The Hole-Drilling Method: Residual Stress Determination

The Hole-Drilling Method
The most widely used modern technique for measuring residual stress is the hole-drilling strain gage method of stress relaxation, illustrated below.

Briefly summarized, the measurement procedure involves six basic steps:

A special three-element strain gage rosette is installed on the test part at the point where residual stresses are to be determined.
The three gage grids are wired and connected to a static strain indicator through a switch-and-balance unit.
A precision milling guide (Model RS-200 , shown above) is attached to the test part and accurately centered over a drilling target on the rosette.
After zero-balancing the gage circuits, a small, shallow hole is drilled through the center of the rosette.
Readings are made of the relaxed strains, corresponding to the initial residual stress.
Using special data-reduction relationships provided here, the principal residual stresses and their angular orientation are calculated from the measured strains.

The foregoing procedure is relatively simple, and has been standardized in ASTM Standard Test Method E837 (Ref. 1 ). Using commercially available equipment and supplies, and adhering to the recommendations in the ASTM standard, the hole-drilling method can be applied routinely by any qualified stress analysis technician, since no special expertise is required for making the measurements. The method is also very versatile, and can be performed in either the laboratory or the field, on test objects ranging widely in size and shape. It is often referred to as a "semi-destructive" technique, since the small hole will not, in many cases, significantly impair the structural integrity of the part being tested [the hole is typically 1/32 to 3/16 in (0.8 to 4.8 mm) in both diameter and depth]. With large test objects, it is sometimes feasible to remove the hole after testing is completed, by gently blending and smoothing the surface with a small hand-held grinder. This must be done very carefully, of course, to avoid introducing residual stresses in the process of grinding.

NOTE: In its current state of development, the hole-drilling method is intended primarily for applications in which the residual stresses are uniform throughout the drilling depth, or essentially so. While the procedures for data acquisition and reduction in such cases are well-established and straightforward, seasoned engineering judgment is generally required to verify stress uniformity and other criteria for the validity of the calculated stresses. This publication contains the basic information for understanding how the method operates, but cannot, of course, encompass the full background needed for its proper application in all cases. An extensive list of technical references is provided in the Bibliography as a further aid to users of the method.