A "bonded strain gauge" is a type of transducer used to measure strain (deformation) in materials. It operates based on the principle of piezoresistivity, where the electrical resistance of a material changes when subjected to mechanical stress. These strain gauges are directly attached or bonded to the surface of the structure under study. Let’s dive into the details:

1. Construction and Working Principle:

• A bonded strain gauge consists of a fine resistance wire (usually made of materials like nichrome, constantan, or nickel) bonded or pasted onto a thin, flexible substrate (such as paper, tissue, Bakelite, or Teflon).
• The substrate acts as a carrier, and the gauge wire is directly attached to the surface of the structure.
• When the structure experiences stress or strain (due to forces, pressure, or deformation), the gauge wire also undergoes changes in length and cross-section.
• As a result, the electrical resistance of the gauge wire changes proportionally to the applied strain.

2. Types of Bonded Strain Gauges:

Bonded strain gauges are further divided into three main types:

• Wire Type Strain Gauge:

  • Available in both bonded and unbonded configurations.
  • In the bonded type, the strain gauge is directly pasted onto the surface of the structure under test using adhesives.
  • Adhesives transmit strain from the structure to the gauge wires.
  • Varieties include:
    • Flat grid wire gauge
    • Wrap-around wire gauge
    • Single wire gauge
    • Woven type gauge

• Foil Type Strain Gauge:

  • Composed of metallic foil (usually made of materials like constantan or nickel) bonded to a flexible insulating carrier.
  • Foil strain gauges are widely used due to their versatility and accuracy.
  • They exhibit excellent linearity and sensitivity.

• Semiconductor Strain Gauge:

  • Utilizes semiconductor materials (such as silicon) with piezoresistive properties.
  • These gauges are sensitive to temperature changes and can be integrated into microelectronic circuits.

3. Advantages and Applications:

• Advantages:
  • Greater accuracy compared to other strain gauge types.
  • Suitable for a wide range of strain measurements.
  • Can be used in the range of ±0.15% strain.
• Applications:
  • Measurement of pressure, acceleration, and force.
  • Used in systems where gauges need placement at different locations and frequent stress or pressure measurements.