8.2 Strain Gauge Types and Installation Techniques

Strain gauges are essential tools in structural health monitoring, measuring tiny deformations in materials under stress. This section covers different types of strain gauges, including foil, semiconductor, and fiber optic, each with unique properties and applications.

Selecting and installing the right strain gauge is crucial for accurate measurements. We'll explore key factors to consider when choosing a gauge and walk through the proper installation techniques, from surface preparation to bonding methods.

Strain Gauge Types and Working Principles

Types of strain gauges

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• Foil strain gauges
  • Consist of a metallic foil pattern (constantan or karma alloy) bonded to a flexible backing material (polyimide or epoxy)
  • Resistance changes when the foil is subjected to strain due to the piezoresistive effect
  • Widely used in general-purpose strain measurement applications (structural testing, load cells)
• Semiconductor strain gauges
  • Made from silicon or germanium substrates doped with impurities (boron or phosphorus)
  • Piezoresistive effect causes resistance to change significantly with applied stress
  • Higher sensitivity (gauge factor of 50-200) compared to foil strain gauges (gauge factor of 2-5)
  • Suitable for measuring small strains and dynamic strain events (impact testing, vibration analysis)
• Fiber optic strain gauges
  • Use optical fibers with Bragg gratings etched into the core to measure strain
  • Bragg gratings reflect specific wavelengths that shift under strain due to changes in the grating spacing
  • Immune to electromagnetic interference and suitable for harsh environments (high temperatures, corrosive conditions)
  • Enable distributed strain sensing over long distances (bridges, pipelines, wind turbine blades)

Strain Gauge Selection and Installation

Selection criteria for strain gauges

• Consider the following factors when selecting strain gauges:
  • Strain range and sensitivity: Choose gauges with appropriate range and sensitivity for the expected strain levels (±1000 με for general purpose, ±50,000 με for post-yield)
  • Temperature range and thermal effects: Select gauges with temperature compensation for the expected operating range (-50 to 200 ℃ for general purpose, up to 1000 ℃ for high-temperature applications)
  • Material compatibility with the test specimen: Ensure gauge backing material and adhesive are compatible with the specimen material (steel, aluminum, composites)
  • Environmental factors: Consider humidity, chemicals, and other environmental factors that may affect gauge performance (waterproof coatings for underwater applications, protective coatings for corrosive environments)
• For high-temperature applications, use special alloys (Nichrome V) or ceramic materials (WC-coated constantan) to minimize thermal drift
• For large strains beyond the yield point, consider using post-yield strain gauges with a larger strain range and improved bonding (epoxy phenolic adhesives)

Installation techniques for strain gauges

1. Surface preparation steps:
   • Clean the surface thoroughly with solvents (isopropyl alcohol, acetone) to remove contaminants (oils, greases, oxides)
   • Abrade the surface with fine grit sandpaper (320-400 grit) to create a smooth, uniform surface for better adhesion
   • Condition the surface with a compatible acidic (phosphoric acid) or alkaline (ammonium hydroxide) solution to improve bonding
   • Neutralize the surface with a neutralizing solution (distilled water) to prevent chemical interactions with the adhesive
2. Bonding techniques:
   • Select an appropriate adhesive based on the operating temperature and environmental conditions (cyanoacrylate for general purpose, epoxy for high temperatures)
   • Apply a thin, uniform layer of adhesive to the prepared surface using a brush or applicator
   • Align and press the strain gauge onto the adhesive, ensuring no air bubbles are trapped between the gauge and the surface
   • Apply pressure (finger pressure or clamping) and heat (if required) to cure the adhesive according to the manufacturer's instructions (24 hours at room temperature for cyanoacrylate, 2 hours at 150 ℃ for epoxy)

Strain Gauge Circuits and Temperature Compensation

Bridge circuits for strain gauges

• Quarter bridge circuit
  • Single active strain gauge with three fixed resistors forming a Wheatstone bridge
  • Temperature compensation achieved by using a dummy gauge in the same thermal environment or a temperature-sensitive resistor (resistor with a known temperature coefficient)
  • Suitable for general-purpose strain measurements with moderate accuracy
• Half bridge circuit
  • Two active strain gauges, typically arranged in a Poisson configuration (one gauge in tension, one in compression)
  • Automatically compensates for temperature effects when gauges are subjected to the same thermal conditions
  • Provides higher sensitivity and better temperature compensation compared to quarter bridge circuits
  • Commonly used in bending and axial strain measurements (beams, columns)
• Full bridge circuit
  • Four active strain gauges, providing the highest sensitivity and automatic temperature compensation
  • Suitable for measuring bending, axial, and shear strains with high accuracy
  • Commonly used in load cells, pressure transducers, and torque sensors
• Temperature compensation techniques
  • Use of dummy gauges in the same thermal environment as the active gauges to cancel out temperature effects
  • Self-temperature-compensating (STC) strain gauges with built-in temperature compensation using a combination of materials with different thermal expansion coefficients
  • Mathematical correction using the gauge factor and thermal expansion coefficients of the gauge and specimen materials to adjust the strain readings for temperature changes
  • $ε_c = ε_m - (α_g - α_s) ΔT$, where $ε_c$ is the corrected strain, $ε_m$ is the measured strain, $α_g$ and $α_s$ are the thermal expansion coefficients of the gauge and specimen, and $ΔT$ is the temperature change