Two-Gage Rosette

While the above analysis of the errors due to misalignment of a single gage may help in understanding the nature of such errors, the 90-degree, two-gage rosette is of considerably greater practical interest.

A two-gage rectangular rosette is ordinarily used by stress analysts for the purpose of determining the principal stresses when the directions of the principal axes are known from other sources. In this case, the rosette should be bonded in place with the gage axes coincident with the principal axes. Whether there is an error in orientation of the rosette with respect to the principal axes, or in the locations of the principal axes themselves, there will be a corresponding error in the principal stresses as calculated from the strain readings.

Biaxial strain field with rosette axes misaligned by the angle from the principal axes.

In the figure above, a general biaxial strain field is shown, with the axes of a two-gage rosette, misaligned by the angle superimposed. The percentage errors in the principal stresses and maximum shear stress due to the misalignment are:

            (511.6)

    (511.7)

            (511.8)

     (511.9)

  (511.10)

  (511.11)

where:

are the principal stresses and maximum shear stress inferred from the indicated strains when the rosette is misaligned by the angle .

is the ratio of the algebraic maximum to the algebraic minimum principal strain, as before.

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