ASTM E251-92(2009)

Strain gauges are the most widely used devices for the determination of materials, properties and for analyzing stresses in structures. However, performance parameters of strain gauges are affected by both the materials from which they are made and their geometric design. These test methods detail the minimum information that must accompany strain gauges if they are to be used with acceptable accuracy of measurement.

Most performance parameters of strain gauges require mechanical testing that is destructive. Since test gauges cannot be used again, it is necessary to treat data statistically and then apply values to the remaining population from the same lot or batch. Failure to acknowledge the resulting uncertainties can have serious repercussions. Resistance measurement is non-destructive and can be made for each gauge.

Properly designed and manufactured strain gauges, whose properties have been accurately determined and with appropriate uncertainties applied, represent powerful measurement tools. They can determine small dimensional changes in structures with excellent accuracy, far beyond that of other known devices. It is important to recognize, however, that individual strain gauges cannot be calibrated. If calibration and traceability to a standard are required, strain gauges should not be employed.

To be used, strain gauges must be bonded to a structure. Good results depend heavily on the materials used to clean the bonding surface, to bond the gauge, and to provide a protective coating. Skill of the installer is another major factor in success. Finally, instrumentation systems must be carefully designed to assure that they do not unduly degrade the performance of the gauges. In many cases, it is impossible to achieve this goal. If so, allowance must be made when considering accuracy of data. Test conditions can, in some instances, be so severe that error signals from strain gauge systems far exceed those from the structural deformations to be measured. Great care must be exercised in documenting magnitudes of error signals so that realistic values can be placed on associated uncertainties.

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

1.2 Test Methods E 251 describes methods and procedures for determining five strain gauge parameters:

1.3 Strain gauges 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 gauge 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 gauge resistance from a known reference value.

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

1.3.3 Changes in gauge 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 gauge's response to strains perpendicular to its measurement axis. Although transverse sensitivity is usually much less than 10 % of the gauge 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 gauge to temperature changes. Thermal output is an additive (not multiplicative) error. Therefore, it can often be much larger than the gauge output from structural loading. To correct for these effects, thermal output must be determined from gauges 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 gauges differ from extensometers in that they measure average unit elongation (ΔL/L) over a nominal gauge length rather than total elongation between definite gauge points. Practice E 83 is not applicable to these gauges.

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

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

1.7 This standard does not purport to address all of the safety concerns, 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.