Enhanced sensitivity of microwave inspection of thin composites at resonance

In this paper, the resonance behavior of layered dielectric media is exploited for microwave non-destructive testing (NDT) of thin composite. Ultra wide band excitation over 1-20 GHz coupled to the composite using open ended coaxial probe was used to identify sample resonance in the absence of the defect. The time gated reflection coefficient (S11) of the sample measured at resonance was used to record the spatial variation in S11 for an insert with low dielectric contrast embedded in the thin composite. The proposed technique was validated using 3.5 mm thick glass fiber epoxy composite of 6 layers for 10 mm × 10 mm × 0.4 mm inclusion with low dielectric contrast (|Δε
_r
| ≤ 2) in between the composite layers. The measurements indicate enhanced sensitivity for the low dielectric contrast at sample resonance which is lost when inspected off resonance in agreement with the simulations.