Residual stress measurements by hole-drilling have developed greatly in both sophistication and scope since the pioneering work of Mathar in the 1930s. Advances have been made in measurement technology to give measured data superior in both quality and quantity, and in analytical capability to give detailed residual stress results from those data. On the technology side, the use of multiple strain gauges, Moire, Holographic Interferometry and Digital Image Correlation all provide prolific sources of high quality data. In addition, modern analytical techniques using inverse methods provide effective ways of extracting reliable residual stress results from the mass of available data. This paper describes recent advances in both the measurement and analytical areas, and indicates some promising directions for future developments.

Advances in Hole-Drilling Residual Stress Measurements

In recent years, the hole drilling method for determining residual stresses has been implemented with optical methods such as holographic interferometry and ESPI to overcome certain limitations of the strain rosette version of hole drilling. Although offering advantages, the interferometric methods require vibration isolation, a significant drawback to their use outside of the laboratory. In this study, a 3D image correlation approach was used to measure micron-sized surface displacements caused by the localized stress relief associated with hole drilling. Residual stresses were then found from the displacements using non-dimensional relations previously derived by finite element analysis. A major advantage of image correlation is that it does not require interferometric vibration isolation. Experiments were performed to check the ability of this new approach for uniaxial and equi-biaxial states of stress. Stresses determined by the approach were in good agreement with computed values and those determined by hole drilling using holographic interferometry.

Residual Stress Determination Using Hole Drilling and 3D Image Correlation

The effectiveness of optical (mostly interferometric) methods for the measurement of residual stresses is largely demonstrated in literature. Nevertheless, these techniques are still confined to optical laboratories due to their high sensitivity to vibrations which makes it very difficult to perform the measurement in an industrial environment. Digital Image Correlation (DIC) has recently been proposed as a possible solution to this problem: this non-interferometric technique is much less affected by vibrations, but its sensitivity is relatively low, thus negatively affecting the accuracy of results. This work proposes to use a variant of Digital Image Correlation, known as Integrated DIC (iDIC), in combination with the hole drilling technique. Since iDIC directly incorporates in its formulation the displacement field related to hole drilling, it overcomes most of the problems of standard DIC; in this way it is possible to obtain accurate results without using interferometric techniques.

Residual Stress Measurement Using Hole Drilling and Integrated Digital Image Correlation Techniques

Most of the full field methods for residual stress investigation combine an optical interferometric technique (grating interferometry, speckle interferometry, holographic interferometry) with the standard hole drilling method. Nowadays many articles describing this kind of approach exist, but most of them focus on the experimental aspects while little attention is devoted to the optimal usage of the huge mass of data that the optical methods make available. This paper, without relying on a specific experimental technique, focuses on the development of a new analytical method that attempts to consistently use all the available data. After a detailed algorithm description, the proposed analysis procedure is tested against numerically generated residual stress fields simulating in-plane speckle interferometry. The results obtained show the reliability and robustness of the algorithm.

A new analytical approach for hole drilling residual stress analysis by full field method

Hole Drilling and Ring Coring

The use of interferometric moire and hole drilling to determine residual stress has been well reported and accepted for stress fields whose principal directions can be predicted well enough to permit the moire grids to be aligned with the principal strain axes. When the principal strains do not align themselves with the grid axes, a third strain component can be obtained by working with the diagonal pitch of the moire grid, but this requires resetting the optical bench to the lower frequency. Diffraction efficiency is lost, with an additional loss in sensitivity. In this paper, the authors determine the shear strain component by observing the rotation of the moire fringes in close proximity to the hole. The results of experiments on a specimen containing a model residual stress distribution are presented and compared with the theoretical prediction. Finally, the isothetic contours, based on elastic theory, were computed and plotted for several cases to verify this proposition. These results and the expected residual stress distribution are also compared to the experimentally obtained moire fringes.

J. M. Papazian

Papers

Measurement of residual stress using interferometric moiré: A new insight