Gage Pattern

The gage pattern refers cumulatively to the shape of the grid, the number and orientation of the grids in a multiple-grid (rosette) gage, the solder tab configuration, and various construction features which are standard for a particular pattern. All details of the grid and solder tab configurations are illustrated in the "Gage Pattern" columns of Standard Gage Series table, is used in conjunction with Catalog 500, Micro-Measurements Precision Strain Gages . The wide variety of patterns in the list is designed to satisfy the full range of normal gage installation and strain measurement requirements.

Uniaxial Gages

With single-grid gages, pattern suitability for a particular application depends primarily on the following:

Solder tabs These should, of course, be compatible in size and orientation with the space available at the gage installation site. It is also important that the tab arrangement be such as to not excessively tax the proficiency of the installer in making proper leadwire connections.

Grid width When severe strain gradients perpendicular to the gage axis exist in the test specimen surface, a narrow grid will minimize the averaging error. Wider grids, when available and suitable to the installation site, will improve the heat dissipation and enhance gage stability -- particularly when the gage is to be installed on a material or specimen with poor heat transfer properties.

Gage resistance In certain instances, the only difference between two gage patterns available in the same series is the grid resistance -- typically 120 ohms vs. 350 ohms. When the choice exists, the higher-resistance gage is preferable in that it reduces the heat generation rate by a factor of three (for the same applied voltage across the gage). Higher gage resistance also has the advantage of decreasing leadwire effects such as circuit desensitization due to leadwire resistance, and unwanted signal variations caused by leadwire resistance changes with temperature fluctuations. Similarly, when the gage circuit includes switches, slip rings, or other sources of random resistance change, the signal-to-noise ratio is improved with higher resistance gages operating at the same power level.

In experimental stress analysis, a single-grid gage would normally be used only when the stress state at the point of measurement is known to be uniaxial and the directions of the principal axes are known with reasonable accuracy ( + 5 deg). These requirements severely limit the meaningful applicability of single-grid strain gages in stress analysis; and failure to consider biaxiality of the stress state can lead to large errors in the stress magnitude inferred from measurements made with a single-grid gage.

Rosettes

For a biaxial stress state -- a common case necessitating strain measurement -- a two- or three-element rosette is required in order to determine the principal stresses. When the directions of the principal axes are known in advance, a two-element 90-degree (or "Tee") rosette can be employed with the gage axes aligned to coincide with the principal axes. The directions of the principal axes can sometimes be determined with sufficient accuracy from one of several considerations. For example, the shape of the test object and the mode of loading may be such that the directions of the principal axes are obvious from the symmetry of the situation, as in a cylindrical pressure vessel. The principal axes can also be defined by PhotoStress® testing.

In the most general case of surface stresses, when the directions of the principal axes are not known from other considerations, a three-element rosette must be used to obtain the principal stress magnitudes. The rosette can be installed with any orientation, but is usually mounted so that one of the grids is aligned with some significant axis of the test object. Three-element rosettes are available in both 45-degree rectangular and 60-degree delta configurations. The usual choice is the rectangular rosette since the data-reduction task is somewhat simpler for this configuration.

Gage Construction

When a rosette is to be employed, careful consideration should always be given to the difference in characteristics between single-plane and stacked rosettes. For any given gage length, the single-plane rosette is superior to the stacked rosette in terms of heat transfer to the test specimen, generally providing better stability and accuracy for static strain measurements. Furthermore, when there is a significant strain gradient perpendicular to the test surface (as in bending), the single-plane rosette will produce more accurate strain data because all grids are as close as possible to the test surface. Still another consideration is that stacked rosettes are generally less conformable to contoured surfaces than single-plane rosettes.

On the other hand, when there are large strain gradients in the plane of the test surface, as is often the case, the single-plane rosette can produce errors in strain indication because the grids sample the strain at different points. For these applications the stacked rosette is ordinarily preferable. The stacked rosette is also advantageous when the space for mounting the rosette is limited.

Page 9 of 20