The hole drilling method has been used for determination of residual stresses in various materials. The released strains measured by the hole drilling method are converted to residual stresses using calibration factors. There is a standard for hole drilling method for isotropic materials. Also for orthotropic materials, there are various methods for converting the released strains to residual stresses. In these methods the numbers of calibration factors vary from two to nine (a three by three matrix). In this research, using an exact solution method, calibration factors for orthotropic plates are determined. For this purpose, by assuming a plane stress condition, the stresses and strains around the area under a rosette strain gauge mounted to an orthotropic plate are calculated. Making a hole in the centre of the rosette strain gauge changes the distribution of the stress and strain forces. Using the closed form solution proposed in this research the released strains and the calibration factors are calculated, and the residual stresses are evaluated. Using the proposed method, calibration factors for any orthotropic material with any level of orthotropy are calculated.
These factors are independent of loading levels and depend on material properties of orthotropic materials and ratio of the rosette diameter to the hole diameter. The material properties and the ratio of the rosette diameter to the hole diameter are considered in the equations, therefore there is no need for presenting the results in tabular form or any interpolations. The results for an orthotropic unidirectional ply made of carbon/epoxy are obtained. Also the calibration factors for three different dunidirectional composites are calculated. The data from the proposed closed form solution are compared with the 'simulated hole drilling method which has been proposed by authors earlier. A good agreement between the results shows the reliability of the closed form solution presented in this study.