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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RR44

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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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