Early studies: Lord Kelvin, Kearns, Ruge and Simmons

The first studies carried out on gauges were recorded in the year 1856,[1]​ when Lord Kelvin discovered the relationship between the deformation and resistance of conductive and semiconductor wires and noticed how their electrical resistance changed. In the early 1930s, Charles Kearns used strain gauges to measure vibratory strains in high-performance blade propellers; However, these gauges were not very precise, as they had problems in the stability of the resistance (R), which was affected by factors such as temperature, and errors were generated in the deformation measurements. Later, in 1938, Arthur C. Ruge and Edward E. Simmons each discovered that small diameter electrical conductors made of alloys could be adhered to surfaces to calculate deformations, and this is how lamellar gauges were created. This type of element has made great advances and constitutes what is known today as strain gauges[2].

Patent Rights: Caltech v. Simmons. Saunders-Roe

Caltech sued for the gauge patent, but Simmons took his case to the California Supreme Court and won the patent rights in 1949. In 1952 the Saunders-Roe company in the United Kingdom sought to improve the performance of gauges by subjecting them to different environments and testing different types of materials for their manufacture. They patented the thin gauges that are currently used in measuring deformations, in different industrial and scientific areas. In this way, the physical model of the gauge could be improved, significantly reducing size and costs.

Samuel Hunter Christie

In 1832, Samuel Hunter Christie invented the electrical resistance measuring instrument (known as the Wheatstone bridge, because it was improved and popularized in 1843 by the British scientist Charles Wheatstone). The electrical model is composed of four resistances in a closed circuit, where one of them is the one to be evaluated. This is used in order to measure the resistance by balancing the arms of the bridge.

Charles Wheatstone

Sir Charles Wheatstone was a British scientist and inventor who during the Victorian era made a great contribution with his inventions, of which the most significant is the Wheatstone bridge, but he also worked on other inventions such as the stereoscope, the Playfair coding technique and the kaleidophone.

Strain gauges take advantage of the physical property of electrical resistance and its dependence not only on the resistivity of the conductor, which is a property of the material itself, but also on the geometry of the conductor. When an electrical conductor is deformed within its elasticity limit, in such a way that no breakage or permanent deformation occurs in it, it will become narrower and elongated. This fact increases its electrical resistance. Similarly, when the conductor is compressed it shortens and widens, thus reducing its resistance to the passage of electric current. In this way, by measuring the electrical resistance of the gauge, the magnitude of the stress applied to the object can be deduced.

Metallic gauges

Metal gauges are made up of a very thin and fine base, to which a very fine metallic thread is attached that can be wound or folded. The two terminals where the wire ends are attached to the transducers. These gauges have the advantage of a low temperature coefficient, since the decrease in electron mobility is compensated by increasing the temperature with the increase in their concentration. In metal gauges the maximum current is about 25 mA if the support is a good heat conductor, and 5 mA otherwise; In any case, in metal gauges there is a great limitation on the current. The main characteristics of metal gauges under normal conditions establish that their size varies between 0.4 mm and 150 mm, and a variable resistance between 120 Ω and 5000 Ω. Its resistance tolerance is in the range of 0.1% and 0.2%.

The electrical resistance of the metal gauge is given by the relationship between the resistivity and the length with respect to the cross-sectional area.

They can be:

The main alloys used by metal gauges are:

Some of the materials used in the support of the metal gauges can be:

Gauges made with glass fiber reinforced epoxy-phenolic backing materials are a good choice for excellent results and performance when working over a wide range of temperatures.

These materials can be used for both static and dynamic measurements from -269 to +290 °C.

The different series of gauges with epoxyphenolic backing material can be found with different acronyms or abbreviations: WA, WK, SA, SK, WD and SD.

Resistance gauges

This type of gauge is an electrical conductor that, when deformed, increases its resistance, since the conductors become longer and thinner. Using the Wheatstone bridge, we can convert this resistance into absolute voltage. and as long as the strain obeys Hooke's law, the strain and absolute voltage will be linearly related by a factor called the gauge factor.

This type of gauge is generally used in laboratory conditions.

Capacitance gauges

These are associated with geometric characteristics and are used to measure stress and deformation. The electrical properties of the materials used for deformation have negligible electrical properties, so the materials of the capacitance gauges can be calibrated according to the mechanical requirements. This allows them to have better calibrations compared to the electrical type.

Photoelectric gauges

By using an extensometer we can amplify the movement of the specimen, while a ray of light is passed through a variable aperture, acting with the extensometer and directly with a photoelectric cell. As the gauge changes its opening, so does the amount of light that reaches the cell, which means that the intensity of the energy generated by the cell presents a variation, which we can measure, and from this we obtain the deformation.

Semiconductor gauges

In semiconductor gauges there is a semiconductor element instead of the metal wire. The big difference with respect to the other gauges is its size, since it is smaller. The maximum power dissipable in semiconductor gauges is about 250 mW. Semiconductor gauges are capable of withstanding high resistance, their fatigue life is longer and they have lower hysteresis, which is the ability for the material to retain its properties under different stimuli.

There are certain characteristic aspects: under normal conditions, its size varies between 1 mm and 5 mm, its resistance is between approximately 1000 Ω and 5000 Ω, and its resistance tolerance is between 1% and 2%.

The most abundant elements to make these gauges are:

Custom strain gauges are easy to install and provide fairly accurate temperature measurements at the point where said sensors are installed; They can also be used in industrial and military processes involving expansion pipe measurements. These sensors are adapted to measure hoop stress around the diameter of a surface to which it is mounted.[6]​.

Length

It corresponds to the active region or grid length sensitive to the stress of a gauge. Elbows and solder pads are not significantly sensitive to stress, due to their large cross section and low electrical resistance. There are gauges with lengths ranging from 0.2 mm. up to 100 mm.

Concentration of effort

One of the most important factors in determining optimal performance of a strain gauge is its length. For example, when you want to determine the stress measurements on some critical part or structure of a machine, these measurements must be made in the parts where the greatest stresses are concentrated, which are generally those that have a higher degree of fatigue. Strain gauges tend to integrate or average the area covered by the grid, since this average, that of the distribution of a non-uniform stress, is always less than the maximum.

A strain gauge that is longer than the stress region will indicate a very low stress magnitude.

To take into account, as a general rule and to the extent possible, the length of a gauge should not be greater than the dimension of the cause of the stress, for said measurement to be acceptable. When the cause of the stress is small (of the order of 13 mm), according to the general rule, very small gauges should be used, and since the use of these brings with it another series of problems, a compromise relationship would have to be reached.

short gauges

Strain gauges whose length is around 3 mm. They tend to experience somewhat degraded performance, especially with respect to their maximum elongation, their stability under static stress conditions, and their durability when subjected to alternating cyclic stresses. When any of these conditions decrease the precision of the measurement to a greater extent than the average stress, it is necessary to use a longer gauge.

Long gauges

When it is necessary to use this type of gauges, it is worth mentioning some advantages that can be obtained with their use:

All of these considerations can be very important when working with plastic materials or some other type of material that has low heat dissipation.

Effort averaging

One of the applications of long strain gauges is the ability to determine stresses in non-homogeneous materials.

Taking concrete as an example, in which we can find a mixture of aggregates, generally stone, and cement; When we measure the forces on a material of this type, it is advisable to use a gauge long enough to cover several pieces of aggregate, in order to take a representative sample of the forces that are being generated on the structure. What is sought in this type of measurements are the averages and not the maximum stress points generated at the aggregate-cement interface. When you want to measure the stresses in this type of non-homogeneous structures, the length of the gauge must be greater than the length of the particles of the non-homogeneous material.

To treat the voltage variation, a Wheatstone bridge is used, which is formed by four resistors joined in a closed circle, and one of them is the resistance under measurement. The Wheatstone bridge can operate in direct and alternating current, allowing measurements of different resistances. The sensitivity of this element depends on how it is composed. In this way, it is possible to measure unknown resistances by balancing the arms of the bridge.

However, this method may have certain errors in its measurement, due to aspects such as the following:

The most common way to obtain an electrical signal as a result of a measurement using the Wheatstone bridge is through the deflection method. This method, instead of assessing the balance of the bridge, what it does is measure the voltage difference between both branches or the current through a detector placed on the central arm.

In order to use the Wheatstone bridge with the gauges, certain aspects must be taken into account, such as the wiring of the bridge. Many times, the gauge and the bridge are not located in the same place: therefore the resistances and temperature changes of the cables can affect the results obtained. To avoid this it is necessary to balance and calibrate the bridge. This procedure means that there can be no voltage at the output of the bridge and the calibration must be done properly, checking that the Wheatstone bridge is correctly producing the results.

Gauges are used for the electronic measurement of different mechanical quantities such as pressure, load, deformation, torque, among others. These measurements can be classified as mixed, dynamic and static. Mixed ones are used for elements subjected to loads that are varying, dynamic ones are used for elements that vibrate or are impacted, and static ones, as their name indicates, are elements subjected to loads that are not in motion.

In the past, metal gauges were used and their structure was made of metal. Nowadays, it is more common to use semiconductor gauges, because they have the ability to withstand greater resistance, especially because this type of gauge has better sensitivity when compared to metallic gauges, although the latter have lower thermal sensitivity.

This measurement system is widely used in construction to see the settlements that the concrete has the following month after being built. It is also used to monitor the deformation that a bridge may suffer and thus prevent it from collapsing without noticing the problem.

There are two types of applications that gauges can have. One of them is that, because of the variable that is intended to be found, the deformation variable is intermediate. And the other is that, when you want to know the stress state of a surface, it involves the direct measurement of the deformation. In this way, gauges are used in many fields of application, depending on the needs.

There are different criteria by which gauge applications can be analyzed. These can be: the type of work, the measurement range or dynamic behaviors. The type of work is due to actions such as tension and compression, which are used for weight, line or general purpose measurements, and actions such as fatigue and impact, used for dynamic tests. The measurement range is divided into micro load cells for high precision and wide range for general use. Finally, the dynamic behavior is attributed to fatigue and high speeds, used for systems subjected to fatigue and vibration and dynamic tests, respectively.

Strain gauges are piezoresistive sensors, generally manufactured with metallic or semiconductor materials (silicon or germanium), whose objective is the micromeasurement of deformations in any direction and with any sense of a specific point of the structure, through the processing of data obtained after the variation of the electrical resistance of the sheet (which occurs when subjected to a mechanical stress). Using the data obtained with this technology (the deformation) and with the mathematical model of the stress-strain relationship of Hooke's law (the deformation of an elastic material is directly proportional to the applied force), the stresses at different external and internal points of the structure can be deduced, and it is also possible to obtain the Young's modulus and the Poisson's coefficient. In the process, where the gauges are subjected to deformations, care must be taken not to exceed the elastic deformation limit of the element, so that the results are true.

Strain gauges are useful for all those situations in which it is necessary to find stresses and deformations in structures that comply with Hooke's law, such as airplanes, train cars, bridges, cranes, reinforced concrete, automobiles and buildings, among others. Frequently, it is important to study a large number of points, which is why the gauge becomes the best option as it is very easy to implant. Typically, gauges are used for research and development purposes.

Strain gauges in the field of civil engineering can be used in the research and debugging of methods to approximate the data obtained in the laboratory with the prediction of deformations and stresses of mathematical models, in addition to the control of deformations in cracks of structural elements (beams, pavements, screens, walls, etc.). An example: in the installation of gauges along a bridge, where the position of the unloaded and loaded points is controlled, in order to compare the real behavior of the structure compared to what was designed. Also in pavements, the deformations in the structure generated by vehicle traffic can be controlled by means of strain gauges. Another example is the control of general and differential settlements in the foundations of structures, embankments, slopes and in soil masses exposed to consolidation, among others.

In medicine, gauges have a wide field of application: dental sensors, ophthalmology devices, blood transfusion, infusion pumps, orthopedic devices, hand clamps, sensors in tendons and ligaments, carpal tunnel transducers, joint simulators "Joint (anatomy)"), verification of torque devices and substance weighing. And through all these sensors, biological parameters such as body pressure and temperature can be measured, as well as flow in different organs and parts of the body.

Strain gauges are also used in agronomy. For example, they are suitable devices to determine the stresses to which the "Tillage (agriculture)" tools are being subjected, and also to determine if there is excess or lack of water in the trees, and to monitor the variation in the diameter of the trunks. They serve to verify the proper use and irrigation control of water, in order not to overexploit the water used in agriculture.

In biomedical engineering, gauges are used to generate devices that can analyze myography, a medical procedure that analyzes muscle behavior. To know the force of the heart's contractions, strain gauges can be used, or to detect complications caused by blood pressure in the hard diaphragm. It is also used to test drugs and treatments in animals and to analyze the cardiac reactions that occur in them, and determine if their use in humans is possible. Precision and electronic scales are made up of gauges inside: as the weight increases or decreases, the sensor varies its resistance. These have too much precision, because by putting too much weight, the gauge exceeds the resistance limit, and the scale generates incorrect results.

Optical fibers can be used as sensors to measure, among other variables, tension, since gauges are used to verify the operation of fiber optic sensors. This is achieved by measuring tests using both optical fibers and gauges and then carrying out a comparison.[8]​.

Magnetostriction is the property of magnetic materials that causes them to change shape when in the presence of a magnetic field. Some of the processes for measuring magnetostriction include the use of gauges, adhering one to the surface of a disk and subjecting it to different magnetic forces. Since magnetostriction is a deformation, we can measure it using the gauge, reaching a precision of up to 10^-6. Another aspect to highlight is the possibility of using strain gauges for this application in a wide range of temperatures, from the liquefaction temperature of helium (4.2°) to temperatures close to 500.[9]​.

Depending on the type of work or specific task to be performed, you must first select the type of gauge to obtain appropriate results, which apparently can be quite simple, although it should not be forgotten that certain elements depend on a rational selection, analyzing the characteristics and specific parameters of the strain gauges, such as:

Many other factors must also be considered: duration, stress range and operating temperatures, to choose the best corresponding strain gauge-adhesive combination.

It should be noted that the strain gauge selection process often comes with a number of trade-offs. This is because, depending on the choice of parameters that tend to satisfy some requirement, it can work to a certain extent against others.

For example, when working in extremely small spaces, where the installation and use of the gauge is a little more complicated and the stress gradient is extremely high, the use of shorter gauges may be the ideal option for this type of work; However, it should be noted that smaller gauges (3 mm) generally have a small maximum elongation, and that gauge life is markedly reduced when subjected to fatigue conditions. For these reasons, it is necessary to reach a compromise that helps satisfy any set of circumstances that may arise, and judge this compromise on the validity and accuracy of the data obtained.

The preparation of the surface "Surface (physical)") does not necessarily follow the same process, because for each objective and material, different considerations are taken.

The preparation process of the surface "Surface (physical)") is as important as the process of installing the gauge, since if the installation surface "Surface (physical)") is not in perfect condition, this will affect the measurement of the gauge, as it cannot achieve perfect adhesion on the surface, or have a surface with inadequate alkalinity. This procedure is carried out by sanding the surface "Surface (physical)"), chemically cleaning it using acetone, marking the direction or directions of the gauge location, and chemical neutralization.

Marking the direction of the strain gauges requires a study that determines the point to be evaluated, the main coordinate planes and the type of mechanical stress (traction or compression) to which the structure will be subjected.[10]​.

Due to the needs of the operator, different designs and techniques can be handled. Usually, the idea is to fix the gauge on the test specimen in the direction or directions in which you want to know deformations when applying a force.

The gauge and the connection pads are extremely delicate elements, so they must be handled appropriately, using tweezers.

When choosing a gauge, it is important to analyze several parameters according to needs such as length, associated with the voltage gradient, voltage peaks and space for installation; the grid pattern, the resistance and the STC number (self-temperature compensation, which is the thermal output output). These parameters considered for the choice of the gauge are influenced and must be evaluated according to the requirements and needs of the operation, where aspects such as precision, durability, stability, temperature, ease of installation, elongation, cyclic resistance and environmental resistance of the gauge are considered.[11]​.

The installation of the gauge consists of achieving perfect adhesion and position of the gauge on the test surface to achieve the most accurate measurement possible. The installation process consists of a perfect location of the pads on the gauge, their proper alignment, placement and gluing on the test surface, and soldering the gauge terminals to a resistance meter or a computer, to obtain data.

Strain gauges are very sensitive to different environmental factors, due to their delicacy and the effect of thermal changes on their precision, which is why it is important that the gauge and its electrical connection are protected with silicones, epoxy resins or even adhesive tape.

Strain gauges depend on the ultimate purpose of the study. Configurations involving one or more gauges facing different directions can be used. This configuration depends on whether the forces to be measured are in uniaxial, biaxial directions or if they are in different directions.

In the case of stresses in a single direction, long and narrow sensors are frequently used, which allow the deformation on the material gauge to be maximized in the direction of interest.

When working in multiple directions, we can achieve simultaneous measurements using multiple gauges configured in the directions of interest. However, it is possible to make this task simpler and more precise through the use of multi-element gauges.

A very convenient configuration of gauges can be the deformation rosette, in which three gauges are located at 45°, each one for the purpose of measuring deformations in all directions.

Contenido

La galga extensiométrica es la parte del sistema de medición que trasforma la deformación inducida por el modelo en una variación de resistencia; a esto se le conoce como efecto piezorresistivo. Este efecto consiste en que un filamento de material semiconductor que se ve sometido a una deformación modificará su resistencia proporcionalmente a la deformación inducida, si la galga esta soldada químicamente, a un modelo de estudio, y si este modelo se deforma por efecto de una fuerza externa, la galga también se deformará. Haciendo una lectura de la variación de resistencia, se puede llegar a conocer la deformación que el modelo indujo a la galga, y por tanto, la deformación unitaria que el modelo experimentó por efecto de la fuerza.

Sensor

The sensor is the device that, when interacting with physical or chemical quantities, called instrumentation variables, undergoes changes in its properties. In the case of strain gauges, the physical magnitude of interaction is the strain, and the altered property is the electrical resistance factor. Typically, two compression sensors and two tensile sensors are connected. This connection is made at the same temperature, to avoid generating changes in the information due to temperature. In this case, this electrical variable is proportional to the instrumentation variable.

This corresponds to the gauges as such. The component of the system that directly receives a deformation due to elongation suffers a variation in its ability to conduct electricity. That is, the deformation of the model generates a linear variation of its resistance in the sensor, until it reaches its flow state.

There are different types of sensors for gauges, but the most common of these is the piezoelectric sensor, which is based on the fact that a piezoelectric material such as quartz or barium titanate, upon receiving a deformation generated by a stress, generates an electrical signal. Another type of sensor is the inductive one, which is based on the principle that a moving core, when moved inside a coil, increases the voltage induced in the secondary winding. An example of this is the crankshaft position sensor of a vehicle, which has a resistance, but when the crankshaft rotates its counterweight approaches the sensor and the metal generates a magnetic field, which causes its resistance to vary and send a signal.

Transducer

The transducer is a device that converts a signal of one physical form into a corresponding signal but of a different physical form.

The resistance variation obtained from the gauges is a direct value of the deformation, but in electronic acquisition systems, it is not correct to try to read the resistance and digitize these values. That is why, with the help of a transducer, this resistance variation is associated with a voltage variation, which is also proportional to the deformation. Then, a resistance variation in the gauge is read by the transducer, and associated with a voltage variation.

Amplifier

The voltage variation that registers the deformation of the gauge is very small, too small to be digitized, so it is necessary to amplify it. The optimal amplification values ​​are related to the digitizer available. The reading range of the digitizer must be taken into account, as if the voltage is extended above this range, data will be lost.

Data Logging

The system that is available until the moment of amplification is a continuous measurement system. However, to store the data, it must be taken every certain time interval, which allows an optical resolution of the record. Finally, the data obtained is a series of time and voltage values.

Temperature is a factor of great relevance, and therefore it must be taken into account and determined with the greatest possible precision how much it influences the material of which the strain gauge is composed when performing a test, although in practice it is very difficult to determine these considerations, and along with this the calculation is complicated by the corrections that must be made due to the effects of temperature. For this reason, when carrying out the test or experiment, these corrections must be determined for the effects of temperature, as it influences the data collected. To carry out this calculation, two strain gauges are used: one that is installed on the test object (which will deform), while another is placed on an element identical to the test object, with the difference that it will not be subjected to stress, so that it does not experience deformations. These two elements and their own gauges are joined to an electrical circuit arranged in the form of a Wheatstone bridge, in such a way that any variation in the resistance of the first gauge, due to temperature, will be nullified by a similar variation in the second gauge, and the unbalance condition observed in the bridge will occur only at the unit deformation of the first. It should be noted that care must be taken when installing the gauges, so that it is done in exactly the same way in the respective specimens and with their respective pieces.[12]​.

All necessary precautions must be taken so that the signal is not altered, especially variations in temperature, which can produce thermal expansion in the gauge and this in turn can affect its resistance, producing erroneous data in the measurement of deformations. The gauge expansion factor or gauge factor (GF) must be taken into account, which is calculated as follows:

where:.

Since electrical resistance is affected by temperature, it is possible to add this decrease factor as follows:

where:.

In practice, strain measuring equipment can be adjusted to work with a quarter, a half and a Wheatstone bridge, that is, with one, two and four gauges respectively. In these models, the temperature differential is assumed by a gauge, whose resistance is included in the electronic circuit. When a balance is achieved between the resistances of the gauges, the effects of temperature are canceled, since it affects each of them equally. The use of these mounts depends on the precision desired.

When a quarter bridge is used, it has low sensitivity and since there is only one gauge, it is without temperature compensation, so an additional one must be installed to compensate for it. When using a half-bridge, the arrangement of the gauges allows for temperature compensation; In addition, there is greater precision in the output signal. When using the full bridge, there is complete temperature and resistance compensation, allowing for high precision.

Many metal materials have a Poisson's ratio of 0.25 to 0.35, which causes the gauge factor to fluctuate between 1.5 and 5, depending on the change in resistivity. Generally, thin film gauges used in civil engineering have a gauge factor of 2 to 5; however, special gauges made of materials such as nickel can register factors of -12.1.

The unit strain can be expressed as follows:

The maximum deformation measured by a gauge depends mainly on the yield stress and the modulus of elasticity of the material from which they are made; Therefore, a relationship can be made between the deformation on the study surface and the maximum deformation of the gauge material, to determine the maximum deformation possible to measure:.

Where:.

When the gauges are purchased, the manufacturer provides the resistance and gauge factor data with their respective tolerances. Commercially, 120 ohm and 350 ohm gauges can be purchased,[13]​ which generally have gauge factors of 2 and 3.18, respectively. Most metals can undergo a maximum strain that does not exceed 0.005 inch/inch, which is 0.5 inch per 100 inches of material, or 0.5%. With this deformation of the material, the temperature change could be up to 1%. In this way and using the previous formulas, the deformation of the element can be calculated.[14]​[15]​.

Early studies: Lord Kelvin, Kearns, Ruge and Simmons

The first studies carried out on gauges were recorded in the year 1856,[1]​ when Lord Kelvin discovered the relationship between the deformation and resistance of conductive and semiconductor wires and noticed how their electrical resistance changed. In the early 1930s, Charles Kearns used strain gauges to measure vibratory strains in high-performance blade propellers; However, these gauges were not very precise, as they had problems in the stability of the resistance (R), which was affected by factors such as temperature, and errors were generated in the deformation measurements. Later, in 1938, Arthur C. Ruge and Edward E. Simmons each discovered that small diameter electrical conductors made of alloys could be adhered to surfaces to calculate deformations, and this is how lamellar gauges were created. This type of element has made great advances and constitutes what is known today as strain gauges[2].

Patent Rights: Caltech v. Simmons. Saunders-Roe

Caltech sued for the gauge patent, but Simmons took his case to the California Supreme Court and won the patent rights in 1949. In 1952 the Saunders-Roe company in the United Kingdom sought to improve the performance of gauges by subjecting them to different environments and testing different types of materials for their manufacture. They patented the thin gauges that are currently used in measuring deformations, in different industrial and scientific areas. In this way, the physical model of the gauge could be improved, significantly reducing size and costs.

Samuel Hunter Christie

In 1832, Samuel Hunter Christie invented the electrical resistance measuring instrument (known as the Wheatstone bridge, because it was improved and popularized in 1843 by the British scientist Charles Wheatstone). The electrical model is composed of four resistances in a closed circuit, where one of them is the one to be evaluated. This is used in order to measure the resistance by balancing the arms of the bridge.

Charles Wheatstone

Sir Charles Wheatstone was a British scientist and inventor who during the Victorian era made a great contribution with his inventions, of which the most significant is the Wheatstone bridge, but he also worked on other inventions such as the stereoscope, the Playfair coding technique and the kaleidophone.

Strain gauges take advantage of the physical property of electrical resistance and its dependence not only on the resistivity of the conductor, which is a property of the material itself, but also on the geometry of the conductor. When an electrical conductor is deformed within its elasticity limit, in such a way that no breakage or permanent deformation occurs in it, it will become narrower and elongated. This fact increases its electrical resistance. Similarly, when the conductor is compressed it shortens and widens, thus reducing its resistance to the passage of electric current. In this way, by measuring the electrical resistance of the gauge, the magnitude of the stress applied to the object can be deduced.

Metallic gauges

Metal gauges are made up of a very thin and fine base, to which a very fine metallic thread is attached that can be wound or folded. The two terminals where the wire ends are attached to the transducers. These gauges have the advantage of a low temperature coefficient, since the decrease in electron mobility is compensated by increasing the temperature with the increase in their concentration. In metal gauges the maximum current is about 25 mA if the support is a good heat conductor, and 5 mA otherwise; In any case, in metal gauges there is a great limitation on the current. The main characteristics of metal gauges under normal conditions establish that their size varies between 0.4 mm and 150 mm, and a variable resistance between 120 Ω and 5000 Ω. Its resistance tolerance is in the range of 0.1% and 0.2%.

The electrical resistance of the metal gauge is given by the relationship between the resistivity and the length with respect to the cross-sectional area.

They can be:

The main alloys used by metal gauges are:

Some of the materials used in the support of the metal gauges can be:

Gauges made with glass fiber reinforced epoxy-phenolic backing materials are a good choice for excellent results and performance when working over a wide range of temperatures.

These materials can be used for both static and dynamic measurements from -269 to +290 °C.

The different series of gauges with epoxyphenolic backing material can be found with different acronyms or abbreviations: WA, WK, SA, SK, WD and SD.

Resistance gauges

This type of gauge is an electrical conductor that, when deformed, increases its resistance, since the conductors become longer and thinner. Using the Wheatstone bridge, we can convert this resistance into absolute voltage. and as long as the strain obeys Hooke's law, the strain and absolute voltage will be linearly related by a factor called the gauge factor.

This type of gauge is generally used in laboratory conditions.

Capacitance gauges

These are associated with geometric characteristics and are used to measure stress and deformation. The electrical properties of the materials used for deformation have negligible electrical properties, so the materials of the capacitance gauges can be calibrated according to the mechanical requirements. This allows them to have better calibrations compared to the electrical type.

Photoelectric gauges

By using an extensometer we can amplify the movement of the specimen, while a ray of light is passed through a variable aperture, acting with the extensometer and directly with a photoelectric cell. As the gauge changes its opening, so does the amount of light that reaches the cell, which means that the intensity of the energy generated by the cell presents a variation, which we can measure, and from this we obtain the deformation.

Semiconductor gauges

In semiconductor gauges there is a semiconductor element instead of the metal wire. The big difference with respect to the other gauges is its size, since it is smaller. The maximum power dissipable in semiconductor gauges is about 250 mW. Semiconductor gauges are capable of withstanding high resistance, their fatigue life is longer and they have lower hysteresis, which is the ability for the material to retain its properties under different stimuli.

There are certain characteristic aspects: under normal conditions, its size varies between 1 mm and 5 mm, its resistance is between approximately 1000 Ω and 5000 Ω, and its resistance tolerance is between 1% and 2%.

The most abundant elements to make these gauges are:

Custom strain gauges are easy to install and provide fairly accurate temperature measurements at the point where said sensors are installed; They can also be used in industrial and military processes involving expansion pipe measurements. These sensors are adapted to measure hoop stress around the diameter of a surface to which it is mounted.[6]​.

Length

It corresponds to the active region or grid length sensitive to the stress of a gauge. Elbows and solder pads are not significantly sensitive to stress, due to their large cross section and low electrical resistance. There are gauges with lengths ranging from 0.2 mm. up to 100 mm.

Concentration of effort

One of the most important factors in determining optimal performance of a strain gauge is its length. For example, when you want to determine the stress measurements on some critical part or structure of a machine, these measurements must be made in the parts where the greatest stresses are concentrated, which are generally those that have a higher degree of fatigue. Strain gauges tend to integrate or average the area covered by the grid, since this average, that of the distribution of a non-uniform stress, is always less than the maximum.

A strain gauge that is longer than the stress region will indicate a very low stress magnitude.

To take into account, as a general rule and to the extent possible, the length of a gauge should not be greater than the dimension of the cause of the stress, for said measurement to be acceptable. When the cause of the stress is small (of the order of 13 mm), according to the general rule, very small gauges should be used, and since the use of these brings with it another series of problems, a compromise relationship would have to be reached.

short gauges

Strain gauges whose length is around 3 mm. They tend to experience somewhat degraded performance, especially with respect to their maximum elongation, their stability under static stress conditions, and their durability when subjected to alternating cyclic stresses. When any of these conditions decrease the precision of the measurement to a greater extent than the average stress, it is necessary to use a longer gauge.

Long gauges

When it is necessary to use this type of gauges, it is worth mentioning some advantages that can be obtained with their use:

All of these considerations can be very important when working with plastic materials or some other type of material that has low heat dissipation.

Effort averaging

One of the applications of long strain gauges is the ability to determine stresses in non-homogeneous materials.

Taking concrete as an example, in which we can find a mixture of aggregates, generally stone, and cement; When we measure the forces on a material of this type, it is advisable to use a gauge long enough to cover several pieces of aggregate, in order to take a representative sample of the forces that are being generated on the structure. What is sought in this type of measurements are the averages and not the maximum stress points generated at the aggregate-cement interface. When you want to measure the stresses in this type of non-homogeneous structures, the length of the gauge must be greater than the length of the particles of the non-homogeneous material.

To treat the voltage variation, a Wheatstone bridge is used, which is formed by four resistors joined in a closed circle, and one of them is the resistance under measurement. The Wheatstone bridge can operate in direct and alternating current, allowing measurements of different resistances. The sensitivity of this element depends on how it is composed. In this way, it is possible to measure unknown resistances by balancing the arms of the bridge.

However, this method may have certain errors in its measurement, due to aspects such as the following:

The most common way to obtain an electrical signal as a result of a measurement using the Wheatstone bridge is through the deflection method. This method, instead of assessing the balance of the bridge, what it does is measure the voltage difference between both branches or the current through a detector placed on the central arm.

In order to use the Wheatstone bridge with the gauges, certain aspects must be taken into account, such as the wiring of the bridge. Many times, the gauge and the bridge are not located in the same place: therefore the resistances and temperature changes of the cables can affect the results obtained. To avoid this it is necessary to balance and calibrate the bridge. This procedure means that there can be no voltage at the output of the bridge and the calibration must be done properly, checking that the Wheatstone bridge is correctly producing the results.

Gauges are used for the electronic measurement of different mechanical quantities such as pressure, load, deformation, torque, among others. These measurements can be classified as mixed, dynamic and static. Mixed ones are used for elements subjected to loads that are varying, dynamic ones are used for elements that vibrate or are impacted, and static ones, as their name indicates, are elements subjected to loads that are not in motion.

In the past, metal gauges were used and their structure was made of metal. Nowadays, it is more common to use semiconductor gauges, because they have the ability to withstand greater resistance, especially because this type of gauge has better sensitivity when compared to metallic gauges, although the latter have lower thermal sensitivity.

This measurement system is widely used in construction to see the settlements that the concrete has the following month after being built. It is also used to monitor the deformation that a bridge may suffer and thus prevent it from collapsing without noticing the problem.

There are two types of applications that gauges can have. One of them is that, because of the variable that is intended to be found, the deformation variable is intermediate. And the other is that, when you want to know the stress state of a surface, it involves the direct measurement of the deformation. In this way, gauges are used in many fields of application, depending on the needs.

There are different criteria by which gauge applications can be analyzed. These can be: the type of work, the measurement range or dynamic behaviors. The type of work is due to actions such as tension and compression, which are used for weight, line or general purpose measurements, and actions such as fatigue and impact, used for dynamic tests. The measurement range is divided into micro load cells for high precision and wide range for general use. Finally, the dynamic behavior is attributed to fatigue and high speeds, used for systems subjected to fatigue and vibration and dynamic tests, respectively.

Strain gauges are piezoresistive sensors, generally manufactured with metallic or semiconductor materials (silicon or germanium), whose objective is the micromeasurement of deformations in any direction and with any sense of a specific point of the structure, through the processing of data obtained after the variation of the electrical resistance of the sheet (which occurs when subjected to a mechanical stress). Using the data obtained with this technology (the deformation) and with the mathematical model of the stress-strain relationship of Hooke's law (the deformation of an elastic material is directly proportional to the applied force), the stresses at different external and internal points of the structure can be deduced, and it is also possible to obtain the Young's modulus and the Poisson's coefficient. In the process, where the gauges are subjected to deformations, care must be taken not to exceed the elastic deformation limit of the element, so that the results are true.

Strain gauges are useful for all those situations in which it is necessary to find stresses and deformations in structures that comply with Hooke's law, such as airplanes, train cars, bridges, cranes, reinforced concrete, automobiles and buildings, among others. Frequently, it is important to study a large number of points, which is why the gauge becomes the best option as it is very easy to implant. Typically, gauges are used for research and development purposes.

Strain gauges in the field of civil engineering can be used in the research and debugging of methods to approximate the data obtained in the laboratory with the prediction of deformations and stresses of mathematical models, in addition to the control of deformations in cracks of structural elements (beams, pavements, screens, walls, etc.). An example: in the installation of gauges along a bridge, where the position of the unloaded and loaded points is controlled, in order to compare the real behavior of the structure compared to what was designed. Also in pavements, the deformations in the structure generated by vehicle traffic can be controlled by means of strain gauges. Another example is the control of general and differential settlements in the foundations of structures, embankments, slopes and in soil masses exposed to consolidation, among others.

In medicine, gauges have a wide field of application: dental sensors, ophthalmology devices, blood transfusion, infusion pumps, orthopedic devices, hand clamps, sensors in tendons and ligaments, carpal tunnel transducers, joint simulators "Joint (anatomy)"), verification of torque devices and substance weighing. And through all these sensors, biological parameters such as body pressure and temperature can be measured, as well as flow in different organs and parts of the body.

Strain gauges are also used in agronomy. For example, they are suitable devices to determine the stresses to which the "Tillage (agriculture)" tools are being subjected, and also to determine if there is excess or lack of water in the trees, and to monitor the variation in the diameter of the trunks. They serve to verify the proper use and irrigation control of water, in order not to overexploit the water used in agriculture.

In biomedical engineering, gauges are used to generate devices that can analyze myography, a medical procedure that analyzes muscle behavior. To know the force of the heart's contractions, strain gauges can be used, or to detect complications caused by blood pressure in the hard diaphragm. It is also used to test drugs and treatments in animals and to analyze the cardiac reactions that occur in them, and determine if their use in humans is possible. Precision and electronic scales are made up of gauges inside: as the weight increases or decreases, the sensor varies its resistance. These have too much precision, because by putting too much weight, the gauge exceeds the resistance limit, and the scale generates incorrect results.

Optical fibers can be used as sensors to measure, among other variables, tension, since gauges are used to verify the operation of fiber optic sensors. This is achieved by measuring tests using both optical fibers and gauges and then carrying out a comparison.[8]​.

Magnetostriction is the property of magnetic materials that causes them to change shape when in the presence of a magnetic field. Some of the processes for measuring magnetostriction include the use of gauges, adhering one to the surface of a disk and subjecting it to different magnetic forces. Since magnetostriction is a deformation, we can measure it using the gauge, reaching a precision of up to 10^-6. Another aspect to highlight is the possibility of using strain gauges for this application in a wide range of temperatures, from the liquefaction temperature of helium (4.2°) to temperatures close to 500.[9]​.

Depending on the type of work or specific task to be performed, you must first select the type of gauge to obtain appropriate results, which apparently can be quite simple, although it should not be forgotten that certain elements depend on a rational selection, analyzing the characteristics and specific parameters of the strain gauges, such as:

Many other factors must also be considered: duration, stress range and operating temperatures, to choose the best corresponding strain gauge-adhesive combination.

It should be noted that the strain gauge selection process often comes with a number of trade-offs. This is because, depending on the choice of parameters that tend to satisfy some requirement, it can work to a certain extent against others.

For example, when working in extremely small spaces, where the installation and use of the gauge is a little more complicated and the stress gradient is extremely high, the use of shorter gauges may be the ideal option for this type of work; However, it should be noted that smaller gauges (3 mm) generally have a small maximum elongation, and that gauge life is markedly reduced when subjected to fatigue conditions. For these reasons, it is necessary to reach a compromise that helps satisfy any set of circumstances that may arise, and judge this compromise on the validity and accuracy of the data obtained.

The preparation of the surface "Surface (physical)") does not necessarily follow the same process, because for each objective and material, different considerations are taken.

The preparation process of the surface "Surface (physical)") is as important as the process of installing the gauge, since if the installation surface "Surface (physical)") is not in perfect condition, this will affect the measurement of the gauge, as it cannot achieve perfect adhesion on the surface, or have a surface with inadequate alkalinity. This procedure is carried out by sanding the surface "Surface (physical)"), chemically cleaning it using acetone, marking the direction or directions of the gauge location, and chemical neutralization.

Marking the direction of the strain gauges requires a study that determines the point to be evaluated, the main coordinate planes and the type of mechanical stress (traction or compression) to which the structure will be subjected.[10]​.

Due to the needs of the operator, different designs and techniques can be handled. Usually, the idea is to fix the gauge on the test specimen in the direction or directions in which you want to know deformations when applying a force.

The gauge and the connection pads are extremely delicate elements, so they must be handled appropriately, using tweezers.

When choosing a gauge, it is important to analyze several parameters according to needs such as length, associated with the voltage gradient, voltage peaks and space for installation; the grid pattern, the resistance and the STC number (self-temperature compensation, which is the thermal output output). These parameters considered for the choice of the gauge are influenced and must be evaluated according to the requirements and needs of the operation, where aspects such as precision, durability, stability, temperature, ease of installation, elongation, cyclic resistance and environmental resistance of the gauge are considered.[11]​.

The installation of the gauge consists of achieving perfect adhesion and position of the gauge on the test surface to achieve the most accurate measurement possible. The installation process consists of a perfect location of the pads on the gauge, their proper alignment, placement and gluing on the test surface, and soldering the gauge terminals to a resistance meter or a computer, to obtain data.

Strain gauges are very sensitive to different environmental factors, due to their delicacy and the effect of thermal changes on their precision, which is why it is important that the gauge and its electrical connection are protected with silicones, epoxy resins or even adhesive tape.

Strain gauges depend on the ultimate purpose of the study. Configurations involving one or more gauges facing different directions can be used. This configuration depends on whether the forces to be measured are in uniaxial, biaxial directions or if they are in different directions.

In the case of stresses in a single direction, long and narrow sensors are frequently used, which allow the deformation on the material gauge to be maximized in the direction of interest.

When working in multiple directions, we can achieve simultaneous measurements using multiple gauges configured in the directions of interest. However, it is possible to make this task simpler and more precise through the use of multi-element gauges.

A very convenient configuration of gauges can be the deformation rosette, in which three gauges are located at 45°, each one for the purpose of measuring deformations in all directions.

Contenido

La galga extensiométrica es la parte del sistema de medición que trasforma la deformación inducida por el modelo en una variación de resistencia; a esto se le conoce como efecto piezorresistivo. Este efecto consiste en que un filamento de material semiconductor que se ve sometido a una deformación modificará su resistencia proporcionalmente a la deformación inducida, si la galga esta soldada químicamente, a un modelo de estudio, y si este modelo se deforma por efecto de una fuerza externa, la galga también se deformará. Haciendo una lectura de la variación de resistencia, se puede llegar a conocer la deformación que el modelo indujo a la galga, y por tanto, la deformación unitaria que el modelo experimentó por efecto de la fuerza.

Sensor

The sensor is the device that, when interacting with physical or chemical quantities, called instrumentation variables, undergoes changes in its properties. In the case of strain gauges, the physical magnitude of interaction is the strain, and the altered property is the electrical resistance factor. Typically, two compression sensors and two tensile sensors are connected. This connection is made at the same temperature, to avoid generating changes in the information due to temperature. In this case, this electrical variable is proportional to the instrumentation variable.

This corresponds to the gauges as such. The component of the system that directly receives a deformation due to elongation suffers a variation in its ability to conduct electricity. That is, the deformation of the model generates a linear variation of its resistance in the sensor, until it reaches its flow state.

There are different types of sensors for gauges, but the most common of these is the piezoelectric sensor, which is based on the fact that a piezoelectric material such as quartz or barium titanate, upon receiving a deformation generated by a stress, generates an electrical signal. Another type of sensor is the inductive one, which is based on the principle that a moving core, when moved inside a coil, increases the voltage induced in the secondary winding. An example of this is the crankshaft position sensor of a vehicle, which has a resistance, but when the crankshaft rotates its counterweight approaches the sensor and the metal generates a magnetic field, which causes its resistance to vary and send a signal.

Transducer

The transducer is a device that converts a signal of one physical form into a corresponding signal but of a different physical form.

The resistance variation obtained from the gauges is a direct value of the deformation, but in electronic acquisition systems, it is not correct to try to read the resistance and digitize these values. That is why, with the help of a transducer, this resistance variation is associated with a voltage variation, which is also proportional to the deformation. Then, a resistance variation in the gauge is read by the transducer, and associated with a voltage variation.

Amplifier

The voltage variation that registers the deformation of the gauge is very small, too small to be digitized, so it is necessary to amplify it. The optimal amplification values ​​are related to the digitizer available. The reading range of the digitizer must be taken into account, as if the voltage is extended above this range, data will be lost.

Data Logging

The system that is available until the moment of amplification is a continuous measurement system. However, to store the data, it must be taken every certain time interval, which allows an optical resolution of the record. Finally, the data obtained is a series of time and voltage values.

Temperature is a factor of great relevance, and therefore it must be taken into account and determined with the greatest possible precision how much it influences the material of which the strain gauge is composed when performing a test, although in practice it is very difficult to determine these considerations, and along with this the calculation is complicated by the corrections that must be made due to the effects of temperature. For this reason, when carrying out the test or experiment, these corrections must be determined for the effects of temperature, as it influences the data collected. To carry out this calculation, two strain gauges are used: one that is installed on the test object (which will deform), while another is placed on an element identical to the test object, with the difference that it will not be subjected to stress, so that it does not experience deformations. These two elements and their own gauges are joined to an electrical circuit arranged in the form of a Wheatstone bridge, in such a way that any variation in the resistance of the first gauge, due to temperature, will be nullified by a similar variation in the second gauge, and the unbalance condition observed in the bridge will occur only at the unit deformation of the first. It should be noted that care must be taken when installing the gauges, so that it is done in exactly the same way in the respective specimens and with their respective pieces.[12]​.

All necessary precautions must be taken so that the signal is not altered, especially variations in temperature, which can produce thermal expansion in the gauge and this in turn can affect its resistance, producing erroneous data in the measurement of deformations. The gauge expansion factor or gauge factor (GF) must be taken into account, which is calculated as follows:

where:.

Since electrical resistance is affected by temperature, it is possible to add this decrease factor as follows:

where:.

In practice, strain measuring equipment can be adjusted to work with a quarter, a half and a Wheatstone bridge, that is, with one, two and four gauges respectively. In these models, the temperature differential is assumed by a gauge, whose resistance is included in the electronic circuit. When a balance is achieved between the resistances of the gauges, the effects of temperature are canceled, since it affects each of them equally. The use of these mounts depends on the precision desired.

When a quarter bridge is used, it has low sensitivity and since there is only one gauge, it is without temperature compensation, so an additional one must be installed to compensate for it. When using a half-bridge, the arrangement of the gauges allows for temperature compensation; In addition, there is greater precision in the output signal. When using the full bridge, there is complete temperature and resistance compensation, allowing for high precision.

Many metal materials have a Poisson's ratio of 0.25 to 0.35, which causes the gauge factor to fluctuate between 1.5 and 5, depending on the change in resistivity. Generally, thin film gauges used in civil engineering have a gauge factor of 2 to 5; however, special gauges made of materials such as nickel can register factors of -12.1.

The unit strain can be expressed as follows:

The maximum deformation measured by a gauge depends mainly on the yield stress and the modulus of elasticity of the material from which they are made; Therefore, a relationship can be made between the deformation on the study surface and the maximum deformation of the gauge material, to determine the maximum deformation possible to measure:.

Where:.

When the gauges are purchased, the manufacturer provides the resistance and gauge factor data with their respective tolerances. Commercially, 120 ohm and 350 ohm gauges can be purchased,[13]​ which generally have gauge factors of 2 and 3.18, respectively. Most metals can undergo a maximum strain that does not exceed 0.005 inch/inch, which is 0.5 inch per 100 inches of material, or 0.5%. With this deformation of the material, the temperature change could be up to 1%. In this way and using the previous formulas, the deformation of the element can be calculated.[14]​[15]​.