Tech Briefs

Strain gauge and temperature-compensation element are exposed to the same temperature.

A relatively simple and inexpensive method of fabricating a temp- erature-compensation element for high- temperature strain gauges has been devised. This element, connected in the adjacent arm of a Wheatstone bridge, provides temperature compensation for an active strain gauge attached to the substrate. A method for accurately measuring structural static strains in harsh environments is an important requirement for future flight research of hypersonic vehicles and ground test articles. Sturdy, flight-worthy strain sensors must be developed for attachment to superalloys, new composite materials, and thermal-protection systems. With little deviation from standard Rokide flame-spray installation procedures, preliminary tests indicate viable data can be produced to operating temperatures of at least 1,700 °F (927 °C).

Figure 1. The Active Strain Gauge and the Temperature- Compensation Element are labeled “RActive” and “RComp,” respectively. The straps that hold down the compensation element have been removed, and the gauge has been lifted for this photograph. Contact with the substrate must be maintained to ensure thermal conduction in the presence of transient heating.
Figure 2. These Apparent-Strain Curves obtained by a half-bridge strain gauge utilizing the presented temperature- compensation element exhibit little zero shift, a low rate of drift at 1,500 °F (≈820 °C), less nonlinearity (in comparison with uncompensated strain gauge), a high degree of cycle-to-cycle repeatability, and no cycle-to-cycle slope changes.
In the present method, the temperature-compensation element is encapsulated and insulated in alumina by the Rokide flame-spray process and used as an inactive element in a half- bridge configuration. An inactive element, or gauge, is often also referred to as a “dummy gauge” because it does not sense surface strains; in other words, there is no mechanical strain transfer from the substrate to the gauge filament. The temperature-compensation element is mounted in close proximity to the attached, or active, strain gauge. Adequate surface contact of the compensation element to the test article must be achieved in order to maintain good thermal conductivity. However, unlike the active strain gauge, the temperature- compensation element is not rigidly attached to the substrate which is to be measured; instead, the temperature-compensation element (see Figure 1) is attached flexibly to the substrate using nickel/aluminum-alloy straps.

Configured as a half-bridge, the temperature-compensation element is connected in an arm of a Wheatstone bridge adjacent to an arm containing the active strain gauge. The temperature-compensation element does not sense mechanical surface strains, but it is subjected to the same temperatures as is the active strain gauge. Inasmuch as equal changes in adjacent arms of a Wheatstone bridge cancel, the equal temperature-induced components of the changes in the resistance of the active strain gauge and the temperature-compensation element cancel, leaving a Wheatstone-bridge output indicative of only the surface strain in the substrate.

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