Correction of errors introduced by hole eccentricity in residual stress measurement by the hole-drilling strain-gage method

The accuracy of the hole-drilling strain-gage technique, the most widely used method for residual stress measurement, is directly related to: (1) the eccentricity between the hole center and rosette center, respectively, and (2) the accuracy of corrections for integration tendency of the strain gages. Although the drilling tool is optically aligned, the eccentricity always cannot be zero and it is a source of error, affecting the accuracy of this technique. An equation for correction of the data affected by hole eccentricity has been proposed by A. Ajovalasit, but it is not precise enough, because it is founded on the wrong hypothesis that in the strain-gage area there is a uni-axial state of stress, instead bi-axial one. The paper presents a new equation for data correction, which take account of the plane (bi-axial) state of stress in the strain-gages area. After the hole drilling and data acquisition, the hole eccentricity will be measured, using an optical microscope. Knowing the eccentricity, the data will be corrected using the proposed equation. This equation is more precise, because it is founded on a realistic hypothesis (bi-axial state of stress in the strain-gages area).