Correction of Principal Strains

With any rosette, rectangular, delta, or otherwise, it is always possible (and often most convenient) to calculate the indicated principal strains directly from the completely uncorrected gage readings, and then apply corrections to the principal strains. This is true because of the fact that the errors in principal strains due to transverse sensitivity are independent of the kind of rosette employed, as long as all gage elements in the rosette have the same nominal transverse sensitivity. Since Equations ( 509.6 ) and ( 509.7 ) apply to any two indicated orthogonal strains, they must also apply to the indicated principal strains. Thus, if the indicated principal strains have been calculated from strain readings uncorrected for transverse sensitivity, the actual principal strains can readily be calculated from the following:

     Eq. (509.16)

     Eq. (509.17)

Furthermore, Equations (509.16) and (509.17) can be rewritten to express the actual principal strain in terms of the indicated principal strain and a correction factor. Thus,

     Eq. (509.18)

     Eq. (509.19)

The indicated strains from three gages with any relative angular orientation define an "Indicated" Mohr's circle of strain. When employing a data-reduction scheme that produces the distance to the center of Mohr's circle of strain, and the radius of the circle, a simple correction method is applicable. To correct the indicated Mohr's circle to the actual Mohr's circle, the distance to the center of the indicated circle should be multiplied by , and the radius of the circle by .The maximum and minimum principal strains are the sum and difference, respectively, of the distance to the center and the radius of Mohr's circle of strain.

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