THERMAL RESPONSE IN FOIL STRAIN GAGES

In experimental stress analysis, heat and cold can be complicating factors. Changes in temperature can change the stability of test parts, causing them to expand when heated and contract when cooled. So, if you bond a strain gauge sensor to the surface of a test part that has been affected by a temperature change, the gauge can potentially produce a response that is independent of any strain due to load and is thus inaccurate.

How do strain gauge manufacturers

mitigate the effect of thermal

response?

During manufacturing, Micro-Measurements goes to great lengths to eliminate this temperature response by heat-treating the strain gauge foil long before it's used to make a strain gauge. Over the years, Micro-Measurements has perfected the process for heat-treating the foil to match the heat response of various materials—from steel to copper to aluminum to carbon fiber, and even some plastics. This matching is known as the self-temperature compensation. The small residual response left over after heat-treating is referred to as apparent strain, or thermal output. Our goal is to reduce the apparent strain to negligible numbers that stay around room temperature, and to keep the total response for most applications at less than 100 µɛ.

So, what causes strain gage

thermal response?

There are two culprits in thermal response:

• The difference in coefficient of thermal expansion between the test material and the strain gage foil.
• A change in the foil's Temperature Coefficient of Resistance (TCR).

These factors make the thermal response non-linear. The good news is that the thermal output data is provided for you on the strain gage package. Micro-Measurements will supply the coefficients for a fourth-order polynomial to fit the curve that is generated by experimental data. In testing, once you quantify the thermal output—either using the data that we provide or generating your own—you can subtract it from your strain gage measurements to remove the error. Problem solved!

What else do I need to know about

strain gauge thermal output?

There are a few key points to keep in mind when doing factoring strain gauge thermal output into your stress analysis testing:

• First, to use the data provided by the strain gauge manufacturer, the strain gauge must be from the same foil lot bonded to the same material as our reference. Otherwise, you may see some inaccuracies.

• Second, if the temperature does not change while you are taking the measurements, thermal output is not an issue and there is no correction needed.

• Lastly, for the most accurate correction, generate your own data using your specific strain gages in the same environment you will use for testing.

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