Measurement of Residual Stresses by the Hole-Drilling Strain Gage Method

Limitations and Cautions Finite-element studies of the hole-drilling method by Schajer and by subsequent investigators ( Refs. 20 , 21 , 22 , 23 ) have shown that the change in strain produced in drilling through any depth increment (beyond the first) is caused only partly by the residual stress in that increment. The remainder of the incremental relieved strain is generated by the residual stresses in the preceding increments, due to the increasing compliance of the material, and the changing stress distribution, as the hole is deepened. Moreover, the relative contribution of the stress in a particular increment to the corresponding incremental change in strain decreases rapidly with distance from the surface. As a result, the total relieved strain at full-hole depth is predominantly influenced by the stresses in the layers of material closest to the surface -- say, in the upper third, or perhaps half, of the hole depth. At hole depths corresponding to  > 0.2, the stresses in these increments have very little effect on the observed strains. This behavior is confirmed (for uniform stress) by the shape of the normalized strain graph , where about 80% of the total strain relief normally occurs in the first half of the hole depth. Because of these characteristics, little, if any, quantitative interpretation can safely be made of the incremental strain data for increments beyond = 0.2, irrespective of the analytical method employed for data reduction. To summarize, the ideal application of the hole-drilling method is one in which the stress is essentially uniform with depth. For this case, the data-reduction coefficients are well-established, and the calculated stresses sufficiently accurate for most engineering purposes -- assuming freedom from significant experimental errors. Incremental drilling and data analysis should always be performed, however, to verify the stress uniformity. If the stress near the surface varies with depth, the procedures given here for approximate incremental analysis offer a simple, convenient alternative to the more rigorous finite-element methods. As illustrated by the example of the cold-rolled steel bar, the described procedures will normally highlight the presence of stress variation with depth and indicate its trend, as well as providing a quantitative estimate of the average principal stresses in the first drilling increment. Error and uncertainty are always present, in varying degrees, in all measurements of physical variables. And, as a rule, their magnitudes are strongly dependent on the quality of the experimental technique as well as the number of parameters involved. Since residual stress determination by the hole-drilling method involves a greater number and variety of techniques and parameters than routine experimental stress analysis, the potential for error is correspondingly greater. Because of this, and other considerations briefly outlined in the following, residual stresses cannot usually be determined with the same accuracy as stresses due to externally applied static loads. (continued...)

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