Rosette Selection Considerations

A comprehensive guide for use in selecting Micro-Measurements strain gages is provided in Vishay Measurements Group Tech Note TN-505, Strain Gage Selection Criteria . This publication should first be consulted for recommendations on the strain-sensitive alloy, backing material, self-temperature-compensation number, gage length, and other strain gage characteristics suitable to the expected application. In addition to basic parameters such as the foregoing, which must be considered in the selection of any strain gage, two other parameters are important in rosette selection. These are: (1) the rosette type - tee, rectangular, or delta; and (2) the rosette construction - planar (single-plane) or stacked (layered).

Two-Element Rosettes
The tee rosette should be used only when the principal strain directions are known in advance from other considerations. Cylindrical pressure vessels and shafts in torsion are two classical examples of the latter condition. However, care must be exercised in all such cases that extraneous stresses (bending, axial stress, etc.) are not present, since these will affect the directions of the principal axes. Attention must also be given to nearby geometric irregularities, such as holes, ribs, or shoulders, which can locally alter the principal directions. The error magnitudes due to misalignment of a tee rosette from the principal axes are given in Vishay Measurements Group Tech Note TN-511, Errors Due to Misalignment of Strain Gages . As a rule, if there is uncertainty about the principal directions, a three-element rectangular or delta rosette is preferable. When necessary (and, using the proper data-reduction relationships ), the tee rosette can be installed with its axes at any precisely known angle from the principal axes; but greatest accuracy will be achieved by alignment along the principal directions. In the latter case, except for the readily corrected error due to transverse sensitivity, the two gage elements in the rosette indicate the corresponding principal strains directly.

Three-Element Rosettes
Where the directions of the principal strains are unknown, a three-element rectangular or delta rosette is always required; and the rosette can be installed without regard to orientation. The data-reduction relationships that follow yield not only the principal strains, but also the directions of the principal axes relative to the reference grid (Grid 1) of the rosette. Functionally, there is little choice between the rectangular and delta rosettes. Because the gage axes in the delta rosette have the maximum possible uniform angular separation (effectively 120 degrees), this rosette is presumed to produce the optimum sampling of the underlying strain distribution. Rectangular rosettes have historically been the more popular of the two, primarily because the data-reduction relationships are somewhat simpler. Currently, however, with the widespread access to computers and programmable calculators, the computational advantage of the rectangular rosette is of little consequence. As a result of the foregoing, the choice between rectangular and delta rosettes is more apt to be based on practical application considerations such as availability from stock, compatibility with the space available for installation, convenience of solder tab arrangement, etc.

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