Strain Gauge Materials and Performance

Selecting a Gauge Material to Match Application Requirements

Strain gauges are sensors that rely on the change in resistance of the gauge to determine the strain within the part that the gauge is attached to.

A strain gauge consists of a wire matrix to maximize the effects of the strain on the gauge. Selecting the right gauge wire material for an application depends on the degree of strain to be measured, environmental variables such as temperature, and non-material considerations such as cost.

Strain Gauge Alloys

There are four primary alloys used in strain gauge fabrication: Constantan, annealed Constantan, isoelastic, and Karma.

• Constantan - Constantan is a copper-nickel alloy and the most commonly used alloy used for strain gauge matrices. Constantan has a very stable resistance over a large temperature range, making it the perfect choice for general-purpose gauges. Constantan can be processed to provide temperature compensation for a variety of coefficients of thermal expansion. Constantan-based gauges are generally the least expensive of the strain gauge alloys, due to their wide-scale usage.
• Annealed Constantan - Annealed Constantan alloy is used when large strains, usually over 5%, are expected. This alloy will experience resistance drift under high cyclic strain, so gauges that use annealed Constantan should not be used in cyclic loading applications.
• Isoelastic - Isoelastic alloy is used for dynamic strain measurements and cyclic loading applications. Gauges that use isoelastic alloys have a larger gauge factor than general purpose gauges, and provide a larger signal to noise ratio. Isoelastic alloy is sensitive to thermal effects, so it is best used in temperature-controlled environments
• Karma - Karma alloy gauges are extremely stable over time and are appropriate for long term monitoring applications. Karma alloy provides better accuracy at extreme temperatures than general purpose gauges, but is difficult to solder. Karma alloy can be processed to provide temperature compensation for a variety of coefficients of thermal expansion.

In addition to these alloys, some other materials are used for very specific applications. For example, titanium wire is used in very high temperature applications due to its high melting point.

Choosing a Strain Gauge Material

Because each of the alloys provide different benefits, the variables within the measurement application need to be considered when choosing a specific gauge material. Some considerations and the appropriate alloy choice include:

• General Purpose Applications - Constantan
• High-Strain Applications - Annealed Constantan
• Dynamic Strain Measurements - Isoelastic
• Long-Term Monitoring - Karma
• Extreme Temperatures - Karma
• Temperature Compensation Needed - Constantan, Karma

The accuracy of a strain gauge measurement depends on the ability of the gauge to maintain stability under environmental and operational effects. Choosing the right strain gauge alloy for a specific measurement application will result in more stable and accurate data.

Sources

eFunda website - Strain Gauge: Materials

Vishay-Micromeasurements website

Vishay Tech Note TN505-4: Strain Gauge Selection