Strain measuring instruments :
These are of the following types :
- Electrical. (Strain gauges)
- Basic working principle of strain gauges :
In 1856, Kelvin observed that resistance of the conductor is directly proportional to the length and inversely proportional to the area of the conductor.
This principle was used in 1933 by defense department of US. They form gauges of alloys of
- Copper + Nickle ( 55 : 45 ).
- Nickle + Chromium + Iron.
Advantages of strain gauges :
- We can use strain gauges after cementing them to structural components. We can use them for a longer time.
- Remote reading is possible.
- The same strain meter can be used to measure the strains at 20 to 30 different places.
- It can measure static and dynamic strains.
- Strain can be measured in any required direction.
- Micro strain measurement is possible.
- It can not be used immediately after cementing it as curing is needed. Curing :It is to put something in favourable conditions to gain strength.
- It measures only surface strain.
- We can not use them beyond the elastic limit of the material of the strain gauge.
- It is very much sensitive to change in temperature.
- Sensitive to change in humidity
Construction of strain gauges :
- Length of strain gauges varies from 2-25 mm.
- Gauges for compression members varies from 2-4 mm.
- Whereas gauges for tension members ranges from 10-25 mm. Diameter of strain gauge wire is 0.025 mm.
- Range of strain gauge resistance is 60-10000 ohm.
- Range of safe current is 25-50 mAmp.
- Range of safe voltage is 35-50 volts
Cementing procedure of strain gauges :
- Clean oil, grease or dust from the surface usually from sand paper. The paint should be removed through sand paper and then paste.
- Make the surface rough for good bond. Structural material and strain gauge should have proper bond. If there will be slippage then some of the strain will not be determined.
- Clean oil, grease and dust from the backside of the strain gauge with tissue paper.
- Apply paste/cementing material on the structural component e.g nitrocellulose and epoxy resins.
- Paste strain gauge very carefully in proper direction and proper location by pressing strain gauge from one end to other end using finger. So that the surplus cementing material should be taken out.
- Apply four coats of water proofing agent to avoid from temperature and humidity effects.
- Give sufficient time for curing, normally 1-2 days.
- Connect strain gauge with wheat stone bridge circuit.