Strain gauges are devices used to measure small changes in dimensions of a structure when force is applied, based on the principle that these changes affect the electrical resistance of a metal wire or foil. Different types of strain gauges exist, including bonded, unbonded, electrical, and piezoelectric, each working on unique principles to measure strain via resistance change. The Wheatstone bridge method is commonly employed to convert changes in resistance to voltage output, which requires amplification due to the relatively small nature of the resistance changes.

1. Strain Gauges

2. Strain
 When a force is applied to a structure, the
components of the structure change slightly
in their dimensions and are said to be
strained. Devices to measure these small
changes in dimensions are called strain
gauges.

3. Strain-gauge
The electrical resistance strain is a metal wire or
metal foil strip which is wafer-like and can be
stuck onto surfaces like a postage stamp.

4. Typical Bonded Strain Gauges

5. When a strain gauge is bonded to an object, and the
object changes in size, the resistance of the strain
gauge will change. The resistance R is given by the
expression:
R= ρ L/A
Where:
 L is the length of the wire in meters
 ρ is the resistivity of the material in ohm meters
 A is the cross-sectional area of the filament in m2

6. When strain gauges measure the changing dimensions of an object, they are measuring
strain. Strain is the ratio of the change in dimension of an object to the original
dimension
Mechanical strain ε = ΔL / L
When subject to strain, its resistance R changes, the fractional change in resistance
ΔR/R being proportional to the mechanical strain i.e.
Electrical strain ΔR/R= G . ΔL/L
where G is the gauge factor (1.8 – 2.2)
R varies between 50 Ω and 2KΩ

10. TYPES
Based on construction :
• Foil strain gauge
• Semiconductor strain gauge
• Photoelectric Strain gauge

11. MECHANICAL STRAIN GAUGE
 It is made up of two separate
plastic layers. The bottom
layer has a ruled scale on it
and the top layer has a red
arrow or pointer. One layer is
glued to one side of the crack
and one layer to the other. As
the crack opens, the layers
slide very slowly past one
another and the pointer
moves over the scale. The red
crosshairs move on the scale
as the crack widens.

12. ELECTRICAL STRAIN GAUGE
 When an electrical wire is
stretched within the limits
of its elasticity such that it
does not break or
permanently deform, it will
become narrower and
longer, changes that
increase its electrical
resistance end-to-end.
 Strain can be inferred by
measuring change in
resistance.

13. PIEZOELECTRIC STRAIN GAUGE
 Piezoelectric generate electric voltage when strain is
applied over it. Strain can be calculated from voltage.
Piezoelectric strain gauges are the most sensitive
and reliable devices.

14. BONDED STRAIN GAUGE
 A bonded strain-gauge element, consisting of a metallic
wire, etched foil, vacuum-deposited film, or semiconductor
bar, is cemented to the strained surface.

15. UNBONDED STRAIN GAUGE
 The unbonded strain gauge consists of a wire stretched
between two points in an insulating medium such as air.
One end of the wire is fixed and the other end is attached
to a movable element.

16. Wheatstone Bridge Method
 The four arms of the bridge
circuit are formed by the
resistance R1 to R4.
 The corner points 1 and 4
are connected to the input
voltage Vin
 The corner points 2 and 3
are connected to the output
voltage Vo.
1
2
4
3
R
2
R
1
R
3
R
4
V
o
Vin

17. Full strain gauge bridge with differential amplifier

18. Strain Gages - Review
 Strain = Elongation / Original Length
 Change in length = Change in electrical resistance
 Electrical Resistance change is very small, too small to be
accurately measured using ordinary voltmeters

19. Wheatstone Bridge
 Converts a change in electrical resistance from a strain gage to a change
in voltage
 Changes in strain are linearly related to a change in voltage output
 Voltage change is too small to be accurately measured and for this
reason an amplifier is used

20. Assignment
 ***** What is Strain Gauge? What is gauge factor? Show that