The residual stresses in a component or structure are self-balanced stresses caused by incompatible internal strains. They may be generated or modified at every stage in the component life cycle. Welding is one of the most significant causes of residual stresses and typically produces large tensile stresses in the weld balanced by lower compressive residual stresses elsewhere in the component.
Residual stresses may be measured by non-destructive techniques (eg X-ray and neutron diffraction, optical, magnetic or ultrasonic methods); by semi- destructive techniques (eg centre-hole and deep-hole drilling, and the ring core method); and destructive techniques (eg block removal, splitting and layering and the contour method). The selection of a measurement technique should take account of various issues such as spatial resolution, material, geometry and access.
Tensile residual stresses may reduce the performance or cause failure of manufactured products. They may increase the rate of damage by fatigue, creep or environmental degradation. They may reduce the load capacity by contributing to failure by brittle fracture, or cause other forms of damage such as shape change or crazing. Compressive residual stresses are generally beneficial, but may cause a decrease in the buckling load.
Measurement of surface or through-thickness residual stresses in difficult locations or complex geometries is a TWI speciality. Recent applications have included tube-to-plate welds, weld cladding, on-site through-wall stress measurements, and butt welds for a range of thicknesses. This has led to the development of special kits, such as Tubestress and Cornerstress. This equipment has been used in major projects for clients in power generation, defence and nuclear waste disposal applications. The equipment can be customised for measurements in other locations with limited access.
A novel, compact system for residual stress measurement using digital image correlation (DIC) has been developed by TWI. Compared with the conventional method using strain gauge rosette, DIC avoids the need for extensive surface preparation and precise drilling of the hole. In contrast with conventional DIC, the new system is convenient, easy to use, and does not require extensive expertise.
TWI stays at the forefront of research in this area, notably via participation in international networks to advance knowledge on measurement and prediction of residual stresses.
Respected for its expertise, professionalism, impartiality and confidentiality, TWI works with the most influential companies worldwide across all industry sectors.
A nuclear equipment manufacturer called in TWI to measure residual stresses on the inside surface of 70 mm diameter nickel base alloy tubes welded to a ferritic steel component. The measurements were made using the centre-hole rosette gauge method with a special purpose jig suitable for small-diameter tubes. Working in the overhead position under the component, the team made measurements at a rate of up to four locations a day. TWI also worked with this company in the early development of the deep-hole method.
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