PRESENTED BY BHOSALE SURAJ UTTAMRAO M.E.(CIVIL-STRUCTURE) M.E.(CIVIL-STRUCTURE) SEM-I
UNDER THE GUIDANCE OF Prof. Dr. Dr. A.N. A.N . BAMBOLE BAM BOLE APPLIED MECHANIC DEPT. DEPT. SARDAR PATEL PATEL COLLEGE OF ENGINERRING, ANDHERI (W), MUMBAI. M UMBAI.
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.
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 with reticle 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
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 µFrozen-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
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.
