Patented SG hole drilling rosette for accurately determining residual stresses

The new RY61M SG hole drilling rosette that HBM is now marketing for this hole drilling method has a patented structure, which appreciably increases the accuracy of residual stress measurement.

The so-called hole drilling method is the one most frequently used to determine residual stresses in components. With the hole drilling method, strain gages (SG) determine the deformation, also called the relaxation strain, all around a drill hole, while a drill hole is being inscribed.

Hole drilling rosettes with three SG measuring grids are normally used for the hole drilling method. The eccentricity of the hole is a possible source of error with this method.

The new RY61M SG hole drilling rosette uses six measuring grids instead of the customary three, to further improve the accuracy of this process. The hole drilling rosette measuring grids are arranged around the drill hole, with each of the opposite SG pairs being interconnected so that their signals are averaged.

Connection of the new hole drilling rosette is no more complex than the old solution, as the measuring grids have already been connected on the carrier foil, or on a small PCB (printed circuit board). But the measurement error resulting from drill hole eccentricity is reduced by a factor of five.

Residual stresses are the mechanical stresses that exist in a material, without external forces or moments acting on the component. They can occur in metallic workpieces during production, for example, caused by uneven cooling during casting, or by rolling.

Residual stresses affect the component in the same way as load-induced stresses, and can reduce its load-carrying capacity from externally acting forces. It is essential to know the residual stresses in the component, to give a satisfactory answer to questions of component reliability.