stress measuremnt

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Anand K Bewoor & Vinay A Kulkarni METROLOGY & MEASUREMENT © 2009 Tata McGraw-Hill Education

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METROLOGY & MEASUREMENT

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Anand K Bewoor & Vinay A Kulkarni




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Chapter 21

StrainMeasurement




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‘Strain has formed a basic component of our life applications which,

has to be sensed, measured and analysed….’

INTRODUCTION TO STRAIN GAUGEThe strain gauge has been in use for many years and is the fundamental sensing element for

many types of sensors, including pressure sensors, load cells, torque sensors, position

sensors, etc. The accurate measurement of strain can be made using strain gauges. Given a

measurement of strain, stress and load may also be calculated via the definition of Young’s

modulus as stress divided by strain, and the definition for stress as force (or load) divided by

area. The most common form of strain gauge is the electrical resistance strain gauge ––

originally invented by Lord Kelvin in circa 1856. Kelvin observed that the resistance of a

conductor varies deterministically when the conductor is stretched (or strained). Therefore, if a

conductor is bonded to a structure such that the change in length of the structure is equal to

the change in length of the conductor, the change of resistance of the conductor is directly

proportional to strain.

The gauges are formed by either a length of wire arranged in an axial grid pattern, or by etching

a thin metal foil into the desired shape. The majority of strain gauges are foil types, available ina wide choice of shapes and sizes to suit a variety of applications. In either case the conductor

is bonded to a backing sheet. In turn, the backing is securely bonded to the structure to be

measured such that a surface strain also strains the conductor. They operate on the principle

that as the foil is subjected to stress, the resistance of the foil changes in a defined way.




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In general, wire gauges are used for high temperature applications, and foil gauges are

used for routine applications. Foil gauges offer the following characteristics.

1. High stability

2. Good proportionality

3. Manufacturing process based on etching which is cheap and allows complex

conductor designs to be obtained4. Low price [per gauge is 25 p to £ 10 (installation and calibration costs are

significantly higher)]

5. Low output voltage ––requires amplification.

A strain gauge’s conductors are very thin—if made of round wire, about 1/1000 inch in

diameter. Alternatively, strain gauge conductors may be thin strips of metallic film

deposited on a non-conducting substrate material called the carrier.




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stress measuremnt

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