In a strain gage application, the carrier matrix and the adhesive must work together to transmit the strains from
the specimen to the grid. In addition, they serve as an electrical insulator and heat dissipator.
The three primary factors influencing gage selection are operating temperature, state of strain (gradient,
magnitude, and time dependence) and stability required.
Because of its outstanding sensitivity, the Wheatstone bridge circuit is the most frequently used circuit
for static strain measurements. Ideally, the strain gage is the only resistor in the circuit that varies and then only due
to a change in strain on the surface.
There are two main methods used to indicate the change in resistance caused by strain on a gage in a
Wheatstone bridge. Often, an indicator will rebalance the bridge, displaying the change in resistance required in micro-strain.
the second method installs an indicator, calibrated in micro-strain, that responds to the voltage output of the bridge.
This method assumes a linear relationship between voltage out and strain, an initially balanced bridge, and known V in.
In reality, the V out-strain relationship is nonlinear, but for strains up to a few thousand micro-strain, the error is not significant.
Potential Error Sources
In a stress analysis application, the entire gage installation cannot be calibrated as can some pressure
transducers. Therefore, it is important to examine potential error sources prior to taking data.
Prime Strain Gage Selection Considerations
- Gage Length
- Number of Gages in Gage Pattern
- Arrangement of Gages in Gage Pattern
- Grid Resistance
- Strain Sensitive Alloy
- Carrier Material
- Gage Width
- Solder Tab Type
- Configuration of Solder Tab
- Availability
Some gages may be damaged during installation. It is important therefore to check the resistance of the
strain gage prior to stress.
Electrical noise and interference may alter your readings. Shielded leads and adequately insulating
coatings may prevent these problems. A value of less than 500 M ohms (using an ohmmeter) usually indicates surface
contamination.
Thermally induced voltages are caused by thermocouple effects at the junction of dissimilar metals
within the measurement circuit. Magnetically induced voltages may occur when the wiring is located in a time varying magnetic
field. Magnetic induction can be controlled by using twisted lead wires and forming minimum but equal loop areas in each
side of the bridge.
Temperature effects on gage resistance and gage factor should be compensated for as well. This may
require measurement of temperature at the gage itself, using thermocouples, thermistors, or RTDs. Most metallic gage alloys,
however, exhibit a nearly linear gage factor variation with temperature over a broad range which is less than ±1% within
±100°C.
Strain gage dimensions
The active grid length, in the case of foil gages, is the net grid length without the tabs and
comprises the return loops of the wire gages. The carrier, dimensions are designed by OMEGA for the optimum
function of the strain gage.
Strain gage resistance
The resistance of a strain gage is defined as the electrical resistance measured between
the two metal ribbons or contact areas intended for the connection of measurement cables. The range comprises
strain gages with a nominal resistance of 120, 350, 600, and 700 Ohms.
Gage Factor (Strain Sensitivity)
The strain sensitivity k of a strain gage is the proportionality factor between the relative
change of the resistance.
The strain sensitivity is a figure without dimension and is generally called gage factor.
The gage factor of each production lot is determined by sample measurements and is given on each package as
the nominal value with its tolerance. Reference Temperature The reference temperature is the ambient temperature
for which the technical data of the strain gages are valid, unless temperature ranges are given. The technical data
quoted for strain gages are based on a reference temperature of 23°C.
Temperature Characteristic
Temperature dependent changes of the specific strain gage grid resistance occur in the applied gage
owing to the linear thermal expansion coefficients of the grid and specimen materials. These resistance changes appear
to be mechanical strain in the specimen. The representation of the apparent strain as a function of temperature is called
the temperature characteristic of the strain gage application. In order to keep apparent strain through temperature changes
as small as possible, each strain gage is matched during the production to a certain linear thermal expansion coefficient.
OMEGA offers strain gages with temperature characteristics matched to ferritic steel and aluminum.
Service Temperature Range
The service temperature range is the range of ambient temperature where the use of the strain
gages is permitted without permanent changes of the measurement properties. Service temperature ranges are different
whether static or dynamic values are to be sensed.
Maximum Permitted RMS Bridge Energizing Voltage
The maximum values quoted are only permitted for appropriate application on materials with good
heat conduction (e.g., steel of sufficient thickness) if room temperature is not exceeded. In other cases temperature
rise in the measuring grid area may lead to measurement errors. Measurements on plastics and other materials with bad
heat conduction require the reduction of the energizing voltage or the duty cycle (pulsed operation).
THE STRAIN GAGE IS ONE OF THE MOST IMPORTANT TOOLS
of the electrical measurement
technique applied to the measurement of mechanical quantities. As their name indicates, they are used for the measurement of
strain. As a technical term "strain" consists of tensile and compressive strain, distinguished by a positive or
negative sign. Thus, strain gages can be used to pick up expansion as well as contraction. The strain of a body is
always caused by an external influence or an internal effect. Strain might be caused by forces, pressures, moments,
heat, structural changes of the material and the like. If certain conditions are fulfilled, the amount or the value
of the influencing quantity can be derived from the measured strain value. In experimental stress analysis this
feature is widely used. Experimental stress analysis uses the strain values measured on the surface of a specimen
or structural part to state the stress in the material and also to predict its safety and endurance. Special
transducers can be designed for the measurement of forces or other derived quantities, e.g., moments, pressures,
accelerations, and displacements, vibrations and others. The transducer generally contains a pressure sensitive
diaphragm with strain gages bonded to it.
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