What is Strain Gauge

A strain gauge is an example of a passive transducer that converts a mechanical displacement into a change of resistance. It is a thin, wafer-like device that can be attached to a variety of materials by a suitable adhesive to measure the applied strain. As the structure is stressed, the resulting strain deforms the strain gauge attached to the structure. It causes an increase in the resistivity of the gauge which produces an electrical signal proportional to the deformation. The strain gauge displacement sensor consists of a structure attached with the strain gauge that elastically deforms when subjected to a displacement shown in Figure 1.32(a).

The change of resistance is very small and it is usually measured using a Wheatstone bridge circuit where a strain gauge is connected into the circuit with a combination of four active gauges for full-bridge, two gauges for half-bridge or a single gauge for the quarter bridge. In half and quarter circuits, the bridge is completed with precision resistors.

Working principle of strain gauge :

Gauge Factor: It is the ration of per unit change in resistance to per unit change in length.

When force is applied to any metallic wire its length increases due to the strain. If L1 is the initial length of the wire and L2 is the final length after application of the force, the strain is given as:

ε =(L2-L1)/L1

Further, as the length of the stretched wire increases, its diameter decreases. so that resistance of the conductor is the direct function of the length. As the length of the conductor increases its resistance increases. This change in resistance of the conductor can be measured easily and calibrated against the applied force. Thus strain gauges can be used to measure force and related parameters like displacement and stress. The input and output relationship of the strain gauges can be expressed by the term gauge factor or gauge gradient, which is defined as the change in resistance R for the given value of applied strain ε.

Requirements of strain gauge material

i. Strain gauge should be small in size with negligible mass.
ii. It should be highly sensitive to strain.
iii. Strain gauge should have a high value of gauge factor.
iv. It should be easily attachable to the specimen.
v. It should have a high speed of response with negligible time lag.
vi. It should be capable to indicate static, transient, and dynamic strain.
vii. It should be capable of remote indication and recording.
viii. It should not be sensitive to ambient conditions such as temperature, humidity, vibration, etc.
ix. It should be inexpensive, reliable and easily available in various sizes

Types of strain gauges

Based on the principle of working, strain gauges are classified into

• Mechanical
• Electrical
• Piezoelectric

Based on mounting, they are classified into

1. Bonded strain gauge
2. Unbounded strain gauge

Based on construction, they are classified into

1. Foil strain gauge
2. Semiconductor strain gauge
3. Photoelectric Strain gauge

• A wide variety of gauge sizes and grid shapes are available. The metallic strain gauge consists of a very fine wire or metallic foil arranged in a grid pattern. The grid pattern maximizes the amount of metallic wire or foil subject to strain in the parallel direction. The cross-sectional area of the grid is minimized to reduce the effect of shear strain and Poisson’s strain. The grid is bonded to a thin backing called carrier which is attached directly to the test specimen.
• The majority of strain gauges are bonded foil types available in a wide choice of shapes and sizes to suit a variety of applications and typical examples are shown in Figure.

• They consist of a pattern of resistive foil which is mounted on a backing material. They operate on the principle that as the foil is subjected to stress, the resistance of the foil changes in a defined way.
• Bonded foil strain gauges can be as small as 16 mm2 and have strain sensitivity or gauge factor of 2.
• Wire wound gauges are made of round wire of copper-nickel, chrome-nickel, or nickel-iron alloys about 0.0064 cm diameter. The length of the wire is 25 mm or less. The figure shows the example of a wire-wound strain gauges.
• The environmental considerations focus mainly on the temperature of the gauge. Since the resistance is a function of temperature, the strain gauges are susceptible to variations in temperature. Thus, if it is known that the temperature of the gauge will vary due to any influence and the temperature compensation is required to ensure that the force measurement is accurate.

Strain gauge load cell

• A load cell is an electromechanical transducer that converts load acting on it into an analog electrical signal. Load cells provide an accurate measurement of compressive and tensile loads.
• Load cells commonly function by utilizing an internal strain gauge that measures deflection. The amount of strain can be calibrated to determine the force upon the load cell because the modulus of elasticity of a load cell is constant.
• Typically, the force creates the strain in the load cell which is measured by a strain gauge transducer.
• The strain gauge is attached to the object or the strained element where the force is being applied. As the object is stressed due to the applied force, the resulting strain deforms the strain gauge attached to it. It causes an increase in resistivity of the gauge which produces electrical signals proportional to the deformation.
• The measurement of resistivity is the measure of strain which in turn gives the measurement of force or load applied on the object. The change of resistance is generally very small and it is usually measured using a Wheatstone bridge circuit where strain gauges are connected into the circuit.
• The strain gauges are serving as resistors in the circuit. The Wheatstone bridge circuit produces an analog electrical output signal. In a typical strain gauge load cell for measuring force, four strain gauges are attached to the surface of the counterforce and they are electrically connected in a full Wheatstone bridge circuit shown in Figure.
• Load cells have different shapes (cylindrical tubes, rectangular or square beams, and shaft) for different applications and load requirements. To ensure that the desired component of force is measured, strain gauges having different shapes are positioned in various orientations upon the load cell body.

  

Diaphragm with strain gauge :

1) Strain gauge is a passive type resistance pressure transducer whose electrical resistance changes when it is stretched or compressed. It can be attached to a pressure sensing diaphragm as shown in fig
2) When diaphragm flexes due to the process pressure applied on it, the strain gauge stretches or compresses due to this resistance changes.

3) As soon as the pressure is applied the strain gauge stretches or compresses accordingly and the bridge circuit in fig is unbalanced due to the change in resistance of the strain gauges.
4) Thus a current flows in the galvanometer, Which is measured by the deflection of the galvanometer, this change in output voltage may be calibrated for the pressure change.

Bonded strain gauge :

Working of Bonded Strain Gauges :

• With the help of an adhesive material, the strain gauge is pasted/ bonded on the structure under study.
• The structure is subjected to a force (tensile or compressive). Due to the force, the structure will change the dimension.
• As the strain gauge is bonded to the structure, the strain gauge will also undergo a change in both in length and cross-section (that is, it strained).
• This strain (change in dimension) changes the resistance of the strain gauge which can be measured using a wheat stone bridge.
• This change in resistance of the strain gauge becomes a measure of the extent to which the structure is strained and a measure of the applied force when calibrated.

Semiconductor Strain gauge.

• These gauges are produced from silicon and germanium crystals in which a certain amount of special impurities are added to impart certain characteristics.
• Two types

a) Negative or n-type, whose resistance decreases in response to tensile strain
b) Positive or p-type, whose resistance increases in response to tensile strain.

• The breaking stress of material rises as the cross-sectional area decreases.
• The gauge is in the form of a single rectangular filament about 0.05 mm thick and 0.25 mm wide and 1.5 mm to 12 mm in length.
• Advantages: Very high sensitivity in comparison of metal gauges, High gauge factor in the range of 100 to 200. , Low hysteresis.

Applications of Strain Gauge : 

(i) Strain measurement
(ii) Residual stress measurement
(iii) Vibration measurement
(iv) Torque measurement
(v) Bending and deflection measurement
(vi) Compression and tension measurement

Advantages of Strain Gauge : 

(i) There is no moving part and hence no wear
(ii) Strain gauges are very precise
(iii) It is small and inexpensive
(iv) It has a high-frequency bandwidth.

Disadvantages of Strain Gauge :

(i) It is non-linear
(ii) It is very sensitive to temperature.
(iii) It needs to be calibrated regularly
(iv) Strain gauges have to be applied manually. Putting them in their place consuming and costly. It is one of their biggest disadvantages

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