What is a Strain Gauge?

An electrical strain gauge is a thin insulated foil grid that is bonded to a material. The change in length of the foil grid results in a change in resistance which is directly proportional to the change in length of the material to which it is bonded. The process of bonding the strain gauge is critical to ensure the change in length of the material is correctly transferred to the strain gauge.

Strain Gauge construction

Figure 1: Strain Gauge construction

How a Strain Gauge Works

When a material elongates or contracts it experiences strain. This elongation and contraction may be caused by:

  • an applied force resulting in mechanical strain on the material,
  • change in temperature of the material resulting in thermal strain,
  • internal forces caused by cooling or material processing methods resulting in residual strain.

How a Strain Gauge manage stress

The change in length of material happens on a micrometre scale and is not possible to detect visually. With the use of a strain gauge, we can measure the change in length of the material accurately to less than a micron.

As the material changes in length there is a proportionate change in resistance of the strain gauges. With the aid of the Wheatstone Bridge (Figure 2) the change in resistance is measured and divided by gauge factor (k-factor) resulting in a strain value (ꙡm/m)

Wheatstone bridge circuit

Figure 2: Wheatstone bridge circuit

Strain Gauge principle

Figure 3: Strain Gauge change measurement principle

Δl/lo = Change in length over original strain gauge length

 ε = Strain

 k = Gauge factor (k-factor)

ΔR/R = Measured change in resistance over original resistance of strain gauge

People also ask

Where are Strain Gauges used?

By the selection of the correct strain gauge and direction of measurement a strain gauge can be used to determine several mechanical quantities such as:

  • principle stress’ and angle with a rosette strain gauge,
  • torque,
  • weight,
  • pressure,
  • force,
  • displacement,
  • residual forces in material.

What are Strain Gauges made of?

The strain gauge measurement grid and insulating carrier material are manufactured from different materials depending on the required application.

  • The “standard” universal strain gauge commonly used for experimental stress analysis (ESA) and simple transducers.
    • Measurement grid material: Constantan
    • Carrier material: Polyimide
  • Strain gauges used for measurements at high temperature.
    • Measurement grid material: Cr-Ni
    • Carrier material: Polyimide
  • For high fatigue measurements and extreme temperatures.
    • Measurement grid material: Cr-Ni
    • Carrier material: Phenolic resin, glass-fibre reinforced

For more information about TANDM’s strain gauges, contact info@tandm.co.za or click the button below. 

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