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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

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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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Log In Sign Up. Jayasimha Reddy. A strain gauge is a strain transducer device for measuring dimensional change on the surface of the structural member that is under test. Measurement of strain has applications in measuring force, pressure, acceleration and torque etc. A strain gauge is a device used to measure the strain on a free surface of a structure.

Strain gages are the prominent tool in stress analysis. Strain gauges of all types are essentially employed to measure the linear deformation over a given gauge length.

They sense the change in length, Magnify and indicate it in some other form. There are different types of strain measuring techniques, they are 1. Mechanical strain gauges 2. Electrical strain gauges 3. Optical strain gauges 4. Pneumatic strain gauges 5. Acoustical strain gauges 4. Berry strain gauge, Huggenbeger extensometer, Johansson extensometer Mechanical strain gauges are also known as Extensometers used to measure static or gradually varying load conditions.

These gauges are usually provided with two knife edges which are clamped firmly in contact with the test component by means of a clamping spring at a specific distance of gauge length. When the specimen under testing is strained the knife edges undergoes displacement, this displacement is amplified by a mechanical linkages and the strain is displaced on a calibrated scale.

Berry Strain gauge These strain gauges uses a lever magnification with dial indicator to show magnified motion. It consists of one rigid frame and two conically pointed contact pointers. One pointer is rigidly fixed to the frame while the other is pivoted at a point on the frame.

The displacement in the lever is magnified and indicated in the dial indicator. These gauges are highly accurate. The movable knife edge rotates the lever at lower pivot, the lever in turn rotates the indicator pointer at upper pivot point with the help of a link. Johansson Extensometer These extensometers uses tension tape or twisted metal strip between two knife edges. Half of the strip is twisted to one direction and remaining half is twisted to other direction and a pointer is fixed at the center of the strip.

On application of load, displacement in the movable knife edge takes place with high amplification due to stretching of twisted metal strip. In electrical resistance strain gauge the displacement or strain is measured as a function of resistance change produced by the displacement in the gauging circuit. When the conductor is stretched, its length will increase and area of cress section will decrease this will result in change in resistance.

Change in resistance per unit strain is defined as Gauge Factor. Gauge factor indicates the sensitivity of the strain gauge. The optical strain gauges are used to measure elongation as well as deflection, following are the two type of optical strain gauges, a. These optical stain gauges employs variety of mirror systems to obtain optical magnification. The pivoted knife edge carries a mirror and the other end of this arm is fastened to specimen as the specimen elongates the measuring knife edge will rotate about its point there by tilting the mirror.

The Reflection of the illuminated scale in this mirror is viewed through the telescope. In this instrument, the relative rotation between the fixed mirror and the movable mirror is measured with autocollimator.

The autocollimator consists of a lamp source to produce parallel beam of rays and a scale to measure the deflection of the reflected ray. A tungsten carbide rocker lozenge acts as a moving knife; one face of this lozenge is polished to act as a mirror. If the specimen deforms, rotates the lozenge which in turn deflects the incident ray back to the reticule.

Actually three images are visible on the reticule one gives the measurement of strain and other two helping alignment of the gauge. The sensitivity of the gauge is 2 micro strains and this gauge is available with a wide range of gauge length of 6mm.

The principal of operation of a pneumatic gauge depends upon the relative discharge of air between a fixed orifice and a variable orifice. Magnification up to , times and the gauge length as small as 1mm are possible to achieve by these gauges. These gauges are suitable for both Static and dynamic strain measurements. These are sensitive, robust and reliable. In an acoustic strain gauge the variation in length of a wire stretched between two gauge points is measured which alters the natural frequency of the wire.

The magnitude of frequency change for a strain gauge can be increased by decreasing the length of the wire or stress in wire. These gauges are highly accurate and long term reliable. Optical strain gauges are used to measure strains in concrete structure, concrete dams, rock, steel structures etc.

In an electrical resistance strain gauge, the device consists of a thin wire placed on a flexible paper tissue and is attached to a variety of materials to measure the strain of the material. In application, the strain gauge will be attached to a structural member with the help of special cement. 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.

Strain Gages

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.

Extra-Long Grid Pattern for Inhomogeneous Materials Strain gages with an extra-long grid patterns for measuring strain in inhomogeneous materials i. Concrete, filled plastics, etc. Measuring strain gage circuits In order to measure strain with a bonded resistance strain gage, it must be connected to an electric circuit that is capable of measuring the minute changes in resistance corresponding to strain.

The Figure shows a typical strain gage diagram.

Design and Construction of a Strain Gauge

A Wheatstone bridge is a divided bridge circuit used for the measurement of static or dynamic electrical resistance. The output voltage of the Wheatstone bridge is expressed in millivolts output per volt input.

The Wheatstone circuit is also well suited for temperature compensation. The number of active strain gages that should be connected to the bridge depends on the application. For example, it may be useful to connect gauges that are on opposite sides of a beam, one in compression and the other in tension. In this arrangement, one can effectively double the bridge output for the same strain.

In installations where all of the arms are connected to strain gages, temperature compensation is automatic as resistance change due to temperature variations will be the same for all arms of the bridge. We understand that our customers may require custom pattern that are manufactured to their specifications. Custom strain gages can be designed to simplify strain gage installation, for a specific application or for an environment where space is limited.

If you do not find what you need in our standard gauge selection please let us know. We can customize your strain gage to fit your needs, including: Modify a standard gage pattern Create a custom gage pattern Put multiple gages on a common carrier Provide non-standard lead length Use custom material Relocate solder pads or provide additional wiring points Produce specific trim dimension or shapes to clear obstructions We can provide custom creep specifications to match your spring element to maximize the performance of your sensor.

Our team will work with you to modify creep compensation higher or lower as required by your test results. We strive to make buying a custom strain gage fast and easy. Just send OMEGA your custom drawing along with your specifications and the quantity of strain gages required.

The OMEGA team will work with you on your application and provide a quotation for the custom strain gages. We can make proof samples of the custom gauges in as little as 2-weeks. With production quantities shortly thereafter. A custom part number will be created for your strain gage to make future ordering fast and easy. Types of Load Cells.

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Please contact the issuing bank for details. Errors were found with the address you provided. Please verify your address. Home Resources Strain Gages. A Strain gage sometimes referred to as a Strain Gauge is a sensor whose resistance varies with applied force; It converts force, pressure, tension, weight, etc.

When external forces are applied to a stationary object, stress and strain are the result.

Stress is defined as the object's internal resisting forces, and strain is defined as the displacement and deformation that occur. The strain gage is one of the most important tools of the electrical measurement technique applied to the measurement of mechanical quantities.

As their name indicates, they are used for the measurement of strain. As a technical term "strain" consists of tensile and compressive strain, distinguished by a positive or negative sign. Thus, strain gages can be used to pick up expansion as well as contraction. Jump to Section. Learn more about strain gages General Purpose Precision strain gages General purpose precision strain gages are encapsulated constantan foil strain gages offered in a wide variety of patterns for scientific, industrial and experimental stress analysis.

Learn More. Concrete, filled plastics, etc Learn More. Related articles to: Strain Gages.

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