Using Optical Interferometry to Restart the Ring-Core Method

Residual stresses are a well-known technical problem because they add to the stress field induced by external loads, thus causing mechanical components to fail at a load level significantly lower than expected. Of the various techniques developed to measure them, the ring-core method is one of the few which in principle can be restarted (by removing the core and re-installing the strain gauge rosette). Thus, it is theoretically able to measure residual stress at significantly greater depth than other methods. Although the idea is interesting, its practical implementation is quite difficult: in particular, re-installing the rosette and re-wiring is almost impossible when depth becomes significant, thus the incremental measurement is more a theoretical possibility than a real experimental approach. In this work we propose to replace the strain gauge rosette with an optical (interferometric) technique. In this way the incremental approach becomes viable, although, depending on the optical technique used, some practical problems have to be addressed.