Strain Gage Pattern, Grid Shape, and Orientation

Solder Tabs & Terminals | Strain Gages

When setting up and reviewing strain gage patterns and orientation, the solder terminals should be fitted between the connecting cables and the strain gauge itself.  That is, for strain gauges containing leads or wires.

As it relates to positioning, Solder tabs should also be consistent in size and orientation with the available area for a given installation site.

It’s also critical that the tab arrangement doesn’t place an undue burden on the installer’s ability to make suitable leadwire connections.

Strain Gauge Grid Width | Wider Grids vs. Narrow Grids

When the test specimen surface has significant strain gradients perpendicular to the gage axis, a small grid will reduce the averaging error.

When accessible and appropriate for the installation site, wider grids will increase heat dissipation and gage stability.

This is especially observable, as noted by our engineers, when the gage is to be put on a material or specimen with poor heat transfer qualities.

Strain Gauge Resistance | High-Resistance vs. Low Resistance

Gage resistance — In some cases, the grid resistance — typically 120 ohms vs. 350 ohms — is the only variation between two gage designs available in the same series.

When the option is available, the higher-resistance gage is preferred since it reduces the rate of heat generation by a factor of three (for the same applied voltage across the gage).

Increased gage resistance also reduces leadwire effects such circuit desensitization induced by leadwire resistance and undesired signal variations produced by leadwire resistance changes with temperature fluctuations.

When the gage circuit incorporates switches, slip rings, or other sources of random resistance variation, higher resistance gages working at the same power level increase the signal-to-noise ratio.

Single-Grid Strain Gages

A single-grid gage would generally be employed in experimental stress analysis only when the stress condition at the measurement location is known to be uniaxial and the directions of the primary axes are known with good accuracy (5°).

These criteria significantly limit the usefulness of single-grid strain gages in stress analysis.  Moreover, failure to account for the biaxiality of the stress state can result in considerable inaccuracies in the level of stress inferred from single-grid gage measurements.

Biaxial Stress States & Strain Gage Alignment

A two- or three-element rosette is necessary to determine the major stresses in a biaxial stress state, which is a common instance requiring strain measurement.

When the principal axes’ directions are known ahead of time, a two-element 90-degree (or “tee”) rosette with the gage axis aligned to correspond with the principal axes can be used. One of several considerations can occasionally be used to establish the directions of the major axes with acceptable accuracy.

For example, in a cylindrical pressure vessel, the shape of the test item and the mechanism of loading may be such that the directions of the primary axes are clear from the symmetry of the situation. Computing photo stress testing can also be used to define the primary axis.

General Surface Applications of Strain Gages

When the directions of the primary axes are not known from other considerations, a three-element rosette must be employed to determine the principal stress magnitudes in the most general situation of surface stresses.

When setting up and reviewing strain gage patterns and orientation, note that the rosette can be put in any orientation.  However, it is most commonly fixed so that one of the grids is aligned with a major axis of the test object. Rosettes with three elements come in 45-degree rectangle and 60-degree delta forms.

The rectangular rosette is the most common choice because the data reduction work is easier in this configuration.

Considerations for Single-Plane and Stacked Rosettes

When using a rosette, the differences in properties between single-plane and stacked rosettes should always be considered.

In terms of heat transfer to the test specimen, the single-plane rosette outperforms the stacked rosette for any given gage length, offering improved stability and accuracy for static strain measurements.

Furthermore, because all grids are as close to the test surface as feasible, the single-plane rosette will yield more accurate strain data when there is a considerable strain gradient perpendicular to the test surface (as in bending). Another thing to keep in mind is that stacked rosettes are less adaptable to curved surfaces than single-plane rosettes.

Determination of High Strain Gradients with the Test Surface

Strain gradients are considered critical and sensitive indicators of structural health.

Yet, the power of singularity for materials with strain gradient effects is often different from that for conventional materials.

As it relates to in-the-field strain gauge testing, it is imperative to determine when there are high strain gradients in the plane of the test surface, as is frequently the case.

The single-plane rosette, on the other hand, can generate strain indication inaccuracies since the grids sample the strain at various positions. The stacked rosette is usually preferred for these applications. When room for installing the rosette is restricted, the stacked rosette is also beneficial in this regard.