The strain gauge element is defined as, “a transducer or device used to measure dimensional change on the surface of a structure under test”. Strain gauge gives indication of strain at only one point.
PURPOSE OF STRAIN GAUGES
Strain gauges are used to measure strains and associated stress. Strain gauge serves two main purposes.
- Strain gauge determines the state of strain existing at a point on a loaded member. This determination is necessary for strain analysis.
- Strain gauge acts as a strain sensitive transducer for measuring force, temperature, displacement and acceleration.
APPLICATIONS OF STRAIN GAUGES
- Measurement of tension, torque and stress in structures.
- Measurement of force in load cell.
- To analyze dynamic strains in complex structures, like bridges, automobiles, roads etc.
THEORY OF STRAIN GAUGES
Strain gauges are the metal conductors. If a strip or rod or bar of elastic metal is subjected to tension, its longitudinal (linear) dimension will increase corresponding to reduction in lateral dimensions. This is called as positive strain, where length increases, while its cross-section area decreases.
We have, R
Therefore, R I—
As the resistance of a conductor is directly proportional to length, the resistance of gauge wire increases with increase in length, i.e. positive strain. Increase in resistance of gauge wire is directly proportional to mechanical stress applied on it, therefore strain gauges are called as positive passive transducers.
Selection of Suitable Strain Gauge
- It is done by considering,
- Its size and location, where the strain gauge is to be mounted.
- High gauge resistance.
- Sensitivity.
- Environmental conditions, at which, stra•n measurement is to be done.
- Cost.
- Requirement of Protection — Needed or not.
- Strain limits of measurements.
Installation procedure of Strain Gauge
- The surface of test specimen should be smooth enough, but not highly polished. Highly polished surface does not provide good adhesion.
- Surface must be made clean by removing rust, paint and scales (deposited layers of solids or liquids) and other contaminations.
- For surface preparation, good quality abrasive paper or emery cloth should be used for cleaning.
- The surface is further cleaned to remove any traces of grease or dirt using solvents like acetone, trichloroethylene with cotton.
- The strain gauge backing should be clean using a suitable solvent to remove oil and grease contamination. After cleaning, strain gauge backing should be dried under electric bulb to remove moisture.
- A fairly generous layer of recommended cement is spread on prepared clean surface of specimen as well as on strain gauge backing.
- The gauge is then laid at desired location very carefully, pressed with thumb to exert pressure. Generally, applied pressure and temperature should be around 35 to 180 kPa and 60 to 200C respectively.
- A care must be taken to avoid air gap between gauge and specimen.