Determination of Calibration Factors of the Hole Drilling Method for Orthotropic Composites using an Exact Solution

Abstract

The hole drilling method has been used for measurement of the 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 the 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 article, calibration factors of the orthotropic plates are determined based on an exact solution method. For this purpose, assuming a plane stress condition, stresses and strains around the area under a rosette strain gage mounted to an orthotropic plate are calculated. A hole in the center of the rosette strain gage changes the distribution of the stresses and strains. Using the closed form solution proposed in this research the released strains and the calibration factors are calculated, and the residual stresses are found. Also, calibration factors for any orthotropic material with any level of orthotropy are calculated. These factors are independent of the loading and depend on the material properties of orthotropic materials and the 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 presented. Also, the calibration factors for three different unidirectional composites are calculated. The results obtained from the proposed closed form solution are compared with the simulated hole drilling method. A good agreement between the results shows the reliability of the closed form solution presented in this study.