3.3 Strain Gauges and Fiber Optic Sensors

Strain gauges and fiber optic sensors are crucial tools in structural health monitoring. These devices measure deformation and strain in structures, providing vital data for assessing integrity and safety. Each type has unique strengths and applications, from simple foil gauges to advanced fiber optic systems.

Choosing between strain gauges and fiber optic sensors depends on specific monitoring needs. Strain gauges excel in localized measurements and are cost-effective, while fiber optic sensors offer distributed sensing and perform well in harsh environments. Understanding their differences helps engineers select the best tool for each monitoring scenario.

Strain Gauges

Principles of strain gauges

Top images from around the web for Principles of strain gauges

• 

  JSSS - Creep adjustment of strain gauges based on granular NiCr-carbon thin films View original

  Is this image relevant?

• 

  JSSS - Creep adjustment of strain gauges based on granular NiCr-carbon thin films View original

  Is this image relevant?

• 

  JSSS - Novel method to reduce the transverse sensitivity of granular thin film strain gauges by ... View original

  Is this image relevant?

• 

  JSSS - Creep adjustment of strain gauges based on granular NiCr-carbon thin films View original

  Is this image relevant?

• 

  JSSS - Creep adjustment of strain gauges based on granular NiCr-carbon thin films View original

  Is this image relevant?

1 of 3

Top images from around the web for Principles of strain gauges

• 

  JSSS - Creep adjustment of strain gauges based on granular NiCr-carbon thin films View original

  Is this image relevant?

• 

  JSSS - Creep adjustment of strain gauges based on granular NiCr-carbon thin films View original

  Is this image relevant?

• 

  JSSS - Novel method to reduce the transverse sensitivity of granular thin film strain gauges by ... View original

  Is this image relevant?

• 

  JSSS - Creep adjustment of strain gauges based on granular NiCr-carbon thin films View original

  Is this image relevant?

• 

  JSSS - Creep adjustment of strain gauges based on granular NiCr-carbon thin films View original

  Is this image relevant?

1 of 3

• Strain gauges measure strain by converting mechanical deformation into electrical resistance change
  • Deformation alters the cross-sectional area and length of the gauge, changing its resistance
  • Relationship between strain and resistance change is given by the gauge factor ($GF = (ΔR/R) / ε$)
• Types of strain gauges:
  • Foil strain gauges: Metallic foil pattern on a flexible insulating backing
    • Commonly made of constantan alloy for its high strain sensitivity and low temperature sensitivity (copper-nickel alloy)
  • Semiconductor strain gauges: Made of silicon or germanium
    • Piezoresistive effect causes resistance change under strain
    • Higher gauge factor and sensitivity compared to foil gauges (up to 100 times more sensitive)
  • Weldable strain gauges: Designed for direct spot welding to the structure
    • Suitable for high-temperature applications and harsh environments (up to 1000°C)
  • Fiber optic strain gauges: Use optical fibers to measure strain (covered in a separate section)

Installation of strain gauges

• Surface preparation:
  • Clean the surface to remove contaminants and ensure proper adhesion (degreasing, abrading)
  • Roughen the surface to improve bonding (sandpaper, acid etching)
  • Apply a suitable bonding agent to attach the gauge (cyanoacrylate adhesive, epoxy)
• Gauge alignment:
  • Orient the gauge along the direction of the strain to be measured
  • Use alignment marks or a special alignment tool for precise positioning (gauge alignment guide)
• Wiring and connections:
  • Use lead wires to connect the gauge to the measurement system
  • Minimize lead wire resistance and ensure secure connections to avoid signal distortion (soldering, crimping)
• Measurement techniques:
  • Quarter-bridge configuration: Single active gauge with three dummy resistors
    • Suitable for general-purpose strain measurements
  • Half-bridge configuration: Two active gauges, one in tension and one in compression
    • Compensates for temperature effects and increases sensitivity
  • Full-bridge configuration: Four active gauges arranged in a Wheatstone bridge
    • Provides the highest sensitivity and temperature compensation
• Data acquisition and processing:
  • Use a strain gauge amplifier to condition and amplify the signal
  • Convert the analog signal to digital using an analog-to-digital converter (ADC)
  • Process and analyze the data using appropriate software tools (LabVIEW, MATLAB)

Fiber Optic Sensors

Fiber optic sensors in monitoring

• Fiber optic sensors use light propagation through optical fibers to measure strain and other parameters
• Operating principles:
  • Fiber Bragg Grating (FBG) sensors:
    • Periodic variations in the refractive index of the fiber core create a grating
    • Strain or temperature changes cause a shift in the reflected wavelength
    • Relationship between wavelength shift and strain is given by $Δλ/λ = (1 - p_e)ε$, where $p_e$ is the photoelastic coefficient
  • Fabry-Perot Interferometric (FPI) sensors:
    • Two partially reflective mirrors create an interferometric cavity
    • Strain changes the cavity length, altering the interference pattern of the reflected light
• Advantages of fiber optic sensors:
  • Immunity to electromagnetic interference (EMI) and radio frequency interference (RFI)
  • High sensitivity and resolution (sub-microstrain)
  • Long-term stability and durability (>20 years)
  • Multiplexing capability, allowing multiple sensors on a single fiber (wavelength division multiplexing)
  • Lightweight and small size, minimally invasive to the structure (diameter <1mm)
  • Wide operating temperature range (-270°C to 1000°C)

Strain gauges vs fiber optic sensors

• Strain gauges:
  • Suitable for localized strain measurements at specific points
  • Lower cost compared to fiber optic sensors
  • Easier to install and replace
  • Limited multiplexing capability (typically <10 gauges per channel)
  • Susceptible to EMI and RFI
  • Shorter service life due to mechanical fatigue and environmental factors (3-5 years)
• Fiber optic sensors:
  • Ideal for distributed and quasi-distributed strain measurements over large areas
  • Higher cost compared to strain gauges
  • More complex installation and interrogation systems
  • Excellent multiplexing capability, reducing cabling requirements (>100 sensors per fiber)
  • Immune to EMI and RFI, suitable for harsh environments
  • Long service life and low maintenance requirements (>20 years)
• Selection criteria:
  1. Measurement requirements (localized vs. distributed, strain range, resolution)
  2. Environmental conditions (temperature, humidity, corrosion, EMI)
  3. Budget and cost considerations
  4. Installation constraints and accessibility
  5. Long-term monitoring and maintenance needs