CAA’s Articulate – Metamaterials for Guided Wave Testing


Context: 

  • Indian Institute of Technology (IIT) Madras and University of Nairobi researchers have used metamaterials to improve detection of defects in large structures by Guided Wave Ultrasound. 


Background:

  • Periodic testing is required to prevent catastrophic failures in many engineering structures like buildings, pipelines and rails. 
  • High-frequency sound waves that travel in the bulk (bulk ultrasound) are widely used for non-invasive and non-destructive testing of structural materials. 
  • About Conventional ultrasound wave testing: The sound waves are sent into the sample, say pipe or pillar, perpendicular to the item, and a detector calculates the time interval between the transmission and reception of the sound waves that are either transmitted or reflected.
  • Principle behind: Sound waves travel at a uniform speed if the object is defect-free, but defects impede or deflect sound waves, which results in delays in reception.
  • Drawback: Conventional bulk ultrasonic inspection is tedious and time-consuming, as it has to be done at multiple regions of the test material. 
    • It is therefore quite difficult to be used with large objects such as train tracks, oil-pipelines and reinforcing structures of tall buildings, etc.

About Guided Wave Testing (GWT)

  • The sound waves are sent along the length of the structure rather than into the structure.This allows the waves to travel long distances.
  • Acoustic metamaterials are useful in manipulating sound waves. With proper selection of the metamaterial, the evanescent waves arising from scattering by a defect can be magnified by a process called Fabry–Pérot resonance.
    • Resonance: It is a phenomenon in which a wave, in this case, the ultrasound wave, is amplified due to a match in frequency between the wave and the frequency generated by the metamaterial.
  • Disadvantage: GWT has poorer resolution than conventional ultrasound-based testing due to diffraction limitations. 

Thus, guided waves are only a long-range screening tool and must be used in conjunction with a testing tool with better resolution for accurate detection of defects.


Metamaterials 

  • They are artificially crafted materials with unique internal microstructures that give them properties not found in nature.
  • Metamaterials are metal–dielectric composites structured on the microscale or nanoscale. 
  • Unique character: For electromagnetic waves with relatively large wavelengths, the composite structure acts as an array of artificial atoms, giving rise to unique and exotic electromagnetic properties.
  • Applications: Metamaterials are used in potentially novel applications in antenna and radar design, subwavelength imaging, and invisibility cloak design.