Strain Measurement

Strain gauges

A strain gauge is defined as “any instrument or device that is used to measure the linear deformation in a given gauge length when it is subjected to loading”.

Strain rosettes

If the direction of principal strain is known, then it is possible to orient the strain gauge in that particular direction to measure the strain. If the direction of principal strain is unknown, three or more gauges can be used to determine the strain occurring at the point. That 3 or 4 strain gauges configuration is termed as strain rosettes.

The various types of strain gauge arrangement to obtain strain rosettes are,

  • Two gauge rosette
  • Rectangular rosette (3 or 4 element)
  • Delta rosette
  • T delta rosette.

Strain gauge circuits and transducers

The sensor circuits are used to convert the strain measured by the strain gauge into an electrical signal. The circuit consist of three fundamental units, they are

  • Sensor
  • Signal conditioner
  • Recorder or indicator

Sensor is used to give the input signal by sensing, while the signal conditioner is used to convert the input signal into usable form. Recorder or indicator is used to perform some basic operations like filtering, differentiation, integration, amplification, recording, indicating.

Semiconductor strain gauges

The ability of the material to conduct electricity is in between good conductor and insulator is known as semiconductor. The conductivity of the semiconductor may be changed when the changes are happen in physical condition; this property of the semiconductor is used to measure the strain. Some of the semiconductor materials used in the strain measurements are germanium, boron, silicon, tellurium…etc.

Two dimensional photo elasticity

Photo elasticity is defined as some materials having the property, they become doubly refractive under the action of stress, and the magnitude of the optical effect has some definite relation to that stress. This optical phenomenon is called as photo elastic effect.

Photo elastic materials should have the following properties,

  • Transparent to light
  • Easily movable
  • High optical sensitivity
  • Linear characteristics with respect stress and strain
  • Free from residual stresses
  • There should be absence of creep
  • High modulus of elasticity
  • High strength, hardness, rigidity.

Three dimensional photo elasticity

Most of the stress analyses are three dimensional in nature, for which the two dimensional photo elastic method cannot be implemented. This problem can be solved either by solving the stresses in model or multilayer reflective technique may be used to determine the stresses at the inner layer of the body.

Three dimensional stress distributions can also be determined by the scattered light method. Types of stress solving methods are, stress freezing method, curing method, creep method, gamma ray irradiation method.

Photo elastic coatings

This method is mostly used to measure the surface strain in opaque bodies like steel, rock etc. It is a field experimental method that is suitable for two and three dimensional models. The coating of thin layer material like polymer is bonded on the flat or curved surface of the model being tested for stress. When the model is coated the surface strains are transmitted to the coatings. Then the strain intensity is analyzed by viewing the model through a light reflection polarascope. The magnitude of the strain being displayed on a digital read out device of polaraiscope.

Brittle coatings

It is also called as stress coat method, in this method brittle lacquer is sprayed on the part to be analyzed and dried overnight. Then loaded in a static , dynamic or impact mode. The brittle lacquer will craw perpendicular to the direction of maximum principal stress. As the load increases the craw pattern enlarges.

By knowing the stresses in the coating, stress in the specimen can be determined. Stresses to an accuracy of ± 15% can be obtained by this method.

Scattered light photo elasticity

A narrow beam of polarized light passes through the three dimensional model. The state and intensity of rays having different directions depends on the stress available inside the model. Polarization produced by the scattering of the light coming out from the model is analyzed to know the intensity of stress.

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