Advantageous and Limiting Attributes of Microwave and Millimeter Testing

Advantageous Attributes

Each NDT technique has its own unique advantageous features, capabilities and limitations, either determined by the physics of the method or for a particular application. Consequently, it is important to be cognizant of the fact that the desirable features or limitations of a method are not universal. Microwave and millimeter wave NDT methods are no different in this regard. The unique features and capabilities offered by each NDT technique, when plurally considered, comprise the NDT toolbox. The high-frequency electromagnetic nature of microwave and millimeter wave NDT methods bring about the following, and in some cases unique, advantageous features:

  • ability to penetrate dielectric (non-conducting) materials,
  • high sensitivity to the presence of relatively small flaws, given the relatively short wavelengths at these frequencies and the availability of large signal bandwidths, which is commonly associated with a certain percentage of frequencies around the operating frequency. For example, 10% bandwidth at an operating frequency of 10 GHz is 1 GHz (1 x 109 Hz) and at 100 GHz it is 10 GHz (10 x 109 Hz),
  • coherent properties of the waves allowing for the use of signal amplitude and phase
  • having varied and controllable wave polarization characteristics,
  • availability of a host of “probes” that can be optimally chosen and their designs optimized for a particular application or purpose,
  • ease of adaptability to in-field measurement requirements,
  • one-sided and non-contact measurement capabilities (i.e., no need for couplants),
  • sensitivity to surface properties of metals and carbon composites (i.e., carbon and glass fiber reinforced polymer),
  • ability to develop robust analytical, numerical or empirical electromagnetic models for:
    • gaining the ever-so-important insight to the fundamental physics of a complex problem
    • optimize measurement approach and parameters prior to assembling a measurement system in order to obtain the highest detection sensitivity
    • analyze expected measurement errors/inaccuracies and their implications on the outcome of a specific inspection
  • ease-of-use, portability of most custom-designed inspection systems, operator friendliness (not requiring microwave engineering knowledge to operate a system), fast data collection of analysis (i.e., real-time measurements), and
  • non-ionizing nature of these waves.

These features capabilities, individually or in combination, have led to an increasing proliferation of microwave and millimeter wave NDT&E techniques.

Limiting Attributes

The most important limiting feature associated with using microwave and millimeter waves for NDT has to do with the fact that the skin depth associated with highly conducting materials (e.g., metals and carbon composites) is very small and in the nano and micrometer range. Therefore, waves at these frequencies do not penetrate such materials and are only capable of inspecting surface anomalies, such as surface-breaking cracks, impact damage, surface fiber condition and alike.

With respect to carbon composites, and only for unidirectional composites, wave polarization can be effectively manipulated to penetrate the composite when the wave polarization vector is orthogonal to the fiber directions. On the contrary, when these two directions are aligned (i.e., parallel) the high conductivity of carbon fibers prevents the wave from penetrating inside.

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