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

Strain Gage Selection and Installation Installing three individual strain gages, accurately spaced and oriented on a small circle, is neither easy to do nor advisable, since small errors in gage location or orientation can produce large errors in calculated residual stresses. The configurations of the residual stress rosettes have been designed and developed by the Micro-Measurements Division specifically for residual stress measurement. The rosette designs incorporate centering marks for aligning the boring tool precisely at the center of the gage circle, since this is critical to the accuracy of the method (Refs. 9 , 10 , 11 ). All configurations are available in a range of temperature compensations for use on common structural metals. However, only the RE design is offered in different sizes ( 031RE , 062RE , and 125RE ), where the three-digit prefix represents the gage length in mils [0.001 in (0.0254 mm)]. The RE design is available either open-faced or with Option SE (solder dots and encapsulation). The UL and UM configurations are supplied in 1/16 in (1.6mm) gage length, and both types are fully encapsulated. Both configurations have integral, copper-coated solder tabs, and offer all advantages of the popular C-Feature strain gage series. All residual stress rosettes are constructed with self-temperature-compensated constantan foil, mounted on a flexible polyimide carrier. Gage resistance is 120 ohms + 0.4%. Surface preparation for installing the rosettes is basically standard, as described in Vishay Measurements Group Instruction Bulletin B-129, Surface Preparation for Strain Gage Bonding . Caution should be observed, however, in abrading the surface of the test object, since abrasion can alter the initial state of residual stress (Ref. 12 ). In general, it is important that all surface-preparation and gage-installation procedures be of the highest quality, to permit accurate measurement of the small strains typically registered with the hole-drilling method. As evidenced by the calibration data , the relieved strains corresponding to a given residual stress magnitude are considerably lower than those obtained in a conventional mechanical test at the same stress level. Because of the small measured strains, any drift or inaccuracy in the indicated gage output, whether due to improper gage installation, unstable instrumentation, or otherwise, can seriously affect the calculated residual stresses.

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