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

Experimental Calibration The needed coefficients for either through-hole or blind-hole analysis can always be determined by experimental calibration. This procedure is particularly attractive since it automatically accounts for the mechanical properties of the test material, strain gage rosette geometry, hole depth and diameter, and the strain-averaging effect of the strain gage grid. When performed correctly, with sufficient attention to detail, it is potentially the most accurate means for determining the coefficients. Its principal disadvantage is that the calibration must be repeated each time a different set of geometric parameters is involved. Calibration for and is accomplished by installing a residual stress strain gage rosette on a uniaxially stressed tensile specimen which is made from the same material as the test part. The rosette should be oriented to align grid no. 3 parallel to the loading direction, placing grid no. 3 along the transverse axis of the specimen. Care must be taken that the tensile stress is uniform over the cross section of the test specimen; i.e., that bending stress is negligible. To minimize edge and end effects, the specimen width should be at least ten times the hole diameter, and the length between machine grips, at least five times the width. When determining and for blind-hole applications, a specimen thickness of five or more times the hole diameter is recommended. For through-hole calibration, the thickness of the calibration specimen is preferably the same as that of the test part. It is also important that the maximum applied stress during calibration not exceed one-half of the proportional limit stress for the test material. In any case, the applied stress plus the initial residual stress must be low enough to avoid the risk of local yielding due to the stress-concentrating effect of the hole. (continued...)

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