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Strain gauge
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STRAIN GAUGES
Stress and Strain• When external forces are applied to a stationary object,
stress and strain are the result• Stress is defined as the object's internal resisting
forces• Strain is defined as the displacement and deformation
that occur.
What is a Strain Gauge
• A Strain gauge is a sensor whose resistance varies with applied force
• Strain gauges are used for the measurement of strain
• “Strain" consists of tensile and compressive strain, distinguished by a positive or negative sign
• Strain gauges can be used to pick up expansion as well as contraction
TYPES OF STRAIN GAUGES
• MECHANICAL
• OPTICAL
• PNEUMATIC
• ACOUSTICAL
• ELECTRICAL
MECHANICAL STRAIN GAUGES:-• The mechanical devices are generally known as extensometer
and are used to measure strain under static or gradually varying loading condition.
PRINCIPLE:-
• Extensometer is usually provided with two knife edges.
• They are clamped firmly in contact with the test component at a specific distance.
• When the test component is strained, the two knife edge undergoes a small relative displacement.
• This displacement is amplified through a mechanical linkage and magnified displacement or strain is displayed on a calibrated scale.
Demountable Mechanical Strain Gauge
Different types of Mechanical strain Gauge:-
a) Berry strain gauge
• Lever arm and dial gauge.
• Measure a strain down to 10microstrain over a 50mm gauge length.
B) Johansson Extensometer
• Torsion tape stretched between knife edges.
• Half end is twisted in one direction, while other half in other direction.
• Measure a strain down to 5microstrain over a gauge length of 50mm.
Ratio Gauge length range
300-2000 6.5 to 100 mm
c) Huggenberger Extensometer
Sets of levers are been used in this to magnify the displacement.
Formula for measurement = L1L2/a1a2
Sensitivity as high as 10microstrain.Suitability: - Where large height does not pose problems of mounting.
Scratch Gauge
d) Scratch Gauge
• Scratch gauge:-
• Minimum strain that a scratch gauge can sense is about 100microstrain.
• Suitability: -
– compact in size,
– weight =30g,
– attached to almost any surface with Clamps, screws or adhesive bonding.
Tuckerman’s Optical Gauge:-
• Auto-collimator is used which carries a source of a parallel beam of light and an Optical system to measure the deflection of the reflected ray.
• Lozenge functions as movable knife edge.
• In this system, any relative motion between the component and the auto-collimator will not affect the measurement.
• Sensitivity is 2microstrain.• Available in wide range starting
from 6mm.• It can reliably measure both
static and dynamic strains
2) Optical Gauges:- Mechanical-Optical Gauges:-
Combination of mechanical and optical levers is used to amplify the relative displacement between knife edges.
Optical strain Gauge
Advantage:-• Reduced wiring as compared to electrical strain resistance.• Light weight as compared to others.• Insensitivity to electromagnetic interference.
Photo elastic
strain Gauges:-
• Components- a strip of plastic with reflection backing with ‘Frozen-in’ fringes pattern.
• Polaroid/Quarter sheet.• Graduated scale is there for
measurement.• Sensitivity- 50microstrain.
Pneumatic strain Gauge
• Pneumatic Strain Gauges:-
• Magnification upto 100000 and gauge lengths as small as 1mm are possible to achieve in these gauges.
• They are sensitive, robust and reliable.
• Suitable for both static and dynamic strain measurements.
Acoustic strain Gauge
Acoustic Strain Gauge
Principle
• It consist of Steel Wire tensioned between two predefined supports
• Variation of distance alter the natural frequency with change in strain.
• An Electro magnet adjacent to wire set the oscillation
• Wire movement will generate the electrical signal
• Calibration of screw senses a Strain
Acoustic Strain Gauges:-
Frequency of vibration in test gauge is measured by comparing it with fundamental natural frequency of the wire.
•Sensitivity:- 0.5microstrain
•Advantages: - Long-term reliability.
• High accuracy.
• Facility for remote measurement.
•Chiefly used at: - Reinforced Concrete structure, dams, steel structure.
Electrical Gauges
• INDUCTANCE GAUGES
• CAPACITANCE GAUGES
• ELECTRICAL RESISTANCE GAUGES
Inductance Strain Gauge
a) Inductance Strain Gauge:-• In this Linear variable differential
transformer (LVDT) used.• Two coils Primary and Secondary are
used is attached to one edge and movable magnetic core is connected to the other.
• Movement of the core in one direction away from the null position produces an output alternating voltage proportional to the displacement from the centre
• Suitability – used over a wide range of temperature below 0 to elevated temperature.
• Sensitivity – range 0.02 to 0.15 V/mm displacement per volt of excitation applied to the primary coil.
• Limitation – magnetic fields, size and mass
Capacitance strain Gauge
Capacitance Strain Gauges:-
• Capacitance changes occur due to axial sliding of an outer cylinder relative to two concentric inner cylinders.
• Temperature compensation is achieved by using rod.
• Function satisfactorily upto 800 C.
• Sensitivity – 1microstrain.
Foil strain gauge
• The strain gauge consists of an insulating flexible backing which supports a metallic foil pattern.
• The gauge is attached to the object by a suitable adhesive. As the object is deformed, the foil is deformed, causing its electrical resistance to change.
• Foil type strain gauges having a zigzag conductor made out of an etched foil• Very widely used as bonded strain gauges • This resistance change, usually measured using a Wheatstone bridge, is
related to the strain by the quantity known as the gauge factor.
Wheatstone BridgeA Wheatstone Bridge is an electrical circuit.
Used in a load cell to measure an overall change in resistance.
Increases sensitivity and reduces the affects of temperature.
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Wheatstone Bridge
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Construction of bonded-wire-type strain gage
Wide range of strain gauges
Experimental Tests
Typical strain gauges types
Two-axis strain gauge
120 degree rosette
45 degree rosette
45 degree stacked rosette
membrane rosette
Adhesives
Cyano-acrylate Z70
Methacrylate mixture X60
Epoxy resin X280
Protective coatings
• It is recommended that strain gauges be protect against external effects (such as humidity, water, oil, mechanical influences)
• To protect the polished metal of our application area against rust
Polyurethane paintNitrile rubberSilicon RubberPermanently plastic putty
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