1Mechanics of Materials
Axial stress:
Strain:
Poisson ratio:
Modulus elasticity:
L
dLstrainaxial a
D
dDstraintransverse t
A
Fa
a
t
E
2Strain Gauges Strain gauges: for measuring force, pressure, torque, and strain.
How ? Convert these forms of input into mechanical strain using an elastic member,
which is then converted into resistance change.
Resistance change is converted into voltage using a bridge circuit, ie.Wheatstone bridge circuit
Must be properly calibrated first.
3Strain Gauges Strain gauges changes resistance due to tension/compression in the
longitudinal direction
4Strain Gauges
Electrical resistance:
Resitance relationship to strain:
A
LR
5Strain Gauges: Calculation Example
6Strain Gauges: Calculation Example
7Strain Gauges: Data Example
8Strain Gauges Typical Values
Electrical Resistance, R 120 Ohm or 350 Ohm
1000 Ohm with plastic materials
A high Gage Factor is desirable because a large change in R is producedfor a given strain
Metal GF= 1.6 to 4 Resistivity does not change with the strain
Semiconductor GF= 80 to 200 Fragile and sensitive to changes in temperature
Axial strain range = 10-6 to 103
dR=0.00024 Ohm to 0.24 Ohm Notice how small this dR
Need to utilize Wheatstone bridge circuit
9Strain Gauges: Gauge Factor Table
Most GF decreases as temperature increase (NTC)
10
Potential Error Sources with Strain Gauges
Application errors:
Gauge may be damaged during installation
Need to verify resistance before stress
Electrical and magnetic field noise
Uneed shielded lead wires and insulated coatings
Utilize twisted lead wires
Temperature effects
Thermal expansion of material
Self heating of strain gages
11
Wheatstone Bridge Circuit
Balanced condition (EAC=0) when R1R3=R2R4or consider R1=R2 =R3 = R4
12
Wheatstone Bridge Circuit: One Active Arm
13
Wheatstone Bridge Circuit: Two Active Arms
14
Wheatstone Bridge Circuit: Two Active Arms
Temperature compensation:
15
Wheatstone Bridge Circuit: Four Active Arms