So, the strain on the surface is measured in order to know the internal stress. Strain gages are the most common sensing element to measure surface strain. measurements using bonded resistance strain gages. We will introduce considerations that affect the accuracy of this measurement and suggest procedures for. rienced by the test specimen is transferred directly to the strain gauge, which Strain gauges are available commercially with nominal resistance values from 30 .
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UNIT-IV STRAIN GAUGES AND MEASUREMENT INTRODUCTION: A strain gauge is a strain transducer device for measuring dimensional change on the. Strain gauges. If a strip of conductive metal is stretched, it will become skinnier and longer, both changes resulting in an increase of electrical resistance. give the user a good working knowledge of strain and strain gages. (available at ruthenpress.info).
The gauge position will be in such a manner that the gauge wires are aligned across the direction of the strain to be measured. The wire used for the purpose will have a diameter between 0. When a force is applied on the wire, there occurs a strain consider tensile, within the elastic limit that increases the length and decreases its area. Thus, the resistance of the wire changes. This change in resistance is proportional to the strain and is measured using a Wheatstone bridge.
When a bar is subjected to a simple tensile loading there occurs an increase in length of the bar in the direction of the load. Strain refers to the relative change in dimensions of the bar under load and it is prescribed as the ratio of the change in length to the unstressed length of the bar.
In practice it is often stated as micro strain mu-strain. The wire strain gauge can be further divided into two. They are bonded and un-bonded strain gauge. As shown in the figure below, an un-bonded strain gauge has a resistance wire stretched between two frames. The rigid pins of the two frames are insulated. When the wire is stretched due to an applied force, there occurs a relative motion between the two frames and thus a strain is produced, causing a change in resistance value.
This change of resistance value will be equal to the strain input. It is connected to a paper or a thick plastic film support. The measuring leads are soldered or welded to the gauge wire. The bonded strain gauge with the paper backing is connected to the elastic member whose strain is to be measured.
If a strip of conductive metal is stretched, it will become skinnier and longer, both changes resulting in an increase of electrical resistance end-to-end.
Conversely, if a strip of conductive metal is placed under compressive force without buckling , it will broaden and shorten. If these stresses are kept within the elastic limit of the metal strip so that the strip does not permanently deform , the strip can be used as a measuring element for physical force, the amount of applied force inferred from measuring its resistance.
Such a device is called a strain gauge. Strain gauges are frequently used in mechanical engineering research and development to measure the stresses generated by machinery. Aircraft component testing is one area of application, tiny strain-gauge strips glued to structural members, linkages, and any other critical component of an airframe to measure stress. Most strain gauges are smaller than a postage stamp, and they look something like this: Alternatively, strain gauge conductors may be thin strips of metallic film deposited on a non-conducting substrate material called the carrier.
The latter form of strain gauge is represented in the previous illustration. The task of bonding strain gauges to test specimens may appear to be very simple, but it is not. It is also possible to use an unmounted gauge wire stretched between two mechanical points to measure tension, but this technique has its limitations. This resistance may change only a fraction of a percent for the full force range of the gauge, given the limitations imposed by the elastic limits of the gauge material and of the test specimen.
Thus, in order to use the strain gauge as a practical instrument, we must measure extremely small changes in resistance with high accuracy. Such demanding precision calls for a bridge measurement circuit. Unlike the Wheatstone bridge shown in the last chapter using a null-balance detector and a human operator to maintain a state of balance, a strain gauge bridge circuit indicates measured strain by the degree of imbalance, and uses a precision voltmeter in the center of the bridge to provide an accurate measurement of that imbalance: Typically, the rheostat arm of the bridge R2 in the diagram is set at a value equal to the strain gauge resistance with no force applied.
The two ratio arms of the bridge R1 and R3 are set equal to each other. Thus, with no force applied to the strain gauge, the bridge will be symmetrically balanced and the voltmeter will indicate zero volts, representing zero force on the strain gauge. As the strain gauge is either compressed or tensed, its resistance will K.
This arrangement, with a single element of the bridge changing resistance in response to the measured variable mechanical force , is known as a quarter- bridge circuit.
With bonded gauges there is always a possibility of slip between the carrier material and the cement. Surface preparation and bonding techniques have been discussed under the following three topics namely: Backing, base or carrier material.
Bonding material or cement. Surface preparation and mounting of strain gauges. The backing material provides support to the resistance wire grid of the strain gauge arrangement. The backing material provides protection to the sensing resistance wire of the strain gauge arrangement. It also provides dimensional stability for the resistance wire of the strain gauge arrangement. Characteristics Required for Backing Materials 1.
The backing material should be an insulator of electricity.
The backing material should not absorb humidity, that is should be non-hygroscopic. In , Prof. Ruge got the idea of bonding a fine wire to. Hans Meier made several specimens with Elinvar wire. Ruge and Hans. Meier had difficulties in their measuring system. They received some help from Prof. V De Forest ME and got a very good galvan ometer. However, the y could not get a.
On the other hand, Simmons being an electrical engineer had developed. Simmons initially thought that his invention was too simple and obvious to.
Baldwin-Southwark prepared the basic patent on behalf of Simmons. He got the. Ruge got four dozen-plus improvement, development and. At first consideration, resistance measure appears to be a straightforward operation. It is a. In the present application, however, the accuracy and resolution needed fo r.
As a result, special circuitry and wiring procedures have. The fundamental formula for the resistance of a wire with uniform cross section, A, and.
Here, L is the wire length. This relation is generall y accu rate for comm on metals and. The change in resistance can be expressed as. This is a complicated expression in its.
The differential expression on the rig ht side is tedious to compute directly but can be. First t aking the natural log ln o f. Where, D is a cross section dimension and C is some constant e. At this point, it can be noted that the longitudinal strain can be written in dif ferential form. Also for linearly elastic and isotropic behavior of the wire:. This expresses the basic proportionality between resistance and strain in the gage element.
A measure of the sensitivity of the material or its resistance change per unit. From the above resistance calculations the Gage Factor can then be determined as. The Gage Factor as expressed above includes effects from two sources. The first term on. The second term represents the. In practice. This is ampl y evident in Figure 3. In this. Pure nickel is also poorly behave d.
This material is seldom used alone but is. The most common material for static strain. At this point a basic difficulty has appeared. The Gage Factor is only of order unity and.
In engineering materials this strain level is typically from 2 to 10, Thus, changes in resistance in the gage of no more than. Table 3. Gage Factors for Various Grid Materials. Gauge length ranges from [0. Gauge factor is supplied by manufacturer and commonly it is 2. It is apparent thus far that quite small resistance changes must be measured if resistance.
Direct measurement of 0. Several measuring techniques are available for this purpose but the. Wheatstone Bridge circuit has proven the most useful for a number of im portant reasons. This circuit will be described shortly but first several funda mental techniques will be. This is a simple and common technique used to make measurements of resistance.
A constant known current is forced through the unknown. This technique is generally used in portable ohmmeters and combination volt -ohm-. A major drawback for the present application is that this circuit indicates the. This circuit shown below is similar to the current injection technique but avoids the need. Instead, a simple voltage source is used and the gage is. This makes the voltage source resemble a constant. The only remaining question is. The sensitivi ty of this circuit can be.
Again, as with the previous circuit, the major drawback is that the strain produces a. The Wheatstone Bridge is the most basic of a number of useful electrical bridge circuits. I t also finds. In the circuit shown below it is. R 3 , R 4 connected so that the initial steady state voltages are cancelled in the.
The output voltage can be written as the difference between two ballast circuits as:. An initial stead y state voltage exists unless the numerator above is zero. Such a. This relationship is not of direct concern here but it is interesting to note that if any three.
For the present, however, it is concerned with the output produced by. If infinitesimal changes occurs in each. It can be used to express de in terms of the strains as: This is the basic equation relating the Wheatstone Bridge output voltage to strain in gages. Several remarks are in order:.
It is valid only for small infinitesimal resistance. Large resistance changes produce nonlinear effects where finite. Increasing either will improve measurement sensitivity. Equal strain in all gages produces no output either. The major intent here is not to suggest electrical measurement techniques but rather to. While there are cases when the entire bridge circuit must be custom. Since all presently available strain measuring.
The equation describing the basic electrical response of the Wheatstone Bridge circuit has. In the following discussions it will be examined. Fluctuations in ambient and in operating temperatures produce the most severe effects. The problems arise primarily from two.
Additionally, for certain bridge circuits in which the. Temperature changes in the gages will obviously be produced by changes in ambient. In addition significant temperature changes may be produced directly in the. The magnitude of this effect is. Gages mounted on thin materials with poor thermal conductivity such. Power generated within a gage in a Wheatstone Bridge is given as:. Due to the. Thus with E and i defined as the Wheatstone Bridge supply voltage and total bridge.
Thus the maximum excitation voltage is given by:. It should be noted that hig her electric output per unit. Thus bridge output is increased, then, by increasing:. Temperature compens ation of the strain gage alone does not generally eliminate thermal. Such compensation is rarely exact and the differences must usually be.
The ability to make. This is. The extraneous effects of temperature and other factors. It should be clear. On the other hand, it should be. When similar strain gages are used in all four arms of the bridge and when they are. In this case the temperature induced. But at the same time, it also follows that an output will only be. This may be difficult. Then, the strain term for these arms vanishes. One way to overcome these problems. If these dummy gages. There are a number of variations in the bridge wiring and the configuration of active and.
These are. In this cas e two dumm y resistances are provided in adjacent arms 3 and 4 as shown. The bridge output is not sensitive to temperature so lon g as any temperature changes. An output is produced only when unequ al resistance changes are. The most useful application of thi s circuit is in the measure. In this case the two gages are mounted opposite. The output is twice that for a single active gage and if the two gages are close to each.
Moreover, if a component of unifor m stretching o r compression is present. The output from a strain gage bridge is proportional to changes in resistance of all of the. In most situations, only one or two arms are active and it is desirable to be able to.
The Wheatstone. Bridge circuit is ideally suited for this purpose because it is relatively eas y to affect a. In order to change the resistance in one arm, two approaches are possible as. A resi stance may be added in series to increase the arm resistance or.
Of these two, the. The series connection requires v ery. It also requires use of very small calibration resistances which are not easily. On the other hand, the parallel connection requires a relatively large resistance. The universal series for strain measurements: More than 2, different types of strain gauges available for nearly any measurement task. C Series extreme temperature.
They are also particularly flexible.
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