Fine increment hole-drilling method for residual stress measurement, proposal of a calibrating apparatus

Hole drilling is one of the most widely used technique for residual stress measurements, due to its precision and low cost. Standard ASTM E 837 is limited to the uniform distribution of residual stresses, whereas it is well known that residual stresses usually feature high gradient along the depth, particularly if residual stresses are induced by surface treatments such us shot peening. The incremental hole-drilling method can be used to evaluate the not-uniform residual stress distribution as shown by Schajer. The strain measurement is repeated at different hole depths to achieve information on the residual stress gradient, from relaxed surface strain. Influence functions for the incremental hole drilling are proposed by Beghini and Bertini by which the relaxed strain can be evaluated analytically if the residual stress distributions are known for any kind of rosette, hole diameter and elastic material. Valentini has developed an apparatus able to produce fine increment hole drilling, by means of a high speed air turbine drill automatically controlled. Valentini et al. used the appartatus to experimentally show the effects on residual stresses measurement by the hole drilling such as eccentricity and sequence of increments steps.