ASTM E251-92(1998)

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

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

Section Part I---General Requirements 7 Part II---Resistance at a Reference Temperature 8 Part III---Gage Factor at a Reference Temperature 9 Part IV---Temperature Coefficient of Gage Factor 10 Part V---Transverse Sensitivity 11 Part VI---Thermal Output 12

1.3 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.

1.3.1 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.

1.3.2 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.

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

1.3.4 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.

1.3.5 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 ([delta]L/L) over a nominal gage length rather than total elongation between definite gage points. Practice E83 is not applicable to these gages.

1.5 These test methods do not apply to transducers, such as load cells and extensometers, that use bonded resistance strain gages as sensing elements.

1.6 Strain gages are part of a complex system that includes structure, adhesive, gage, leadwires, instrumentation, and (often) environmental protection. As a result, many things affect the performance of strain gages, including user technique. A further complication is that strain gages once installed normally cannot be reinstalled in another location. Therefore, gage characteristics can be stated only on a statistical basis.

1.7 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

1.8 The values stated in SI units are to be regarded as the standard.