In the process of material processing and manufacturing, residual stress is a common phenomenon, which has an important impact on the performance and service life of materials. Understanding the causes of residual stress, spatial distribution characteristics and its impact on material properties, to improve product quality, prevention of safety accidents is of great significance.
I. Causes of residual stress
Residual stress refers to the end of material processing, the workpiece still retains the internal stress. This stress arises mainly from the following aspects:
1. Uneven plastic deformation
When the workpiece is subjected to complex loads (such as cold rolling, drawing, extrusion and surface shot peening, rolling, etc.), due to the uneven force on the cross-section, the place of large force may produce plastic deformation, while the place of small force may still be elastic deformation. When unloaded, these uneven plastic deformations can lead to residual stresses within the workpiece. Specifically, stretched areas are subjected to compressive stresses after unloading, while adjacent areas are subjected to tensile stresses. This residual stress due to uneven plastic deformation is a common form of stress in machining processes.
2. Thermal effects of inhomogeneous temperature field
In thermal processing, the heating and cooling processes are usually very complex, involving rapid heating and cooling as well as localised heating and cooling. Since heat can only be transferred through the surface, the workpiece will have differences in thermal expansion due to temperature inhomogeneity during heating and cooling, which in turn generates transient thermal stresses. When the value of these transient thermal stresses exceeds the high temperature yield limit of the material, plastic deformation occurs in certain areas. When the heating process is over, although the workpiece has cooled to room temperature, residual stresses are generated within the workpiece due to uneven plastic deformation during the heating process. Such residual stresses due to uneven temperature fields are particularly common in heat treatment processes.
3. Phase transition effects
During material processing, changes in the metallurgical organisation (e.g. austenite to martensite transformation during quenching) lead to changes in the internal specific volume of the material, which in turn generates phase transition stresses. This phase transition stress will also remain inside the workpiece after processing, forming residual stress.
Second, the spatial distribution of residual stress characteristics
The spatial distribution of residual stress depends on its cause and the specific conditions of the processing. Generally speaking, residual stress can be divided into three categories: macro residual stress, micro residual stress and ultra-micro residual stress.
1. Macro residual stress
Macroscopic residual stress is in the workpiece as a whole or the macro range of residual stress to achieve equilibrium. The distribution of this stress is usually more uniform, the overall performance of the workpiece has a greater impact. In the process, due to uneven plastic deformation or uneven temperature field of the thermal effect, the workpiece will produce macro tensile or compressive residual stress.
2. Micro residual stress
Microscopic residual stresses are residual stresses that reach equilibrium within a few grains within the workpiece. The distribution of this stress is usually more complex, and the shape, size and orientation of the grain and other factors. During plastic deformation, microscopic tensile or compressive residual stresses are generated due to grain interactions and the presence of grain boundaries. These stresses have an important influence on the local properties of the material, such as fatigue strength and crack extension rate.
3. Ultra-microscopic residual stress
Ultra-microscopic residual stress is a large number of atomic surfaces in the workpiece, atomic columns near the equilibrium of the residual stress. The distribution of this stress is more complex, and is related to the microstructure of the material and defects and other factors. During phase transformations, ultramicroscopic tensile or compressive residual stresses are generated due to lattice distortions and defects. These stresses have an influence on the physical and chemical properties of the material.
The spatial distribution characteristics of the residual stresses not only depend on their causes, but are also closely related to the specific conditions of the process. For example, during the welding process, the weld seam and its neighbouring areas undergo rapid heating and cooling, resulting in a complex distribution of residual stresses in these areas. The inhomogeneity of these stress distributions can have an important effect on the strength and toughness of the weld.
Third, the impact of residual stress on material properties
Residual stress has a significant effect on the mechanical properties of materials. In terms of yield strength, when there are residual tensile stresses within the material, it will reduce the yield strength of the material, making the material more prone to plastic deformation. Conversely, residual compressive stresses increase the yield strength of the material to some extent. For tensile strength, the presence of residual stress will change the stress distribution of the material in the tensile process, thus affecting its tensile strength. In addition, the residual stress will also have an impact on the plasticity, toughness and hardness of the material and other indicators. For example, residual tensile stress increases the elongation of the material, while residual compressive stress decreases the elongation of the material. In terms of hardness, residual tensile stress decreases the hardness measurement, while residual compressive stress increases the hardness measurement.
The effect of residual stress on the fatigue life of a material is even more critical. Under the effect of alternating load, residual tensile stress will be superimposed with the applied load, accelerating the emergence and expansion of cracks, significantly reducing the fatigue life of the material. Therefore, in the design and manufacture of mechanical parts, the impact of residual stress must be fully considered in order to avoid fatigue failure caused by excessive residual stress.
2. Impact on physical properties
Residual stress will also have an impact on the physical properties of the material. For example, in terms of the coefficient of thermal expansion, residual stress will change the internal lattice structure of the material, which in turn affects its thermal expansion characteristics. When residual stresses are present in a material, the coefficient of thermal expansion may change, which in some temperature sensitive applications (e.g., thermal control systems in aerospace) can lead to problems with the fit of components. In addition, residual stress has an effect on the thermal and electrical conductivity of the material. Studies have shown that residual stresses can distort the atomic arrangement within a material, increasing the probability of scattering of phonons and electrons, thereby reducing the thermal and electrical conductivity of the material. In electronic devices, where heat dissipation and electrical conductivity are critical, this effect from residual stress may lead to a decrease in the thermal efficiency and an increase in the resistance of the device, which in turn affects the performance and stability of the device.
