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

Mechanical Properties As in any form of experimental stress analysis, the accuracy of residual stress measurement is limited by the accuracies to which the elastic modulus and Poisson's ratio are known. But typical uncertainties in the mechanical properties of common steel and aluminum alloys are in the neighborhood of 1 to 3% and, as such, are minor contributors to potential errors in residual stress analysis. Much larger errors can be introduced by deviations from the assumptions involved in the basic theory, as described previously . A key assumption, for instance, is linear-elastic material behavior. If the stress/strain relationship for the test material is nonlinear, due to yielding or other causes, the calculated residual stresses will be in error. When the initial residual stress is close to the yield strength of the test material, the stress concentration caused by the presence of the hole may induce localized yielding. It is therefore necessary to establish a threshold level of residual stress below which yielding is negligible. This problem has been studied both experimentally and analytically, and there is substantial agreement among the different investigations (Refs. 10 , 17 , 18 ). That is, errors are negligible when the residual stress is less than 70% of the proportional limit of the test material -- for both blind holes and through holes. On the other hand, when the initial residual stress is equal to the proportional limit, errors of 10 to 30% (and greater) have been observed. The error magnitude obviously depends on the slope of the stress/strain diagram in the post-yield region; and tends to increase as the curve becomes flatter, approaching the idealized perfectly plastic behavior (Ref. 18 ).

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