Easy-to-use high-temperature strain-sensor systems based on strain gauges made from Pd/Cr-alloy wires have been developed. These systems include strain-gauge units comprising Pd/Cr wires bonded to high-temperature metal-alloy shims by flame spraying of ceramic materials. Optionally, a strain-gauge unit can be supplied alone, but ordinarily, it is delivered as part of a system that also includes an integral, weldable-terminal high-temperature cable and a bridge-circuit-completion module at the cool end of the cable (see figure).

A Strain-Measurement System comprises a strain-gauge unit, a cable, and bridge-circuit-completion module. All critical steps in fabrication, installation, stabilization, and calibration are performed under controlled conditions prior to delivery of the system.

Strain gauges made from Pd/Cr-alloy wires, and techniques and materials used to fabricate them, have been described in a number of previous articles in NASA Tech Briefs. Pd/Cr-wire strain gauges can be used to measure static strains at temperatures up to 1,400 °F (760 °C). However, the successful use of these strain gauges entailed considerable difficulty prior to the development of the present systems. This is because in order to realize the full potential of Pd/Cr strain gauges, it is necessary to adhere strictly to installation, stabilization (heat treatment), and calibration procedures unlike those of conventional practice for lower-temperature strain gauges. The major advantage afforded by the development of present systems does not lie in any single fundamental physical concept, but, rather in the establishment of the practice of performing all critical steps in a controlled laboratory environment at the factory, before use in the field.

The prefabrication of strain-gauge units containing Pd/Cr wires bonded to shims overcomes the installation difficulty. The critical flame-spraying steps are performed under controlled factory conditions, rather than by the end user. Each unit includes a margin of shim material around its periphery; the margin can be used to attach the strain-gauge to a test structure (a structure on which strain is to be measured) by ordinary spot-welding at the end-use site.

Prior to calibration, each strain-gauge unit is prestabilized by heating at a temperature of 1,440 °F (782 °C) for 50 hours. Care is taken to maintain this temperature within a narrow margin of error; underheating causes changes in electrical resistance to be smaller than expected, while even momentary overheating destroys a temperature-compensation feature. In other words, both underheating and overheating introduce calibration errors.

To prepare for calibration, a strain-gauge unit is tack-welded to a bar of the same material on which it is to be used to measure strain. Strain readings are taken at temperature intervals spanning the full test temperature range, using a calibrated ballast (bridge-completion) resistor inserted in the bridge-circuit-completion module. If necessary, the calibration procedure is repeated with different bridge-completion resistors until a satisfactory calibration curve is obtained.

Once calibration has been performed in the laboratory, the strain-measurement system (including the final chosen bridge-completion resistor) is ready for use. The strain-gauge unit can be spot-welded to a test structure, and the system plugged into any common strain-measuring instrument and operated without need for further stabilization or calibration.

This work was done by J.F. Lei of Lewis Research Center and S. P. Wnuk, Jr., and V. P. Wnuk of Hitec Products, Inc. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Lewis Research Center
Commercial Technology Office
Attn: Tech Brief Patent Status
Mail Stop 7 - 3
21000 Brookpark Road
Cleveland
Ohio 44135.

Refer to LEW-16572.


NASA Tech Briefs Magazine

This article first appeared in the August, 1998 issue of NASA Tech Briefs Magazine.

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