For many years, there has also been a great interest in using the hole drilling method to measure the variations of residual stresses with depth. At present, all the procedures involve measuring the strain relaxations after successive small increments of the hole depth.

The present paper investigates the feasibility of employing interferometric techniques in alternative to the strain gauge, thus eliminating the need of incremental drilling. The information carried by the whole fringe pattern due to the drilling of a hole may, in fact, enable uniform or non-uniform states of stress to be reconstructed, provided that proper calibration constants are assessed. Moreover, the use of interferometric techniques removes the main source of inaccuracy in the hole-drilling strain-gauge method, i. e. a possible misalignment between the hole and the rosette.

The paper reports the experimental results obtained utilizing holographic and speckle interferometry to detect the surface displacements around a blind hole drilled in a specimen supported by a self-contained loading device. The device enables a linear or constant stress distribution to be established which is controlled by a set of strain-gauges applied on the free sides of the specimen. Fringe patterns depicting normal, oblique and in- plane components of displacement are processed on a general purpose image processing computer system interfaced with a high resolution TV-camera. The variation of residual stress with depth is reconstructed by a simple superposition of a restricted number of elementary stress states. In the present paper the feasibility of approximating the stress distribution using elementary stress states expressed by the terms of a power series is investigated. Such approach avoids numerical instabilities without excessively reducing the accuracy of the solution. The procedure requires numerically determined calibration constants allowing the weight of the single elementary state to be calculated from the full field fringe pattern which can be obtained by any interferometric techniques.