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
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.
c) Huggenberger Extensometer Sets of levers are been used in this to magnify the displacement. Formula for measurement = L1L2/a1a2 Ratio
Gauge length range
300-2000
6.5 to 100 mm
Sensitivity as high as 10microstrain. Suitability: - Where large height does not pose problems of mounting.
d) Scratch Gauge
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.
2) Optical Gauges:Mechanical-Optical Gauges:Combination of mechanical and optical levers is used to amplify the relative displacement between knife edges.
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 autocollimator will not affect the measurement. • Sensitivity is 2microstrain. • Available in wide range starting from 6mm. • It can reliably measure both static and dynamic strains
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.
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 Bridge A 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.
R1 +
-
R4 V0
VEX R2
R3
Wheatstone Bridge R1 +
-
R4 V0
VEX R2
R3
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 paint Nitrile rubber Silicon Rubber Permanently plastic putty
