Enhanced Sensitivity Residual-stress Measurements Using Taper-hole Drilling by M. Tootoonian and G.S. Schajer ABSTRACT--A novel method for enhancing the strain sen- sitivity of the hole-drilling method for measuring uniform re- sidual stresses is examined. Such enhanced strain sensi- tivity is important because it improves the accuracy of the residual-stress evaluation. The new method involves en- larging the effective hole size by drilling a reverse taper hole. A simple practical technique for drilling reverse taper holes is described. The strain sensitivity for this new method is compared with that of the conventional hole-drilling method. Experimental results show excellent correspondence with theoretical results. The reasons for the sensitivity improve- ment are explained. Introduction The hole-drilling method is a widely used technique for measuring residual stresses. Simplicity, reliability in prac- tice, and limited damage to the specimen are among the reasons for the popularity of the method. The method in- volves drilling a small hole in the stressed material at the center of a specially designed strain-gage rosettC shown in Fig. 1. This rosette measures the resulting strain reliefs in the material adjacent to the hole. The measured strain re- liefs are used to calculate the original residual stresses that existed before the hole drilling. The strain sensitivity of the hole-drilling method de- pends directly on the size of the hole relative to the rosette size. Maximum sensitivity for a given rosette size is achieved when the hole has the maximum allowable size. This max- imum size is determined by the requirement that the edge of the hole does not touch the strain-gage grids. Unfortu- nately, even with the maximum size hole, the sensitivity of the hole-drilling method is not very high. This modest strain sensitivity means that small strain-measurement errors can cause significant errors in the calculated residual stresses. Taper-hole drilling is a method proposed here to improve the strain sensitivity and stress-calculation accuracy Without exceeding the limit for hole size at the surface of the spec- imen. In the proposed method, a truncated cone shape hole is drilled instead of a conventional cylindrical hole, as shown in Fig. 2. By modifying the geometry of the hole, the ef- fective size of the hole and the flexibility of the material in the region close to the hole are increased. This increase i [ stress after relieved stress~ I ~ before drilling ~ i ~strain gauge Fig. 1--Hole-drilling and strain-gage arrangement in flexibility of the material causes larger strain reliefs and increased sensitivity. An additional advantage is that the maximum strain relief occurs at smaller hole depth than when using a straight hole. Thus, a shallower hole can be used and less damage is done to the specimen. This paper describes and explains the sensitivity im- provement that is achieved by taper-hole drilling. A prac- ileal drilling procedure is also briefly described. The strain sensitivity of taper-hole drilling is measured experimentally and is compared with numerical results from finite-element calculations. Throughout this paper, a uniform stress pro- file with respect to depth is assumed. M. Tootoonian is Research Assistant, and G.S. Schajer (SEM Mem- ber) is Associate Professor, Department of Mechanical Engineering, 2324 Main Mall, University of British Columbia, Vancouver, British Columbia, V6T 1Z4, Canada. Original manuscript submitted: November 5, 1993. Final manuscript received: August 1, 1994. Review of the Hole-drilling Method The relation between the strain reliefs measured by each element of the strain-gage rosette shown in Fig. 1 and the original residual stresses is 2'3 E = fit(O-ma x -~- O'min) + n(O'ma x -- Ilmin)COS2qO (1) 124 ~ June 1995