Strain gauge and rosettes Introduction
A strain gauge is a device which is used to measure strain (deformation) on an object subjected to forces. Strain can be measured using various types of devices classified depending upon their principle of operation. Some of them are as follows:
1. Mechanical type,
2. Optical type
3. Pneumatic type
4. Electrical type
Earlier, mechanical type of device such as extensometer or extension meter was used to measure strain by measuring change in length. Photoelectric strain gauge was also introduced which uses a light beam to produce electric current corresponding to deformation. The most commonly used strain gauge is an electrical resistance strain gauge. This strain gauge works on the principle that when a metallic wire type gauge is strained (here due to forces on object in contact), the resistance of the wire will be changed due to changes in its length, diameter and resistivity.
Resistance (R) = ρL A
Where ρ is resistivity; L is length of wire; A is area of cross-section of wire.
This change in resistance will be in proportion with the strain produced which can be easily measured using Wheatstone bridge. Drawbacks of Strain gauge 1. A strain gauge is capable only of measuring strain in the direction in which gauge is oriented. 2. There is no direct way to measure the shear strain or to directly measure the principal strains as directions of principal planes are not generally known. Strain rosettes Since for strain analysis in biaxial state of stress we should know strain in three directions and due to drawbacks in a strain gauge, Strain rosettes came in to picture. Strain rosette can be defined as the arrangement of strain gauges in three arbitrary directions. These strain gauges are used to measure the normal strain in those three directions. Depending on the arrangement of strain gauges, strain rosettes are classified in to:-
1. Rectangular strain gauge rosette
2. Delta strain gauge rosette
3. Star strain gauge rosette
https://intertechnology.com/Vishay/pdfs/TechNotes_TechTips/TN-515.pdf
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