ASTM International

Standard Test Methods for Performance Characteristics of Metallic Bonded Resistance Strain Gages
 N E251

Annotation

The purpose of these test methods are to provide uniform test methods for the determination of strain gage performance characteristics. Suggested testing equipment designs are included.

Test Methods E251 describes methods and procedures for determining five strain gage parameters:

Strain gages are very sensitive devices with essentially infinite resolution. Their response to strain, however, is low and great care must be exercised in their use. The performance characteristics identified by these test methods must be known to an acceptable accuracy to obtain meaningful results in field applications.

Strain gage resistance is used to balance instrumentation circuits and to provide a reference value for measurements since all data are related to a change in the gage resistance from a known reference value.

Gage factor is the transfer function of a strain gage. It relates resistance change in the gage and strain to which it is subjected. Accuracy of strain gage data can be no better than the precision of the gage factor.

Changes in gage factor as temperature varies also affect accuracy although to a much lesser degree since variations are usually small.

Transverse sensitivity is a measure of the strain gage's response to strains perpendicular to its measurement axis. Although transverse sensitivity is usually much less than 10 % of the gage factor, large errors can occur if the value is not known with reasonable precision.

Thermal output is the response of a strain gage to temperature changes. Thermal output is an additive (not multiplicative) error. Therefore, it can often be much larger than the gage output from structural loading. To correct for these effects, thermal output must be determined from gages bonded to specimens of the same material on which the tests are to run, often to the test structure itself. 1.4 Bonded resistance strain gages differ from extensometers in that they measure average unit elongation ( L/L) over a nominal gage length rather than total elongation between definite gauge points. Practice E83 is not applicable to these gages.