior.
Grain elongation: In the tensile zone, the grains are stretched by the plastic deformation. This grain elongation leads to an anisotropic grain structure that influences the mechanical properties in different directions. Strength increases along the stretch direction, while weakening may occur across the stretch direction.
Distorted grain boundaries: In areas of high plastic deformation, distorted grain boundaries can occur. This leads to an increase in the number of obstacles to dislocation movement, which further increases work hardening. However, this can also increase the susceptibility to cracking along the grain boundaries, especially under cyclic loading.
Texture development: In areas of high deformation, a preferred texture can develop in which the grains adopt a preferred orientation in the deformation direction. This texture can lead to anisotropic mechanical properties, as the material shows different strength values in different directions.
2.3 Residual stresses and springback After bending, residual stresses remain in the material. These are the result of the uneven plastic deformation and the resulting internal stress fields.
Tensile residual stresses: OnTensile residual stresses remain on the outside of the bent wire because the material has been plastically stretched here. These stresses are critical because they can make the material more susceptible to microcracking. Under repeated loading, these cracks can grow and lead to premature failure of the spring.
Compressive residual stresses: On the inside of the wire, compressive residual stresses remain, which tend to be viewed as beneficial. Compressive stresses can reduce the risk of cracking because they inhibit the growth of microcracks.
Springback: The springback that occurs after the wire is unloaded is an indicator of the residual stresses remaining in the material. This springback is a result of elastic deformation and shows how much the material can return to its original state. The greater the springback, the more elastic energy remains stored in the material.
3. Stress states after bending
After bending, complex stress states remain in the material that affect long-term behavior, especially fatigue strength and service life.
3.1 Residual stresses and their effects
The stresses created during bending lead to residual stresses that remain in the material. These can occur in the form of tensile or compressive stresses and have a significant impact on the service life of the spring.
Residual tensile stresses:
Tensile stresses on the outside of the wire can be dangerous under cyclic loads as they can contribute to cracking. Cracks can grow faster, particularly in areas of high dislocation density or at grain boundaries, which greatly shortens the service life of the spring.
Residual compressive stresses:
On the inside of the wire, compressive stresses can increase fatigue strength because they reduce the risk of crack propagation. Compressive stresses can therefore extend the service life of the spring to a certain extent.
3.2 Stress relaxation
Over longer periods of time, stress relaxation can occur, in which the residual stresses stored in the material are partially reduced. This happens particularly at elevated temperatures or under constant load. Stress relaxation leads to a decrease in the spring's restoring force and can impair the functionality of the spring over longer periods of time.
4. Modern wire bending techniques to minimize disadvantages
In order to minimize the adverse effects of material-mechanical changes during bending, modern bending techniques have been developed that control the residual stresses and optimize the structural changes.
4.1 CNC-controlled bending
CNC-controlled bending processes can increase the precision and reproducibility of the bending processes. By precisely controlling the bending force and the bending radii, the stress distributions can be specifically influenced, which leads to a more even distribution of the residual stresses.
4.2 Inductive heating
Inductive heating before bending can be used to reduce the deformation resistance of the material and reduce the risk of brittle fracture. The targeted heating softens the material locally, resulting in fewer dislocations and minimizing the risk of cracking.
4.3 Roll bending
Roll bending is another process in which the wire is bent over several rolls. This results in a more even distribution of stresses and reduces the risk of local overloads that could lead to plastic deformation or cracking.
4.4 Vibration-assisted bending
Vibration-assisted bending can be used to facilitate the movement of dislocations and prevent the formation of dislocation congestion. The vibrations during bending distribute the plastic deformation more evenly, which increases the strength of the material and reduces the risk of microcracks.
In summary, it can be seen that bending spring wire triggers a series of complex material-mechanical processes at different levels. These processes have a significant influence on the strength, spring behavior and service life of the spring. However, modern bending techniques offer ways to minimize these negative effects and optimize the mechanical properties of the material.
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Reiner Schmid Produktions GmbH Specialist and expert for the manufacture, production, production, development and sample production of torsion springs, double torsion springs and bent wire parts.
Abstract:
The manufacture, production and production of torsion springs, double torsion springs and bent wire parts is carried out in small series, large series and variant production.
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Keywords:
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