Short Answer:

A bonded strain gauge is a type of strain gauge that is firmly attached (bonded) to the surface of a material using a special adhesive. It is designed to measure the strain or deformation that occurs in the material when it is subjected to external force or stress.

As the material stretches or compresses, the bonded strain gauge deforms along with it, causing a small change in its electrical resistance. This change is measured and used to calculate the strain, making bonded strain gauges useful in structural testing, machine monitoring, and many engineering applications.

Detailed Explanation:

Bonded Strain Gauge

A bonded strain gauge is one of the most common and reliable tools used for measuring strain in materials. It consists of a fine wire or metallic foil arranged in a grid pattern and mounted on a flexible backing material. This entire assembly is bonded to the test surface using a strong adhesive so that any strain in the material is directly transferred to the strain gauge.

Bonded strain gauges are widely used in mechanical, civil, aerospace, and electrical engineering to monitor stress and deformation in structures, components, and machines.

Structure of a Bonded Strain Gauge

1. Resistive Element
   The key component is a thin metallic wire or foil (often made of constantan), which acts as the sensing element. This element changes its electrical resistance when stretched or compressed.
2. Backing Material
   The resistive element is attached to a thin insulating and flexible backing, usually made of plastic or paper. This backing allows the gauge to be easily handled and bonded to surfaces.
3. Adhesive Layer
   A special adhesive (like epoxy resin) is used to bond the strain gauge firmly to the surface of the test object. The adhesive must ensure good strain transfer and resist temperature or environmental effects.
4. Lead Wires
   Wires are connected to the resistive element to allow electrical connection to external measurement instruments.

Working Principle

• When the bonded strain gauge is attached to a surface, it experiences the same strain as the material itself.
• As strain occurs (either tension or compression), the length of the resistive wire changes.
• This causes a change in its electrical resistance.
• The change in resistance is usually very small and is measured using a Wheatstone bridge circuit.
• The strain (ε) is calculated from the resistance change using the gauge factor.

Advantages of Bonded Strain Gauges

• High accuracy in measuring strain
• Small size and light weight
• Reliable bonding ensures accurate strain transfer
• Suitable for both static and dynamic measurements
• Can be used on many materials, including metals, plastics, and composites

Applications of Bonded Strain Gauges

• Bridge and building monitoring for stress and safety
• Aircraft and spacecraft testing
• Load measurement in machines and structures
• Automotive testing (chassis, engine parts)
• Pressure sensing using diaphragm-mounted strain gauges

Limitations of Bonded Strain Gauges

• Requires careful surface preparation for proper bonding
• Temperature changes can affect readings if not compensated
• Limited life in harsh or corrosive environments without protection
• Adhesive may degrade over time in extreme conditions

Conclusion

A bonded strain gauge is a precision sensor that is glued to the surface of a structure to measure strain by detecting small changes in electrical resistance. It works by transferring the strain from the surface to the resistive element of the gauge. Bonded strain gauges are widely used in engineering due to their accuracy, simplicity, and ability to provide direct and reliable strain measurements. They are essential tools in stress analysis, material testing, and structural monitoring.