The incremental centre-hole drilling (ICHD) technique is widely used for surface residual stress measurements with limited damage to the specimen. The technique is a mechanical strain relaxation method where stressed material is removed by drilling a small blind hole in the area of interest. This results in local redistribution of residual stresses and distortion of the surface near the drilled hole. The relaxed strains are measured using a variety of techniques for example a specially designed strain gauge rosette, electronic speckle pattern interferometry (EPSI) or digital image correlation (DIC). To measure the variation of residual stress with depth the hole is drilled incrementally and the relaxed strains are recorded at each increment. The measured strains allow for the back calculation of the residual stresses that were originally present prior to drilling the hole.
The arrangement of centre hole drilling in the vicinity of a strain gauge rosette is shown below. The rosette includes three strain gauges aligned around the rosette centre which is attached concentrically around the hole axis. The readings of three strain gauges in a rosette permit the measurement of surface in-plane residual stress state. The maximum standard depth in which the centre hole drilling is used to measure residual stress profile is about 1-2 mm.
The ICHD technique is relatively simple, inexpensive, quick and versatile and portable; It is considered as ‘semi-destructive’ as the volume of material removed is relatively small and can often be tolerated or adequately repaired.