Gage Element Numbering

"Numbering", as used here, refers to the numeric (or alphabetic) sequence in which the gage elements in a rosette are identified during strain measurement, and for substitution of measured strains into data-reduction relationships such as those given in Section 4.0. Contrary to a widely held impression, the subject of gage numbering is not necessarily a trivial matter. It is, in fact, basic to the proper, and complete, interpretation of rosette measurements. With any three-element rosette, misinterpretation of the rotational sequence (CW or CCW), for instance, can lead to incorrect principal strain directions. In the case of the rectangular rosette, an improper numbering order will produce completely erroneous principal strain magnitudes, as well as directions. These errors occur when the gage user's numbering sequence differs from that employed in the derivation of the data-reduction relationships.

To obtain correct results using the data-reduction relationships provided later, the grids in three-element rosettes must be numbered in a particular way. It is always necessary in a rectangular rosette, for instance, that grid numbers 1 and 3 be assigned to two mutually perpendicular grids. Any other arrangement will produce incorrect principal strains. Following are the general rules for proper rosette numbering. With a rectangular rosette, the axis of Grid 2 must be 45 degrees away from that of Grid 1; and Grid 3 must be 90 degrees away, in the same rotational direction. Similarly, with a delta rosette, the axes of Grids 2 and 3 must be 60 degrees and 120 degrees away, respectively, in the same direction from Grid 1.

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