Residual strain is an internal stress that remains within a material even after all external forces have been removed. It is present in many manufactured goods and influences how materials behave and perform over time.
Understanding Residual Strain
Residual strain represents internal forces locked within a material, distinct from the temporary deformation caused by external loads. Unlike applied strain, which disappears when an external force is removed, residual strain persists indefinitely.
There are two primary types of residual strain: tensile and compressive. Tensile residual strain involves internal forces pulling the material apart, similar to a stretched rubber band. In contrast, compressive residual strain involves internal forces pushing the material together. Both types can exist simultaneously within a single component, balancing each other out.
How Residual Strain Develops
Residual strain develops during manufacturing processes involving heating, cooling, or deforming materials. When a material is subjected to thermal gradients, such as during welding or quenching, different parts cool at varying rates. This uneven cooling leads to differences in thermal contraction, creating internal stresses that become residual strain. For instance, during welding, the heated weld metal cools and contracts, constrained by cooler surrounding material, resulting in tensile residual strain in the weld and compressive strain in adjacent areas.
Mechanical deformation processes, including machining, forging, rolling, or bending, also introduce residual strain. These processes cause non-uniform plastic deformation across the material’s cross-section. When the external force is removed, elastic portions attempt to recover their original shape, but are resisted by permanently deformed plastic regions. This incomplete recovery locks in residual stresses.
Another source of residual strain is phase transformations within a material. As a material transforms from one metallurgical phase to another, there can be a change in volume. If different parts transform at different times or rates, the resulting volume differences can lead to internal stresses that become residual strain.
Impact on Material Performance
Residual strain influences the properties and longevity of materials, with both detrimental and beneficial outcomes. Tensile residual strain reduces a material’s resistance to fatigue failure, where cracks grow under repeated loading. It also increases susceptibility to stress corrosion cracking, where cracks form and propagate in corrosive environments under sustained tensile stress. Tensile residual strain can also cause distortion or warping in manufactured components.
Conversely, compressive residual strain improves material performance. It enhances fatigue life by counteracting applied tensile stresses, requiring a higher external load to initiate or propagate cracks. Compressive residual strain also increases resistance to stress corrosion cracking by suppressing crack initiation and growth. Processes like shot peening, which intentionally introduce compressive residual strain by bombarding a surface with small, hard particles, improve wear resistance and durability of components.
Detecting and Managing Residual Strain
Measuring residual strain involves specialized techniques. X-ray diffraction is a common non-destructive method that measures the spacing between atoms in a material’s crystal lattice, allowing for the calculation of residual stresses in shallow surface layers. Another technique, hole drilling, is a semi-destructive method where a small hole is drilled into the material, and the resulting deformation is measured with strain gauges to estimate residual stresses. This method can provide information on stress distribution with depth.
Managing residual strain involves either reducing undesirable stresses or intentionally introducing beneficial ones. Stress relief heat treatment is a method to reduce tensile residual strain by heating the material to a specific temperature and holding it there, allowing internal stresses to relax. This process is performed at temperatures below the material’s transformation temperature to avoid further microstructural changes.
Conversely, techniques like shot peening are used to intentionally introduce compressive residual strain into the surface of a material. This process involves bombarding the surface with small, spherical media at high velocity, creating a shallow layer of compressive stress that improves fatigue resistance and wear properties. Mechanical working, such as rolling or forging, can also be controlled to induce beneficial residual strain distributions.