Microwave Nondestructive Testing (NDT)
of Dielectric Materials

With the advances in materials science and technology, the aerospace industry has introduced new dielectric materials such as glass-fiber reinforced-plastic laminates, honeycomb structural composites, thermoplastics, reinforced and non-reinforced rubber, ceramics and others. The advantages of these new materials over metals are increased strength-to-weight ratio, increased fatigue resistance and elimination of corrosion problems. Typical flaws introduced during material joining, manufacture and service are delaminations, disbonds (cold fusion), foreign material inclusions (e.g., talc and dust), voids, changes in thickness, moisture or other liquid contamination, mechanical damage, physical changes due to chemical attack, aging and others. Most of them are difficult to detect with traditional nondestructive testing (NDT) techniques due to access requirements, flaw orientation, high attenuation of interrogating energy and material thickness.

Recently, a new microwave (MW) technique was introduced at EWI. The transmitter probe generates a standing electromagnetic wave in the volume of the material at fixed frequency. Two receiver probes tuned to measure the standing wave at two points separated by λ/4 distance (λ - wave length) are used to form three (two absolute and one differential) channels of data. Any flaw that changes the dielectric properties (dielectric permittivity) of the material will affect the amplitude and phase of the standing wave and will be detected in all or at least one of the channels. Any inclusions, especially those with good electrical conductivity will produce strong indications and will be easily detected. The equipment allows presentation of the data in C-scan and 3D isometric view formats increasing the speed and reliability of signal interpretation. In addition, the MW is perfectly suited for field inspections. Automated MW scanning and results for inspection of a 40-mm thick electro-fusion joint across the length and circumference in plastic pipe are shown in figures 1 through 3. Joint conditions under the plastic coupler such as “short stab” are easily detectable and sized. Inclusions ranging from 2 to 10 mm in diameter are also detected, separated from other geometric features and accurately sized on both the isometric and C-scan images.

The technique has the following advantages over other NDT techniques for dielectric materials testing:

• No couplant and/or direct contact with the inspection surface are required.
• The propagation of electromagnetic wave is almost independent of the temperature in the common temperature range.
• The standing wave is non-directional and the MW image resolution and sensitivity depends solely on the probe position control which is easily achievable.
• Equipment is small and portable.
• Inspection of complex geometries is possible.

For more information, contact Evgueni Todorov at 614.688.5268 or evgueni_todorov@ewi.org.