6.2 Frequency Response Function-Based Techniques

Frequency Response Functions (FRFs) are key tools in structural health monitoring. They show how structures react to forces at different frequencies, helping detect damage by comparing healthy and potentially damaged states. FRFs are sensitive to changes in stiffness, mass, and damping.

Measuring FRFs involves exciting structures and recording responses. Various techniques, like transmissibility and damage index methods, analyze FRF data to spot damage. These methods can detect different damage types but depend on measurement quality and the ability to distinguish damage from environmental changes.

Frequency Response Function (FRF) Fundamentals

Frequency response functions in damage detection

Top images from around the web for Frequency response functions in damage detection

• 

  Frontiers | Structural Health Monitoring of a Cable-Stayed Bridge Using Regularly Conducted ... View original

  Is this image relevant?

• 

  Frontiers | Structural Health Monitoring of a Cable-Stayed Bridge Using Regularly Conducted ... View original

  Is this image relevant?

• 

  Frontiers | Structural Health Monitoring of a Cable-Stayed Bridge Using Regularly Conducted ... View original

  Is this image relevant?

• 

  Frontiers | Structural Health Monitoring of a Cable-Stayed Bridge Using Regularly Conducted ... View original

  Is this image relevant?

1 of 2

Top images from around the web for Frequency response functions in damage detection

• 

  Frontiers | Structural Health Monitoring of a Cable-Stayed Bridge Using Regularly Conducted ... View original

  Is this image relevant?

• 

  Frontiers | Structural Health Monitoring of a Cable-Stayed Bridge Using Regularly Conducted ... View original

  Is this image relevant?

• 

  Frontiers | Structural Health Monitoring of a Cable-Stayed Bridge Using Regularly Conducted ... View original

  Is this image relevant?

• 

  Frontiers | Structural Health Monitoring of a Cable-Stayed Bridge Using Regularly Conducted ... View original

  Is this image relevant?

1 of 2

• FRFs represent a system's input-output relationship in the frequency domain describing how a structure responds to excitation forces at different frequencies (impact hammers, shakers) and capturing its dynamic behavior
• Compare FRFs of a healthy structure to those of a potentially damaged structure to indicate the presence, location, and severity of damage as changes in FRFs are sensitive to variations in structural properties such as stiffness, mass, and damping

FRF measurement for structural monitoring

• Excite the structure with a known input force (impact hammers, shakers, ambient vibrations) and measure the output response using accelerometers or laser vibrometers
• Calculate FRFs by dividing the output response by the input force in the frequency domain requiring Fourier transform of time-domain signals and assess the quality of the FRF measurements using the coherence function
• Compare the FRFs of the structure in its current state to a baseline or reference state obtained from the healthy structure or a finite element model using statistical methods and pattern recognition techniques to identify significant changes

FRF-Based Damage Detection Techniques

Comparison of FRF-based detection techniques

• Transmissibility methods
  • Ratio of FRFs measured at different locations on the structure sensitive to changes in the dynamic behavior between the measurement points
  • Do not require knowledge of the input force and can be used with ambient excitation but may not be sensitive to localized damage and require multiple measurement points
• Damage index methods
  • Quantify the difference between the FRFs of the healthy and potentially damaged structure (Coordinate Modal Assurance Criterion COMAC, Frequency Response Assurance Criterion FRAC)
  • Provide a scalar value indicating the degree of damage and can be used to localize damage but require a baseline FRF of the healthy structure and may be sensitive to environmental and operational variations

Robustness of FRF methods for damage types

• Effective in detecting various types of damage
  • Cracks, delaminations, and disbonds in composite structures
  • Corrosion and fatigue damage in metal structures
  • Loosening of bolted connections and damage to joints
• Robustness depends on
  1. Quality and repeatability of the FRF measurements
  2. Sensitivity of the FRFs to the specific type and location of damage
  3. Ability to distinguish damage-induced changes from environmental and operational variations
• Applicability depends on the structure and monitoring conditions
  • More suitable for structures with well-defined dynamic behavior and limited environmental variability
  • May require a dense sensor network for large or complex structures
  • Update baseline FRFs periodically to account for long-term changes in the structure