The resistive strain gauge has been around for over seventy-five years; according to Ramsey (1996) the first resistive strain gauges were wound fine copper wire on a thin paper tube, which was then flattened and bonded to a metal surface to determine the strain in the metal. In 1843, Charles Wheatstone described the effect of the change of the resistance in an electrical conductor due to the effects of mechanical stress. Almost a century later, in 1938, Arthur Ruge invented the first wire strain gauge where a tiny piece of high-resistance filament was bent in a zigzag pattern and fixed in a rigid base. In the early 1940s, the bonded resistance strain gauge was introduced; then, in 1952, the “printed circuit” technique was mentioned for the first time and its refined form led to the development of the “metal foil strain gauge”. Resistive strain gauges are used across the world in a vast range of diverse engineering applications. These include scales of various types; hoppers; load cells for weighbridges; torque, force and pressure transducers, whilst also being used in the medical and educational industries. They are used in experimental stress analysis, including fatigue life and residual stress analysis, as well as determining values and direction of mechanical stress. They also have a vast array of uses in mechanical engineering study and development, in order to examine the condition and safety of aeronautical and automotive equipment; as well as machinery and equipment used in the gas, oil and power generation industries. Some bridges and buildings are also mounted with resistive strain gauges to keep their structural integrity and safety under continuous inspection.Below figure.1 shows Arthur Ruge monitoring a model of a water tank on a vibrating table with the first strain gauges during his investigation into behavior during earthquakesIn order to explain how resistive strain gauge’s work, the actual workings of resistance must be understood. According to Rizzoni ,(2004) when electric current ?ows through a metal wire or through other circuit elements, it encounters a certain amount of resistance, the magnitude of which depends on the electrical properties of the material, practically all circuit elements exhibit some resistance.Resistance is measured with a device called an ohmmeter. An instrument for the direct measurement of electrical resistance (Gibilisco, 2001). Ohmmeters operate by passing a small current through the circuit and measuring the voltage drop therefore finding the resistance via Ohm’s law (V=IR) According to Physics Classroom, (2017) when an electron travels through the wire of a circuit it will encounter resistance. Resistance is the obstruction to the flow of charge. For an electron, the trip from the beginning terminal to the end terminal of a circuit is not a straight path. It is in fact a meandering route that results from limitless collisions with static atoms within the conducting material. The electrons encounter resistance which is an obstruction to their movement. As well as the type of conducting material, there are two other main features that affect the rate at which charge flows from terminal to terminal.Firstly, the length of the wire will have an effect on the amount of resistance. The longer the length of the wire, the greater the resulting resistance will be. There is a direct association between the length of wire the charge must negotiate and the amount of resistance encountered by the charge. Ultimately, if resistance happens as the outcome of clashes between charge carriers and the atoms of the wire, then there is likely to be more clashes in a longer wire. More clashes would mean more resistance.Secondly, the cross-sectional area (CSA) of the wire will have an effect on the amount of resistance. A thicker wire would have a larger cross-sectional area. Similar to a liquid flowing through a wider pipe at a higher velocity than it will flow through a narrower pipe; the thicker a wire is, the less resistance there will be to the flow of electrical charge. Therefore, if all other attributes are the same, the charge will flow at a higher rate through a thicker wire with greater CSA than through a thinner wire.A resistive strain gauge uses resistance to measure strain. It is a device that takes advantage of the physical property of electrical conductance and its dependence on the conductor’s geometry; thus converting a change in applied force into a change in produced resistance. This enables relative changes in length i.e. strain (the ratio: elongation/original length) to be measured. According to Hughes et al., (1994) the resistive strain gauge is essentially a length of resistance wire, coiled for compactness into a flat coil. The strain gauge is stuck via a special adhesive to the surface of the material for which the strain is being measured. When the surface suffers a strain, and stretches, the strain gauge is also stretched (within the limits of its elasticity such that it does not break or permanently deform). Although resistive strain gauges are transducers (Ramsey, 1996) they are unique in that they convert changes in mechanical force into changes in electrical resistance, which are then converted into measurements of strain. Commonly strain gauges display electrical resistance ranging from 3 to 30 ?.Strain is referred to as positive if the wire is stretched by the material in tension, thus increasing in length and decreasing in cross-sectional-area (electrical resistance increases), and negative if the wire is compressed by the material in compression, thus decreasing in length and increasing in cross-sectional-area (electrical resistance decreases). Fig. 3a. shows a breakdown of how this occurs, notably it can also be seen that the gauge pattern lines are running parallel with the induced tensional or compressional stress; this is conclusive with Adams (1975) who states that resistive strain gauges are most sensitive in this direction. The gauge should be positioned so that its active axis is along the direction of the measured strain.
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