The present invention relates to a computer assisted system for calibration of
high temperature strain gauges at 600°C.
BACKGROUND OF THE INVENTION
The strain gauges are used in the stress analysis of engineering components.
The dedicated procedure for the strain gauging needs to be followed for the
better results. Before going for the experimental stress analysis the strain gauges
and its analysis technique is usually calibrated. This exercise is simple when the
analysis is carried out at room temperature. When the analysis is carried out at
higher temperatures, the thermal strain is to be considered which is included in
the measured strain. Especially at very high temperatures such as at 600 deg C
and above, the strain becomes nonlinear and segregation of thermal and
mechanical strain becomes complicated. Hence calibration of the strain gauges
when the component is at high temperature becomes essential.

US Patent 2357643 teaches a calibration system for strain gauge at room
temperature. The system is based on the bending of specimen manually. The
invention is not suitable to the calibration of high temperature strain gauges.
US patent 3005332 describes a method and an apparatus for calibrating strain
gauges at extreme conditions. The constant strength cantilever beam is used for
the calibration where simultaneously many number of gauges can be calibrated.
However when the test is carried out at very high temperature such as 600°C,
the system is not suitable as the whole set up will be heated instead of heating
the specimen alone which leads to error in the calibration results.
US Patent 3293909 discloses a method for manufacture of high temperature
strain gauges suitable up to 1000°F which are sensitive and accurate.
US Patent 3448607 teaches a strain gauge temperature compensation system.
The system includes at least one strain gauge connected in a bridge with other
resistance members, preferably strain gauges, to compensate for temperature
changes. The bridge is provided with characteristics to maintain a constant
current from a source of constant voltage even when the member is subjected to
a variable strain. The output voltage from the bridge provides an indication of
such variations in strain.

US Patent 6973837 B2 describes a temperature compensated strain sensing
apparatus or installation, and in particular to such an apparatus or installation for
use in the presence of thermal gradients.
All of these prior art are related to the high temperature strain gauge
developments and a method of compensating high temperature strains.
US Patent 3956919 discloses a high temperature strain gauge calibration
fixture. The fixture consists of a dead weight loading mechanism to apply
bending load on the specimen and to the strain gauge also. The tensile load
calibration is not brought out clearly. The loading mechanism is manual and
controlled strain loading is not feasible.
OBJECTS OF THE INVENTTpN
It is therefore an object of the invention is to propose a computer assisted
system for calibration of high temperature strain gauges to 600°C and above.

Another object of the invention is to propose a computer assisted system for
calibration of high temperature strain gauges to 600°C and above which can
calibrate high temperature strain gauges with controlled strain mode
SUMMARY OF THE INVENTION
According to the invention, a system for the calibration of high temperature
strain gauges at various temperatures and loads is disclosed. The high
temperature strain gauge is to be welded/pasted with the specimen before
positioning of the specimen on the system. The specimen to be tested is
mounted on a computer controlled machine through which the load is applied.
The loading machine is having job mounting jaws capable of holding flat and
round specimens. One of the jaws is fixed and other is movable. Different
specimens according to the requirement can also be held with proper selection of
fixture. The load to be calibrated on the specimen is applied by load control
system. The load is transferred to the specimen through the movable jaw. The
load is applied and controlled through a computer program so that an accurate
control on both load strain can be achieved during the calibration of high
temperature strain gauges.

A computerized calibration system for the high temperature stress analysis has
the capability to calibrate the mechanical and thermal strains separately and
together with online data capturing facility which enables the measurement of
transient strain during calibration.
The load control system available in the calibration system works based on the
computer program according to the calibration requirement. The load could
either be the tensile or compressive in nature. Also the transient and fluctuating
loads can be applied through the system. The advantage of applying load
through the computer controlled program enables the better calibration as both
load and strain can be controlled precisely along with temperature.
After the proper position of the specimen on the system, a sliding cylindrical
furnace is moved through the specimen and the furnace is positioned exactly at
the specimen location. The electrical circuits for the strain gauge and
thermocouple is brought out through opening provided at the end of the
cylindrical furnace. The thermocouple circuit is used to monitor the temperature
around the specimen on which gauge is mounted. Also the thermocouple
feedback given to the control system which maintains the preset temperatures
for the calibration. The electrical furnace can be operated to different
temperatures.

The strain gauge circuitry is connected to the strain data acquisition system. The
strain control system can capture the strain during the application of both
thermal and mechanical loads. A computer attached with the system displays
both the strain gauge reading and the strain applied on the specimen by the load
control system. Because of the integration of strain gauge, load control system
and furnace control, both the thermal and mechanical strains can be calibrated in
a single trial. Finally, the size of the specimen, applied strain on the specimen,
strain gauge reading and temperature on which the testing was carried out all
are printed out by the system.
?PTEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows a schematic view of a high temperature strain gauge calibration
system according to the invention.
Figure 2 - schematic view of a sample test specimen with the strain gauge
calibrating system.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
According to the invention, there is provided an improved method and a
computer assisted system for calibration of high temperature strain gauge, by
which a computer controlled calibration is carried out to avoid any uncertainties
during the calibration process.
The high temperature strain gauge calibration system consists of a load control
device, a furnace control device, a strain data acquisition device and a data
display device. The high temperature strain gauge to be calibrated is
pasted/welded on a test specimen. The specimen is positioned on the load
control device in such that, it is located on the center of the furnace. Strain
gauge circuitry is connected to the strain data acquisition device. Furnace ends
are closed. Furnace power switch is on to reach a preset temperature and the
computer controlled load is applied once the preset temperature is reached. Both
mechanical and thermal strains are captured by the data acquisition device. The
strain captured by the strain meter and the load control device are compared for
calibration.

The constructional details pertaining to the high temperature strain gauge
calibration system is explained with reference to the accompanying drawings.
The high temperature strain gauge calibration system comprises :
1) a High temperature strain gauge,
2) a Specimen with the strain gauge.
3) a Load control device,

4) Fixed Jaws,
5) Moving Jaws,
6) Sliding cylindrical furnace,

7) Furnace control means,
8) Strain gauge circuitry,
9) a Strain data acquisition device,

10) a Computer apparatus, and
11) a Table
The typical high temperature strain gauge calibration system is shown in
Figure. 1. The strain gauge (1) is pasted/welded on the sample specimen (2). The
specimen (2) is to be positioned on the load control device by fixing the
specimen between fixed jaw (4) and moving jaw (5). After the specimen (2) is
fixed on the load control device, the sliding cylindrical furnace (6) is properly
positioned in such a way that it encloses the specimen (2) completely. The
temperature of the sliding cylindrical furnace (6) is controlled by the furnace
control means (7). The temperature in which the high temperature strain gauge
is to be calibrated is set on the furnace control means (7). The strain gauge
circuitry (8) is brought out of the sliding cylindrical furnace (6) through an
opening at the end of the furnace. The strain gauge circuitry (8) is connected to
the strain data acquisition device (9). The strain gauge circuitry (8) and the load
control device (3) are integrated by means of a computer apparatus (10).
The loading plan according to the calibration requirement is set on a computer
program which is then loaded to the computer (10). The different types of loads
and loading pattern can be selected from the computer panel (10). The load

control device (3) activates and apply the load on the specimen (2) through the
moving jaw (5). The moving jaw (5) clutches the specimen (2) after the
specimen has reached to its preset temperature. The corresponding strain
variation on the specimen (2) is recorded by the computer (10) through
feedback control panel (7) of the load control device (3). By the same time the
strain experienced by the specimen (2) is captured by the strain gauge (1). This
includes the thermal strain and mechanical strain experienced by the specimen
(2). The total strain captured by the strain gauge is recorded on the computer
(10) through the strain data acquisition device (9). The strain recorded by the
load control device (3) is subtracted from the strain data acquisition system (9)
readings to get the thermal strain experienced by the specimen if the strain
gauges (1) are not temperature compensated. The strain captured by the strain
data acquisition device and strain captured by the load control feedback are
same if the gauges (1) are self-temperature compensated. The gauge can be
calibrated for the different temperature and different loads. All the control
systems are table (11) mounted.

WE CLAIM :
1. A computer assisted system for calibration of high temperature strain
gauges to 600°C and above, comprising:
t
- a load control device (3) having a feedback panel (7), a fixed jaw (4), and
a moving jaw (5);
- a high temperature strain gauge (1) to be calibrated pasted or welded on
a sample specimen (2), the strain gauge (1) having circuitry (8);
- a sliding furnace (6) controllable by said feedback panel (7), the sliding
furnace (6) being disposable to entirely cover the specimen (1, 2) when
fixed between said fixed and moving jaws (4, 5) of the load control device
(3);
- a strain data acquisition device (9) connectable to said strain gauge
circuitry (8);
- a computer apparatus (10) having a program incorporated with loading
including calibration instructions,

wherein the load control device (3) is activated to apply load on the
specimen (1) when reached at the preset temperature and corresponding
strain variation on the specimen (2) is recorded through the load control
device (3) by the computer apparatus (10) wherein the thermo-
mechanical strain experienced by the specimen is simultaneously captured
through the strain data acquisition device (9) by the computer apparatus
(10), the said two sets of data as recorded are compared to calibrate or
otherwise the high temperature strain gauge (1).
2. The system as claimed in claim 1, wherein the device are integrated via
the computer apparatus (10).