What is Strain Gage?
• A device whose electrical resistance varies in proportion to the
amount of strain in the device.
Why strain gauges??????
• It is difficult to measure the stress directly, but a strain
gage can be used to measure the strain, and then the
stress can be determined using the Hooke’s law. That
is
σ = Eε
Principle of Working of Strain Gauges
• When force is applied to any metallic wire its length increases due to
the strain.
ε =(L2-L1)/L1
• Resistance of the conductor is the inverse function of the length.
• This change in resistance of the conductor can be measured easily
and calibrated against the applied force.
• The term dD/D is known as the transverse strain, εt.
• Solid mechanics provides the following relationship between the axial and transverse
strain
where, v is known as Poisson’s ratio and it is the property of material.
• The negative sign indicates that as the wire becomes longer, the transverse dimension
decreases.
Combining the above equations we get
• The above equation shows the relationship between the change in resistance of the
wire, strain, and the change in resistivity of the wire.
• The strain gage factor, S, is defined as
• Combining the above two equations we get
STRAIN-GAGE CONSTRUCTION
• Foil used in a strain gage must be very fine or thin to have a
sufficiently high electrical resistance (usually between 60 and 350
ohms), it is difficult to handle.
• Some use has been made of wire filaments in strain gages, but this
type of gage is seldom used except in special or high temperature
applications. In order to handle this foil, it must be provided with a
carrier medium or backing material, usually a piece of paper, plastic,
or epoxy.
• The backing material performs another very important function in
addition to providing ease of handling and simplicity of application.
• The cement provides so much lateral resistance to the foil that it can
be shortened significantly without buckling; then compressive as well
as tensile strains can be measured.
• Lead wires or connection terminals are often provided on foil gages,
as illustrated in the typical foil gage
Types of strain gauge transducer
• Unbounded strain gauge: a resistance wire
is stretched between two frames one
being the moving frame and other being
the fixed one.
• Bounded strain gauges: widely used for
measuring force torque, vibration etc. it
can be of metal foil or small rods
Measuring Strain (Strain Gages)
• Two common arrangements of the three strain gages are:
- Rectangular rosette
- Equiangular rosette
• In rectangular rosette, the gages are placed at angles of 0, 45 and 90
degrees.
• In equiangular rosette, the gages are arranged at 0, 60 and 120 degrees.
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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.
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.
• Strain can be inferred by measuring change in resistance.
• 12. 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.
• 13. BONDED STRAIN GAUGE • A bonded strain-gage element, consisting
of a metallic wire, etched foil, vacuum-deposited film, or semiconductor
bar, is cemented to the strained surface.
• 14. UNBONDED STRAIN GAUGE • The unbonded strain gage 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.
• 15. FOIL STRAIN GAUGE • The foil strain gage has metal foil photo-etched in a
grid pattern on the electric insulator of the thin resin and gage leads attached,
• 16. SEMICONDUCTOR STRAIN GAUGE • For measurements of small strain,
semiconductor strain gauges, so called piezoresistors, are often preferred over
foil gauges. Semiconductor strain gauges depend on the piezoresistive effects
of silicon or germanium and measure the change in resistance with stress as
opposed to strain.
• 17. PHOTOELECTRIC STRAIN GAUGE • The photoelectric gauge uses a light
beam, two fine gratings, and a photocell detector to generate an electrical
current that is proportional to strain. The gage length of these devices can be
as short as 1/16 inch, but they are costly and delicate.
two-gauge method
• To measure the shaft bending moment, the shaft is rotated by 360°,
and the amount of deformation at this time can be represented by a
value corresponding to the shaft rotation angle.
• The strain of the sine wave shape related to
• bending moment,
• distance between the neutral axis of the shaft and the strain gauge.
• The axial stress σ can be expressed by the beam relation used in the
calculation of uniaxial stress on the shaft surface.
• In the hollow axis and the solid axis, I is to be respectively and c=
do/2
• vertical and horizontal moments are expressed by
• to investigate the influence of the ship operation on the shaft motion
• two non-contact displacement sensors are installed at 90° intervals
from the shaft centre and an additional displacement sensor is
installed to record the rotational speed
Configuration of measuring equipment
