SG hole drilling rosette accurately determines residual stresses

23 August 2010

HBM has introduced a hole drilling rosette to increase the accuracy of residual stress measurement by a factor of five. In the hole drilling method, which is one of the most common methods of determining residual stresses in components, strain gauges (SGs) determine the deformation, or relaxation strain, around a drill hole while it is being drilled.

Residual stresses are mechanical stresses that exist in a material without external forces or moments acting on the component. They can occur in metallic work pieces during production and may be 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. It is thus essential for manufacturers to determine the residual stresses in any component so that they can be assured of its operational reliability.

HBM’s new RY61M SG hole-drilling rosette has a patented structure that utilises six measuring grids instead of the customary three to determine the eccentricity of the hole which may be a potential source of error. 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.

The measurement error resulting from drill-hole eccentricity is reduced by a factor of five because HBM uses six measuring grids. The RY61M SG is easily connected as the measuring grids are already connected on the carrier foil, or on a small printed circuit board.

High-precision measurement of compressive and tensile forces is also now possible with HBM's new S9M force transducer, which works with SGs and is extremely accurate because of its so-called S-type geometry. The S9M has an accuracy class of 0.02; the earlier S9 model had an accuracy class of 0.05.

The device is compensated for parasitic influences, such as lateral forces or bending moments, and measurement reliability is markedly improved. The transducer is also unaffected by temperature variations, including temperature gradients. The S9M is available with different nominal (rated) force ranges, from 500N to 50kN.