Many Directions, One sensor... The 3 Element Rosette

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.

Many Directions, One sensor... the 3 Element Rosette

Imagine a firefighter trying to put out three fires at once by spraying a hose back and forth over them all. There is never enough water in one place to extinguish the blaze. Now try to imagine a stress analyst trying to measure strain with three discrete linear strain gages in different directions.

3-element rosettes are used where the direction of peak strain (known as the maximum and minimum principle strains) is unknown. By recording the three separate strains, a calculation can additionally tell the user direction of the strains, shear strains, and if the material properties are known the stresses can also be calculated – not possible with a single strain measurement.

Example of an automotive steering knuckle which can be loaded due to road surface changes, steering, and braking. All of which will likely have unique directions of principal strains/stresses.  When simultaneously sampled the three element rosette can show how the principal strain directions and magnitude can change with different loading conditions.

Knowledge in the selection and application of rosettes is critical to their successful use in experimental stress analysis. Knowledge in the selection and application of rosettes is critical to their successful use in experimental stress analysis

The simple determination for when a three-element rosette should be used is the answer to the question, “Are the directions and magnitudes of the principal strains and stresses known?”  If the answer is no, then a three-element rosette most likely is needed.

Our TechNote 515 provides guidance regarding rosette selection, application and data reduction for all types for rosette gages.

Rosettes are available in two distinct types – rectangular (0-45-90° between grids) and delta (0-60-120° between grids) with the former being the most common. Additionally, they can be planar (grids next to each other) or stacked (grids on top of one another). Accuracy comparisons can be discussed, but each type can suit different applications.

They are available as pre-wired miniature for PCBA testing (as small as 2 x 4mm with 0.36mm grids), to large planar versions best suited for plastic composites, including GFRP, CFRP (carbon composite) and sandwich structures.