Hilbert-Curve Array Sensor for Nondestructive Testing of CFRP Composites

A novel Hilbert-curve (HC) based microwave nondestructive testing (NDT) array sensor is presented for carbon fiber-reinforced polymer (CFRP) composites. The proposed sensor is designed on a cost-effective Rogers RT5880 by using second order Hilbert-curve. The transmission coefficient (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{S}_{21}$</tex>) response and electric field distribution are examined. The result works as a bandstop filter when cracks are present and bandpass filter when cracks are absent within the sensing band for the sample-under-test (SUT). The HC-based unit sensor uses a fringing field sensing capacitor (FFSC) as a “sensing pad” and interacts directly with the SUT to detect surface cracks. The designed sensor can detect the minimum crack size of 0.5 mm while operating in the frequency range of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{0.25-2.75~GHz}$</tex>. The designed sensors have potential applications for detecting surface cracks in valuable infrastructure including aircraft, bridges, pipelines and strengthening their safety and integrity.