Residual stress is an internal “locked-in” stress within a component in the absence of any external loading or thermal gradient. Residual stress is mostly caused by non-uniform plastic deformation or material phase change, which force the surrounding materials to deform elastically to preserve dimensional continuity and therefore creating residual stress. This often occurs during manufacturing processes such as forging, extrusion, rolling, welding, casting, quenching, or surface treatment process such as machining and peening.
Residual stress can be detrimental to structural integrity, e.g., tensile residual stress added up to the applied stresses to cause yield and lead to premature failure. On the other hand, residual stress can be engineered to give beneficial effects, e.g., compressive stress introduced to the surface of components and act against applied tensile load, improving the fatigue life.
The appreciation of residual stress is of the utmost importance for designing and manufacturing critical engineering components. Therefore, various residual stress measurement techniques have been developed, offering different ranges of capabilities. The destructive and semi-destructive technique works by removing some material from the component to relieve the locked-in stress. This relief is translated to the deformations in the remaining component, which are then recorded and analysed to back-calculate the original residual stress state. The non-destructive method works on the contrast between the stressed and stress-free material in responding to the various probes, e.g., neutrons, x-rays or ultrasound waves.
Semi-destructive and destructive techniques include: Incremental hole drilling, contour method, slitting, Sachs boring.
Non-destructive methods include: Neutron diffraction, Synchrotron/ Laboratory X-ray diffraction, Ultrasonic, Laser ultrasound, Barkhausen Noise.