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Types of Resistance Strain Gauge
1. Unbonded Gauges:
a) Non-Metallic
b) Metallic
2. Bonded Gauges:
a) Non-Metallic
b) Metallic
i) Wire type ii) Foil Type
3.Weldable Gauges
4. Piezoresistive Gauges
Unbonded Non-Metallic Gauge
• It is a mechanically actuated gauge contains
resistance element.
• Arranged in such a way that when one part of
the gauge displaced with respect to another
devloping a change in pressure on the
measuring gauge.
• This change in pressure changes the resistance
of the element and the recorded electrically.
-Contn....
• The gauge is composed of series of plates
arranged in a stack.
• Change is displacement creats pressure
difference in the stack and finally the
resistance changes.
Applications
• To determine displacement, loads, strain in
flexible cables, airplanes, bridges, vibrating
members, dynamometers, pressure gauges.
• The disadvantages are
Mechanical Friction
Hyserisis
Unbonded Metallic Gauge
• The principle is based on the change in
electrical resistance of a metallic wire is due to
change in tension of the wire
• Designed by Carlson and Eaton in 1930.
• It is constructed by winding wire in three coils
which are placed under initial tension into
four arm wheatstone Bridge.
-Contn.
• As the compressive strain is applied the
prestrain would simply be relieved and the
unbonded element would remain taut.
• The types of the gauges are rarely used for
experimental stress analysis.
• The
mechanism
is
incorporated
in
Accelerometers and pressure pick ups.
Bonded Non Metallic Gauge
• A strain gauge of non- metallic resistor
element is bonded (Using common glue)
directly the material in which strain is to be
measured.
• For metallic structures the surface is first
coated with non-conducting material.
• In this type carbon coating is applied directly
to the surface of the structure in which strain
is to be measured.
• When the Carbon Coating Stretched, the
carbon particles move apart.
• It the carbon coating compressed the particles
would move closer creates change in
resistance.
• This change in resistance is measured as
strain.
• Carbon sheets are cut into strips of 6mm wide
and 25mm long
• On each end silver band is plated to attach
lead wire
• The sensitivity and resistance of the gauge are
affected by temperature and humidity.
Bonded Metallic Gauge
• This type of strain gauge consist of a strain
sensitive conductor mounted on a backing
paper.
• The gauge is cemented on the surface to be
tested.
• This gauge can be wire type or foil type.
-Contn..
• The wire grid may be wither a flat type or
wrap around type.
• Attach the lead wires to the two ends of the
grid.
• Second layer of paper is cemented over the
wire grid.
• In wrap type of wire gauges the wire is wound
around the cylindrical core in form of helix
Disadvantages
• Comparatively costly
• Time consuming
• Complicated Bonding
In order to overcome the limitations led to the
development of weldable wire resistance strain
gauge
Weld able strain gauges
Description
• The weldable strain gauge consist of a strain
sensitive element Nickel Chrome or Platinum
Tungsten housed with a small diameter
stainless steel tube.
• The strain element is insulated with highly
compacted ceramic insulation or metallic
oxide powder.
-Contn…
• The basic construction of quarter bridge or
half bridge includes integral flexible lead wire
configurations.
• When the gauge is welded to the specimen
and the test specimen put into tension or
compression, the stress is transmitted through
the weld to the mounting flange into the
strain tube through the MgO powder
Piezo resistive strain gauge
• Crystals of Silicon, Germanium, quartz and
Rochelle salt shows the change in resistance
when deformed by applying pressure