Measuring complex geometries for residual stress
French nuclear equipment manufacturer, Framatome, 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 centrehole rosette gauge method with a special purpose jig suitable for tubes down to 50mm internal diameter. Fittings were designed and built at short notice to accurate location of the strain gauges and measurement equipment inside tapered and parallel tubes at distances up to 130 mm from the tube end.
Working in the overhead position under the component, the team made measurements at a rate of up to four locations a day, not including calibration tests. The job, including design and fabrication of fittings and on site stress measurements, was completed in six weeks.
The residual stresses varied from compression to tension around the tube diameter and correlated well with the observed distortions caused by the tube attachment welds.
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, crack tip stresses, on-site through-wall stress measurements, and butt welds from 10 to 235 mm thick.
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