Abstract
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.
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Notes
- 1.
Actually the required magnification is not significantly larger. Indeed, in the ring-core method the active area is the core, which should fill the imaged area, whereas in hole-drilling no theoretical limitation exists, but displacements decay rapidly with distance from the axis of the hole, thus imaging more than 5–6 diameters is practically useless.
- 2.
Using moiré interferometry is obviously impossible. Indeed, using this technique would simply move the problem from gluing the rosette to replicating a grating at the bottom of a deep hole.
- 3.
Note that also the optical strain rosette presents significant problems: indeed, it requires micro-indenting the surface at the bottom of the hole; moreover the diffracted beams quite probably will be blocked by the lateral surface of the hole.
- 4.
Note that all previous phase fields result from the subtraction of current phase from the reference one (i.e. the phase field of the original surface). Because we removed the core, we had to shift the reference field, which now corresponds to the bottom of the hole at 2.8 mm depth.
References
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Baldi, A., Bertolino, F. (2015). Incremental Ring Core by Optical Methods: Preliminary Results. In: Sottos, N., Rowlands, R., Dannemann, K. (eds) Experimental and Applied Mechanics, Volume 6. Conference Proceedings of the Society for Experimental Mechanics Series. Springer, Cham. https://doi.org/10.1007/978-3-319-06989-0_17
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DOI: https://doi.org/10.1007/978-3-319-06989-0_17
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