An in-house lab was established for developing, testing, and evaluating high-temperature strain gauges and to aid in in-house applications of high-temperature strain instrumentation. The lab is automated to provide computer control of oven temperatures, imposed strain, and data sampling

Hobart, Howard F.

Will, Herbert A.

English

NASA Technical Reports Server

N88" 11146 i
THE LEWIS STRAIN GAUGE LABORATORY - STATUS AND PLANS
Howard F. Hobart and Herbert A. Will
NASA Lewis Research Center
Cleveland, Ohio
Several recent and ongoing HOST programs have the goal of developing electrical
resistance strain gauge systems for measuring static strain in the hot-section compo-
nents (combustors and turbine blades and vanes) of gas-turbine engines. A typical
goal is to be able to measure static strain up to ±2000 microstrain to within ±10 per-
cent at temperatures to 1250 K over a 50-hr period.
The approach to the problem has been to first develop an alloy with suitable
hi$h-temperature characteristics. Once a suitable alloy has been identified, gauges
made of sputtered thin films or small diameter (0.025 nun) wire will be fabricated and
evaluated.
The ongoing HOST programs are a combination of parallel in-house and contract
work to achieve these goals. An in-house lab has been established for developing,
testing, and evaluating high-temPerature strain gauges and to aid in in-house appli-
cations of high-temperature strain instrumentation. To accomplish these tasks, data
must be taken over a wide range of temperatures, times, and strain levels. If done
manually, this becomes a very labor-intensive effort; therefore, the lab is automated
to provide computer control of oven temperatures, imposed strain, and data sampling.
Test Apparatus
The basic equipment of the lab consists of two ovens, a test fixture for holding
a constant strain beam, an actuator for deflecting the beam, and a computer controller
and data system. One oven is dedicated to the test fixture. The computer-controlled
actuator is mounted at the rear of the oven and deflects the beam via an extension
rod. This system allows determination of gauge factor at various strain levels and
temperatures all under computer control. The second oven is presently set up with a
holding-cooling fixture to support an in-house experiment in the burner cyclic ri_
(ref. 1). Both ovens have been modified by the addition of an air mixer to eliminate
temperature gradients.
Computer Controller and Data System
A standard IBM PC is set up as a system controller and data collector for the
testing of high-temperature strain gauges. A block diagram of the strain gauge test-
ing system is shown in figure 1.
The testing system consists of a 10-channel digital thermometer, a 2-channel
digital-to-analog converter in progran_ning the oven temperatures, a digital multimeter
for measuring strain gauge resistance, a single-axis linear actuator for mechanically
bending the strain gauge beam, and a 10-channel strain gauge bridge for measuring
strain directly. The computer communicates with the strain gauge instrumentation by
means of an IEEE-488 bus and an RS-232 serial interface.
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The computer program (written in Fortran 77) is a very versatile time-sequencing
controller. The operator can control each instrument by putting a series of times and
commands in a dataset. The computer program then controls the instruments according
to this series of commmnds. Commands are written in any order in an easy to under-
stand English language format.
Currently, the program records data from the instruments and prints it out on a
sheet of paper. In the future the data will be recorded on a floppy disk for analysis
by other computer programs.
Lab Status and Future Plans
Today the strain gauge laboratory, is nearly operational. The computer control
program is operational, with only a few minor bugs.
Several tests will be conducted in the near future. One involves characterizing
strain gauges mounted on a Hastelloy X plate. Four Chinese-type gauges (ref. 2) and
four Kanthal A-I wire gauges (ref. 3) will be used in this in-house experiment on
combustor simulation. The other test is related to a contractor effort to develop
gauge systems from a palladium - 13-weight-percent-chromium alloy (ref. 4); NASA has
ordered 0.025-mm-diameter wire for winding gauges of this material. Also, a sputter-
ing target of Pd-Cr is in use to develop in-house expertise in the thin film sputter-
ing area.
The strain gauge laboratory as designed is expected to perform these types of
automated tests to yield maximum data with minimum manpower expended.
REFERENCES
I. Thompson, Robert L.: High Temperature Stress-Strain Analysis. Turbine Engine
Hot Section Technology 1984, NASA CP-2339, 1984, pp. 259-272.
2. Hobart, Howard F.: Evaluation Results of the 700 °C Chinese Strain Gages.
Turbine Engine Hot Section Technology 1984, NASA CP-2339, 1984, pp. 77-84.
3. Grant, Howard P.; Anderson, Wilbur L.: Demonstration Test of Burner Liner Strain
Measurements Using Resistance Strain Gages. Turbine Engine Hot Section Technol-
ogy 1984, NASA CP-2339, 1984, pp. 85-100.
4. Hulse, Charles; Bailey, Richard; Grant, Howard: High Temperature Static Strain
Gage Program. Turbine Engine Hot Section Technology 1984, NASA CP-2339, 1984,
pp. 71-76.
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STRAINGAUGETESTINGSYSTEM
I THERMOCOUPLESI
I STRAIN GAUGERESISTANCE
J STRAIN GAUGES I
I OVEN 1I OVEN 2
k -I
J SINGLE AXISACTUATOR
IO-CHANNEL
DIGITAL
THERMOMETER
DIGITAL TO
ANALOG CON-
VERTEROVEN
PROGRAMMER
MULTIMETER
ACTUATOR
CONTROLLER
IO-CHANNEL
STRAIN GAUGE
READOUT
I
RS 232
IEEE 488
F I PERIPHERALSI
CS-85-3288
Fi gure I
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The Lewis Strain Gauge Laboratory: Status and plans

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