Experimental stress analysis is based on the principle of strain measurement. Initially, bulky mechanical devices were used for measuring strain that displayed strain using a lever ratio of one thousand or more. These devices were the only type available for performing measurements so essential for stress analysis.
The Measurement System
Normally, the strains determined with strain gauges are very small. The change in resistance is also minimal and direct measurement is not possible.
The strain gage must be included in a measurement system where accurate determination of the strain gage's change of resistance is possible.
The components include a strain gauge, which converts mechanical strain into a change in electrical resistance and a measuring circuit that is shown as the Wheatstone bridge having the strain gage as one arm.
Both the measuring circuit and the strain gauge are passive components. When the strain gage's resistance changes due to a strain, the bridge circuit loses its symmetry and loses its balance. A bridge output voltage is obtained which is proportional to the bridge's unbalance.
The measuring system includes an amplifier as the third component which amplifies the bridge output voltage to a level suitable for indicating instruments.
In some cases amplifiers are designed to give an output current proportional to the bridge output voltage, but some models can provide either voltage or current outputs. The fourth component in the measuring system is the display. Here the output signal of the amplifier is converted into a form which can be understood by the user.
These are just the essential elements. The system is extended through the use of additional equipment, e.g. scanners, filters, peak value storage, limit switches, transient recorders, etc.
Types of Strain Gauges
Metal Strain Gages
The change of resistance in an electrical conductor due to the impact of mechanical stress, as discovered by Wheatstone, became the subject of study in the 1930s.
A number of advancements were experimented, and finally Ruge took a very thin resistance wire, stuck it in a meander shape on to some thin tissue paper and terminated the ends with thicker connections. In order to study the characteristics of this prototype device, he glued it to a bending beam and compared the measurements with a traditional strain measuring device. He discovered a good correlation with a linear relationship between strain and the displayed values over the complete measurement range, both with positive and with negative, i.e. compressive, strain, including good zero point stability. Hence the “electrical resistance strain gage with bonded grid” was invented (figure 1).
Figure 1. Arthur Claude Ruge, the inventor of the strain gage, working on his measurements.
Semiconductor Strain Gages
There are other kinds of electrical resistive strain gauges other than metal strain gauges. The measurement principle of semiconductor gauges is based on the semiconductor piezoresistive effect discovered by C.S. Smith in 1954 (figure 2).
First germanium was used and later silicon. Semiconductor strain gauges are just like metal strain gauges in construction.
The measuring element includes a strip a few tenths of a millimeter wide and a few hundredths of a millimeter thick which is fixed to an insulating carrier foil and is provided with connecting leads.
A thin gold wire suppresses diode effects as a connection between the semiconductor element and the connecting strips. Semiconductor strain gauges are used for measuring very small strains. The large signal given by this kind of strain gauge is beneficial in the presence of strong interference fields.
Figure 2. Diagrammatic representation of a semiconductor strain gage.
Vapor-Deposited (Thin-Film) Strain Gages
This is provided by vapor deposition techniques. In this case, the measuring element is directly deposited onto the measurement point under a vacuum by the vaporization of the alloy constituents. The applications are restricted to the production of transducers.
Figure 3. Thin-film strain gage on the spring body of a transducer.
Capacitive Strain Gages
The capacitive strain gage (figure 4) is considered as an alternative to conventional strain gages for use at high temperatures beyond the limit of metal strain gages. There are three known versions presently:
- A British development by the Central Electricity Research Laboratories (C.E.R.L.) in cooperation with the company Planer. Here a plate capacitor is used where the plate separation changes depending on the strain to be measured.
- An American development from Boeing Aircraft which is constructed as a differential capacitor.
- A German development from Interatom. This is also constructed as a plate capacitor. Capacitive transducers are fixed to the test object with spot welding techniques. It is possible to obtain good results with capacitive strain gages in the temperature range up to about 500°C. The results are in the range up to 800°C.
Figure 4. Diagram of a capacitive strain gage from Interatom.
Piezoelectric Strain Gages
Piezoelectric strain gages are active devices. The strain sensing material is barium titanate.
When compared to piezoelectric transducers that use quartz as the sensing material, the strain gage provides an electrical charge on its surfaces which is proportional to strain and that can be measured with charge amplifiers.
Photoelastic Strain Gages
A strip fabricated from optically stressed active material exhibits an isochromatic field as a result of a “frozen”, continually increasing stress.
The isochromatics get displaced as a result of the strain. The degree of displacement which is read off a scale is a measure of the strain. These kind of strain gages are made in the USA. They do not have any specific benefits and are no longer commercially available.
Mechanical Strain Gages
These devices are seen infrequently but have a long tradition. They can usually only be applied to larger objects due to their construction. The measurement effect is shown by a trace scratched on a metal plate or on a glass cylinder, which can only be studied at the end of the test under a microscope. This disadvantage is offset somewhat by the large temperature range.
Figure 5. Mechanical extensometer.
This information has been sourced, reviewed and adapted from materials provided by HBM, Inc.
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