What is residual stress? Definition and properties
Residual stress refers to internal, mechanical stresses within components. It is not induced by external forces, but by plastic deformation.
Causes
Residual stress is always caused by plastic deformation. They can take the form of: cold deformation, inhomogeneous, structural change, thermal change and phase transformations, which can emerge during manufacturing processes. To different degrees, residual stress is therefore present in every component.
Types of Residual Stress
Depending on the extent of stress, there are three different types of residual stress. They are distinguished on the basis of crystallites, which are also known as grains in metallurgy.
- Macroscale residual stress (Type I): Macro-residual stresses can be found in several grains. Signs for the type I residual stress are cracks and permanent plastic deformation.
- Microscale residual stress (Type II): Micro-residual stresses can be found in a single grain. They are often found in martensitic transformation with partially austenites being formed on the surface. Micro-residual stress be be spotted by cold deformation and the Bauschinger effect.
- Nanoscale residual stress (Type III): Nano-residual stress (or sub-micro residual stress) is developed within a grain itself. Signs for this type of crystalline defects are dislocations.
Measurement
In general, measurements are distinguished by destructive and nondestructive methods. In contrast to regular mechanical stresses, residual stress cannot be measured by usual methods. In lieu, side effects, such as plastic deformation, are determined. Thus, conclusions about residual stresses can be drawn.
Non-destructive methods
Components are not harmed using non-destructive methods. Possible techniques include x-ray diffractions, ultrasonics, and neutron diffractions. Since these methods do not irrupt particularly deeply into materials, non-destructive measurements do not deliver results that are as effective as (semi) destructive methods.
Semi destructive measurement methods
Components are only partly damaged when it comes to semi destructive measurement methods. Damages can furthermore be repaired. Common treatments are deep and centre hole drilling.
Destructive measurement methods
As its name already reveals, destructive measurement inflicts major damage on components to the point of being non-repairable. Parts can be cut open, drilled out or become subject to the cut compliance method. Components cannot be used again after destructive measurement, but they act as an example for components that were manufactured the same way.
Removal of residual stress
Although residual stress will likely never be removed completely, there are two methods to reduce internal stress: movement and temperature.
One of the most common procedures is stress relief annealing. Stress is relieved by slowly heating up components. Steel, for instance, reaches temperatures between 550 °C and 650 °C. A new procedure to relief stress is through vibrations. Components are subject to oscillations at low frequencies to relief inner stress.
