Strain Gauge Performance for Inhomogeneous Materials

Strain can be defined as the deformation of a material resulting from applied force, and it is measured by change in length. It can be either compressive or tensile. A strain gauge is a device that changes its electrical resistance in proportion to strain, which may result from a number of external or internal influences, including temperature, pressure or structural change. The most frequently used gauges are bonded metallic strain gauges, which comprise of foil or fine wire assembled in a grid pattern.

The grid is bonded to the test specimen and identifies changes in length that occur when a load is applied. This results in a change in resistance, which is measured by an electric circuit. Inhomogeneous materials may present various complexities in achieving accurate strain measurements.

Properties of Inhomogeneous Materials

Until lately, most of the strain measurements were applied to metals such as aluminum and steel alloys. The present trend of replacing metals with composites and polymers has brought about an increasing number of reinforced plastics with considerably different chemical, thermal and mechanical properties. One major difference in the physical properties of polymers and metals is found in the elastic modulus. A composite material with a plastic or polymer matrix may have an elastic modulus of more than twice that of metals, based on the volume of reinforcement and fiber material.

As a result, strain measurements for these inhomogeneous materials are significantly larger than for metals and typically require special wiring techniques and strain gauge bonding.

The thermal conductivity of polymer matrix composites could be as much as two orders of magnitude lower when compared to that of metals. When the gauge and test specimen undergo deformation, the gauge resistance changes in response to strain and produces a calibrated voltage offset. Strain gauges with a higher resistance result in less heating for the generated voltage. Polymer matrix composites, however, do not conduct heat well and may allow heat to accumulate in the gauge. This rise in temperature leads to an increase in resistance and, then, an error in strain measurement. In some cases, this may need temperature compensation.

As some inhomogeneous materials are hygroscopic, they may contract or expand as moisture content changes. These dimensional fluctuations are not differentiated from thermal output and they result in false strain measurements. Plastic composite matrices change considerably in hygroscopic properties.

For instance, while polyethylene absorbs practically no moisture, acrylic plastic has a strong tendency to absorb moisture. Another inhomogeneous material prone to absorbing moisture is wood, which will expand and contract in accordance with atmospheric changes. Moreover, the shrinkage that occurs during drying is less in a direction parallel to the grain when compared in a direction across the grain. This not only underscores the necessity of proper specimen selection for compensating strain gauges, but it also offers consistent atmospheric conditions for both the compensating and active gauges.

Gauge Length for Inhomogeneous Materials

The gauge length is defined as the receptive region of the grid. End loops and solder tabs are excluded from what is regarded as the strain sensing area of the gauge due to the relatively large size of their cross-sectional area and low electrical resistance. Strain gauge lengths are usually chosen depending on the shape and size of the sample and expected strain distribution. The gauge length also plays an essential role in the accuracy of strain measurements. Usually, strain gauges of 0.125 inches or greater offer greater stability and measurement range. In addition, larger gauges offer better heat dissipation, as they have a lower wattage per unit of grid area.

Another important consideration in strain measurements for inhomogeneous materials, such as concrete and reinforced plastics, is gauge length. The length must be large in relation to the size of the inhomogeneities in the sample to offer strain measurements representative of the structure. Measurements of inhomogeneous materials generally seek an average strain instead of the inconsistencies that arise at the peripheries of matrix and aggregate materials.

Gauge Pattern

Choosing the accurate gauge pattern is crucial in optimizing strain measurements. The gauge pattern designates the configuration of the solders tabs and grid. Grid width may be either wide or narrow, based upon the application. A narrow grid width reduces the averaging error for strain gradients aligned vertically to the gauge, while wider grids enable heat dissipation for specimens that show poor heat transfer properties. The solder tab configuration should concur with the position and size of the installation and enable the connection of lead wires.

Normally, identical gauge patterns are provided with a different resistance; the most familiar are 120 and 350 Ω. The higher resistance gauge is most often preferred because it reduces the amount of heat produced and lowers signal noise from lead wires and other sources of resistance changes.

Adhesives

TT300, SG401, SG496 Strain Gauge Adhesives

The performance of the strain gauge can be affected by the adhesives selection, which becomes part of the measurement system. Therefore, it is wise to use adhesives suggested by the gauge manufacturer. Epoxy adhesives that cure at room temperature are suitable for the majority of inhomogeneous materials, such as composites. They are available in several formulations, with properties to accommodate variations in time, temperature and elongation capability.

If users are applying an epoxy to glue composite sheets together, the epoxy employed would be a good choice to adhere the gauge. Generally, unfilled adhesives are preferred to reduce creep. Still, composite materials with fibers that result in an irregular surface may present problems in bonding. In such conditions, a partially filled adhesive may be applied to smooth the surface before bonding the specimen to the gauge with unfilled adhesive.

Strain Gauges Designed for Inhomogeneous Materials

SGD-30/120-LY40 Extra-Long Strain Gauge

The OMEGA® SGD-30/120- LY40 is an extra-long strain gauge explicitly developed for inhomogeneous materials. Its carrier is 40 by 12 mm and the grid is 25 by 8 mm. The nominal resistance is 120 Ω, and it stops with solder pads. Constantan foil, which is sealed in a polyimide carrier, is used to form the grid. OMEGA’s SGD-30/120-LY40 is strong and flexible to offer extremely accurate static and dynamic measurements.

The SGD-30/350-LY40 strain gauge is made from the same quality materials as the SGD-30/120-LY40. Its 50 mm length enhances the accuracy of strain measurements for inhomogeneous materials. The carrier is 36 by 5 mm and the grid is 30 by 3 mm. 350 Ω is the nominal resistances for the SGD-30/350-LY40. This linear pattern strain gauge is exclusively developed to measure strain in a single direction.

Conclusion

SG1-KIT Strain Gauge Application Kit

Extra-long strain gauges offer multiple advantages for strain measurements of inhomogeneous materials. They support a larger area for measurement, which enables a better representation of the structure of the material. Additionally, the greater surface area facilitates heat dissipation to provide more stable measurements.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.