Strain Gauge - Part 1

Contents: 
1.0 Definition of stain gauge
2.0 Classification of strain gauges
          2.1 Mechanical strain gauges
                 2.1.1 Berry Strain gauge 
                 2.1.2 Huggenbeger Extensometer
                 2.1.3 Johansson Extensometer
          2.2 Electrical Strain gauges
          2.3 Optical strain gauges
          2.4 Pneumatic strain gauges 
          2.5 Acoustical strain gauges

1. Definition :
A strain gauge is a device used to measure the strain on a free surface of a structure. Strain gages are the preeminent tool in stress analysis. Strain gauges of all types are essentially employed to measure the linear deformation over a given gauge length. The sense the change in length, Magnify and indicate it in some other form. Strain Gauge is invented by Edward E Simmons and Arthur C Ruge in the year 1938.

2. Classification of Strain Gauges
Depending up on the magnification system, the strain gauges are broadly classified as under,
     a. Mechanical strain gauges
     b. Electrical strain gauges
     c. Optical strain gauges
     d. Pneumatic strain gauges
     e. Acoustical strain gauges

2.1 Mechanical strain gauges
Mechanical strain gauges are also known as Extensometers used to measure static or gradually varying load conditions. These gauges are usually provided with two knife edges which are clamped firmly in contact with the test component by means of a clamping spring at a specific distance of gauge length. When the specimen under testing is strained the knife edges undergoes displacement, this displacement is amplified by a mechanical linkages and the strain is displaced on a calibrated scale.

Types of Mechanical strain gauges
2.1.1 Berry Strain gauge
These strain gauges uses a lever magnification with dial indicator to show magnified motion. It consists of one rigid frame and two conically pointed contact pointers. One pointer is rigidly fixed to the frame while the other is pivoted at a point on the frame. The displacement in the lever is magnified and indicated in the dial indicator.

2.1.2 Huggenbeger Extensometer
This extensometer has a set of compound levers which are relatively small in size and high magnification factor. These gauges are highly accurate. The movable knife edge rotates the lever at lower pivot, the lever in turn rotates the indicator pointer at upper pivot point with the help of a link.

2.1.3 Johansson Extensometer
These extensometers uses tension tape or twisted metal strip between two knife edges. Half of the strip is twisted to one direction and remaining half is twisted to other direction and a pointer is fixed at the center of the strip. On application of load, displacement in the movable knife edge takes place with high amplification due to stretching of twisted metal strip.