Incremental ring core by optical methods: preliminary results

The ring core method is a well-known technique for residual stress measuring. It consists of milling a circular ring around the point of interest and measuring the surface deformations of the core. The method is more sensitive than hole drilling, but its sensitivity decreases with depth to become null when ring depth is equal to one third of the diameter. To overcome this problem, in literature an incremental version of the technique has been proposed consisting of removing the core, re- installing the strain gauge rosette and re-performing the measurement. Although the idea is interesting, its practical implementation is quite difficult, in particular re-installing the rosette is almost impossible when depth becomes significant, thus the incremental measurement is never performed. In this work we propose to replace the strain gauge rosette with an optical technique. In this way the incremental approach becomes viable, even though, depending on the optical technique used, some practical problems have to be addressed.