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Residual Stress Measurement

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).

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.

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Residual Stress Measurement

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 industries. 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 a 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.

For more information about TWI's Structural Integrity and Corrosion Management services, please contact us.