Construction
All three types of rosettes (tee, rectangular, and delta) are manufactured in both planar and stacked versions. As indicated (for the rectangular rosette) below, the planar rosette is etched from the strain-sensitive foil as an entity, with all gage elements lying in a single plane. The stacked rosette is manufactured by assembling and laminating two or three properly oriented single-element gages.

   

Rectangular rosettes (of the same gage length) in planar and stacked construction.

When strain gradients in the plane of the test part surface are not too severe, the normal selection is the planar rosette. This form of rosette offers the following advantages in such cases:

• Thin and flexible, with greater conformability to curved surfaces.
• Minimal reinforcing effect.
• Superior heat dissipation to the test part.
• Available in all standard forms of gage construction, and generally
• Accepts all standard optional features.
• Optimal stability.
• Maximum freedom in leadwire routing and attachment.

The principal disadvantages of the planar rosette arise from the larger surface area covered by the sensitive portion of the gage. When the space available for gage installation is small, a stacked rosette may fit, although a planar one will not. More importantly, where a steep strain gradient exists in the surface plane of the test part, the individual gage elements in a planar rosette may sense different strain fields and magnitudes. For a given active gage length, the stacked rosette occupies the least possible area, and has the centroids (geometric centers) of all grids lying over the same point on the test part surface. Thus, the stacked rosette more nearly approaches measurement of the strains at a point. Although normally a trivial consideration, it can also be noted that all gages in a stacked rosette have the same gage factor and transverse sensitivity, while the grids in a planar rosette will differ slightly in these properties, due to their different orientations relative to the rolling direction of the strain-sensitive foil. The technical data sheet accompanying the rosettes fully documents the separate properties of the individual grids.

It should be realized, however, that the stacked rosette is noticeably stiffer and less conformable than its planar counterpart. Also, because the heat conduction paths for the upper grids in a stacked rosette are much longer, the heat dissipation problem may be more critical when the rosette is installed on a material with low thermal conductivity. Taking into account their poorer heat dissipation and their greater reinforcement effects, stacked rosettes may not be the best choice for use on plastics and other nonmetallic materials. A stacked rosette can also give erroneous strain indications when applied to a thin specimen in bending, since the grid plane of the uppermost gage in a three-gage stack may be as much as 0.0045 in (0.11 mm) above the specimen surface. In short, the stacked rosette should ordinarily be reserved for applications in which the requirement for minimum surface area dictates its selection.

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