3-ELEMENT ROSETTE STRAIN GAGES.
A strain gage rosette is, by definition, an arrangement of two or more closely positioned gage grids, separately oriented to measure the normal strains along different directions in the underlying surface of the test part. Rosettes are designed to perform a very practical and important function in experimental stress analysis.
It can be shown that for the not-uncommon case of the general biaxial stress state, with the principal directions unknown, three independent strain measurements (in different directions) are required to determine the principal strains and stresses. Moreover, even when the principal directions are known in advance, two independent strain measurements are needed to obtain the principal strains and stresses.
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