A strain gage is a sensor that is used to measure strain generated in a body because of stress that is applied on it. Strain Gages measure strain which is a physically detectable quantity as it is the change in length or shape of a body as compared to its initial length or shape, and then by using the value of that strain, stress can be calculated if needed.
There are many types of Strain Gages. However, they all can be classified into either of following four types, on the basis of their working principle and construction:
- Mechanical Strain Gages
- Optical Strain Gages
- Acoustical Strain Gages
- Electrical Strain Gages
These four types exist because of various nature of conditions in which strain gages are needed to be operational. So, for some conditions one type strain gages are suitable while for other condition, some other type works better than the rest. However, still Electrical Strain gages and specifically, the Resistance based electrical strain gages are the most used in the world.
Mechanical Strain Gages
Mechanical Strain Gages, as the name suggests, measure strain produced in a body in the form of some mechanical properties of a material.
Mechanical Strain Gages are not commonly used nowadays, as the Electrical Strain Gages are much better in terms accuracy, cost and ease of use, comparatively. However, Extensometers, which are mechanical strain gages are still widely used in tensile tests of materials for obtaining the stress-strain curves.
The other example of mechanical strain gage which is still used is called a Demec Gage or Whittemore Gage.
The primary reason for still using mechanical strain gages in spite of their lower accuracy and ease of use as compared to electrical strain gages, is that they are independent of time and temperature. This is because electrical strain gages and more specifically, resistance strain gages work on basis of their resistance change with respect to strain generated in it. But resistance also changes as the temperature of environment in which gage is operating.
An extensometer is usually equipped with a knife edge and wire spring that that forces the knife edge to move along the material points in case of extension or compression.
This movement of specimen moves the arms of extensometer, and as these arms move, they bend a small cross-flexural element ensuring center-point bending over the entire range of the extensometer. The cross-flexural member, which is the sensing element, also provides good lateral stability and requires low actuating forces. The extensometer provides an accurate response to specimen strain with maximum nonlinearity of 0.3 percent of range and maximum hysteresis of only 0.1 percent of range.
The demountable mechanical strain gauge (DEMEC) was developed at the Cement and Concrete Association to enable strain measurements to be made at different parts of a structure using a single instrument.
The DEMEC consists of a standard or a digital dial gauge attached to an Invar bar.
A fixed conical point is mounted at one end of the bar, and a moving conical point is mounted on a knife edge pivot at the opposite end. The pivoting movement of this second conical point is measured by the dial gauge.
A setting out bar is used to position pre-drilled stainless-steel discs which are attached to the structure using a suitable adhesive.
Each time a reading has to be taken, the conical points of the gauge are inserted into the holes in the discs and the reading on the dial gauge noted. In this way, strain changes in the structure are converted into a change in the reading on the dial gauge.
The gauge has been designed so that only minor temperature corrections are required for changes in ambient temperature, and an Invar reference bar is provided for this purpose.
Although originally designed for use on concrete structures, the gauge is just as useful on any type of structure. In the case of steel structures, the locating holes can be drilled directly into the steel if required.
OPTICAL STRAIN GAGES
Optical strain gauges (also called fiber optical (strain) sensors or FOS, optical (strain) sensors or Fiber Bragg Grating sensors) are used to measure strain, but can easily be integrated into different types of transducers, such as in temperature, acceleration or displacement. Compared to traditional electrical strain gauges, optical strain gauges do not need electricity. Instead, the technology is based on light that propagates through a fiber. Therefore, the sensors are completely passive and immune, for example, to electromagnetic interference. This is just one of the reasons why optical strain gauges are superior to electrical ones in certain applications.
An optical fiber usually consists of a glass or silica fiber and a plastic coating. It is much like regular telecommunication fiber and can be up to several kilometers long with many measuring points along its length. The fiber itself consists of two layers: the core and a surrounding cladding with lower density. A plastic coating is wrapped around the silica fiber for protection.
A laser is used to send light through the fiber. The two different fiber material densities create a barrier that channels the light inside the fiber so that it doesn’t scatter. For this to work, it’s important that the fiber is not bent too much.
To create the actual strain sensor, the optical fiber is inscribed during production with a so-called Fiber Bragg Grating (FBG). This is basically a pattern of material interferences, which reflects the light differently from the rest of the fiber. For better understanding, you can visualize the fiber as a cylindrical length of transparent material, with a number of thin slices in it. When the light from the laser hits this pattern, certain wavelengths are reflected, while others pass through.
The material interferences, called the slices, are placed at certain intervals. When the fiber is stretched or compressed—and is therefore subjected to positive or negative strain—these intervals change. When the fiber is stretched, it lengthens and the spaces get bigger and vice versa.
Not only does the reflected light take a little longer or shorter to travel back when the FBG is under strain, but the wavelength that is reflected also changes. In scientific terms, the FBG has a certain refractive index. The refractive index of a material describes how much light is bent or refracted when passing through the material. When the grating changes shape due to strain, its refractive index changes as well.
When the optical fiber is applied to a material, it will be strained along with this material. The measured strain will in turn allow an analysis of the mechanical stress in the material, which is the aim of most strain measurements.
To give a practical example, when the fiber is applied to the walls of a long tunnel, it is strained when there is stress in the material of the walls. This can be, for example, due to the vibrations of trains rushing by. When the walls settle or even develop weak points or fissures over the years, this becomes visible from the information about strain and thus mechanical stress acquired by the sensors—a useful early indication as to where maintenance is needed, or measurements, the optical fiber needs to be connected to a so-called interrogator; it continuously sends out light in different wavelengths, one at a time, thus covering a wide spectrum. This is called “sweeping laser”. Light propagates through the fiber, is reflected at some point by a FBG and returns to the interrogator.
Thanks to the different periods of individual FBGs, it is possible to distinguish between the signals of different sensors. The rest of the light is refracted when reaching the end of the fiber so that it doesn’t interfere with the measurement. The actual strain and, in turn, the material stress can be deduced from the raw light signals which return from the FBGs.
ACOUSTICAL STRAIN GAGES
Acoustical strain gages have been employed in a variety of forms ill several countries since the late 1920s. To date, they have been largely supplemented by the electrical-resistance strain gage. However. they arc unique among all forms of strain gages in view of their long-term stability and freedom from drift over extended time periods. The acoustical gage described here, due to R. S. Jarrett. was developed in 1944 and is typical of the devices currently being employed. The strain-measuring system is based on the use of two identical gages identified as a test gage and a reference gage.
One end of a steel wire is attached to the movable knife edge while the other end of the wire passes through a small hole in the fixed knife edge and is attached to a tension screw. The movable knife edge is connected to a second tension screw by a leaf spring. This design permits the initial tension in the wire to be applied without the transmission of load to the knife edges.
The wire passes between the pole pieces of two small electromagnets. One of these magnets is used to keep the wire vibrating at its natural frequency; the other is employed to pick up the frequency of the system. Electrically both magnets operate together in that the signal from the pickup magnet is amplified and fed back into the driving magnet to keep the string excited in its natural frequency. The reference gage is identical to the test gage except that the knife edges are removed and a micrometer is used to tension the wire. A helical spring is employed in series with the wire to permit larger rotations of the micrometer head for small changes of stress in the wire.
To operate the system, the test gage is mounted and adjusted and the reference gage is placed near it to attempt to compensate for temperature effects. Both gages are energized, and each wire emits a musical note. If the frequency of vibrations from the two gages is not the same, beats will occur. The micrometer setting is varied on the reference gage until the beat frequency decreases to zero. The reading of the micrometer is then taken and the strain is applied to the test gage.
The change in tension in the wire of the test gage produces a change in frequencies, and it is necessary to adjust the reference gage with the micrometer until the beats are eliminated. This new micrometer reading is proportional to the strain. If the test gage is located in a remote position and the beat signals from the test and reference gages cannot be developed, it is possible to balance the two gages by using an oscilloscope. The voltage output from the pickup coils of each gage are displayed while operating the oscilloscope in the xy mode. The resulting Lissajous figure provides the readout which permits adjustment of the micrometer on the reference gage to match the frequency of the test gage. The natural frequency I of a wire held between two fixed points is given by the expression
where L = the length of wire between supports
J = the stress in the wire
p = the density of the wire
The sensitivity of this instrument is very high. with possible determinations of displacements of the order of O. t /lin (2.5 IUTI). The range is limited. in general. to about one-thousandth of the wire length before over- or under-stressing of the sensing wire becomes critical. The gage is temperature-sensitive unless the thermal coefficients of expansion of the base and wire are closely matched over the temperature range encountered during a test. Finally. the force required 10 drive the transducer is relatively large, and it should not be employed in high-compliance systems where the large driving force will be detrimental.