Gage Linearity

A discussion of high-elongation strain gage measurements is not complete without including the subject of gage linearity under plastic straining conditions.

Although constantan alloy is one of the most linear of all strain gage alloys, it does exhibit small changes in gage factor during high elongation.

Without treating this subject in detail, it can be shown in theory that the gage factor of the strain gage approaches a value of 2 +  when the gage alloy is strained beyond its elastic range. The elastic range for constantan is approximately 0.5%.

Using the 2  +  strain guideline, the gage factor at a strain level of 10% would be close to 2.1 in tension and 1.9 in compression. At a 20% strain level, the gage factor approaches 2.2 in tension and 1.8 in compression.

While these numbers are correct in theory, they have not, to our knowledge, been substantiated by actual test results.

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