Hole-Within-a-Hole Method for Determining Residual Stresses

Abstract

The strain gauge rosette hole drilling method is often used for determining residual stresses versus depth to depths on the order of 0.5–1.5 mm. Frequently, it is of interest to find stress profiles to greater depths. To provide such a capability, a new approach is presented. Residual stresses versus depth are found by drilling a small diameter hole incrementally to a depth of half of its diameter. The profile of stresses versus depth is found from changes in surface displacements associated with the stress relief from introducing the hole, observed by optical means. Next, a larger diameter, square-bottomed hole is milled directly over the small hole to a depth equaling that of the smaller diameter hole. The bottom of the larger hole provides a fresh surface for optical observation and incremental drilling of a new small hole. This procedure is repeated until a desired total depth is reached. A computational approach is described for correcting the stresses found from the small holes to account for the perturbation of stresses by the material removed by the larger diameter hole. Results of applying this method to find stresses versus depth in a plate subject to uniaxial bending stress and a plate with biaxial residual stresses that vary from compression to tension through the thickness are shown.