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 aracer.mobi).
A brief historical review about strain gage inventions is looking at the "who, when and how". The comprehensive bibliography leads to additional background information. Particular consideration is given to the stress analysis in order to verify the mechanical properties and capacity of components with focus on stability and serviceability, optimization, and safety checks, as well as in order to foresee inspection and monitoring.
Particular emphasis is laid on the correct planning and assessment of measurements, and on the interpretation of the results. Step-by-step guidance is given for many application examples, and comments help to avoid typical mistakes.
The book is an indispensable reference work for experts who need to analyze structures and have to plan measurements which lead to reliable results. The book is instructive for practitioners who must install reliable measurement circuits and judge the results. The book is also recommended for beginners to get familiar with the problems and to learn about the possibilities and the limits of the strain gage technique.
Author Bios Prof. Stefan Keil played an authoritative part in the development of modern experimental strain analysis. Stefan Keil was born in After graduation he researched on strength of materials and leaded fatigue tests on structural components of aircrafts with Hamburger Flugzeugbau GmbH.
He then returned to the RWTH Aachen Institute of Material Science as assistant professor where he was active in research on material behavior under service loads.
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: K.
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.
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.
The backing material should be very thin. It should go along with the adhesive material used to fix bond it on the structure under study. It should not be affected by temperature changes.
It should be strong enough to transmit the force from the structure under study to the sensing resistance wire. These adhesive are called as bonding material or cements. The different adhesive, their composition and the temperature for which they can be used are shown in following table. The characteristics required for a bonding material are listed.
The bonding material should be an insulator of electricity. The bonding material should not absorb humidity, that is, it should be non-hygroscopic. It should go along with the backing material so that the backing material is fixed bonded rigidly on the structure under study. It should have good shear strength to transmit the force from the structure under study to the sensing resistive wire.
It should be easy to apply and should spread easily and should provide good bonding adhesion. The bonding material should have a high creep resistance. The structure under study is made even and free from dust and dirt by rubbing with an emery sheet or by sand blasting.
The bottom side of the backing gauge carrier is also cleaned by a solvent using a cloth. Unless compensated for, changes in temperature will cause the item to which the strain gauge is attached to expand or contract, which is then indicated as a change in strain. Changes in temperature will cause the strain gauge itself to expand or contract, independent of any strain in the part to which it is attached.
That is, as the temperature changes, the resistance of the strain gauge and connecting wires will change independently of any change in strain.
Some texts treat the first two items as the same effect. After all, if the coefficients of expansion of the gauge and the item under test are the same, they will contract or expand at the same rates in response to a temperature change. On the other hand, if the parameter of interest is really stress, or its close relative, force, any strain caused by temperature changes would induce a true error in the result. In a more simple case, the load cell used to measure the force placed on a postal scale should not induce errors simply because the scale is next to the window on a sunny summer day!
However the dummy gauge is bonded to a separate unstrained component identical to that of the loaded member. Evidently the dummy gauge remains unstrained throughout the test run and suffers change in resistance due to temperature only. The two gauges are placed close to each other so that they are influenced equally by the ambient temperature changes.
For this reason, it is good practice to mount the dummy gauge adjacent to gauge being measured. However, it should be attached in such a way as not to be subjected to the induced strain of the tested part. In some cases, with relatively thin subjects and when measuring bending strain as opposed to pure tensile or compressive strain , it may be possible to mount the dummy gauge on the opposite side of a bar or beam.