Planar Microwave Sensor for Detection of Localized Discontinuities in Polylactic Acid (PLA) Materials

Material discontinuities and defects can profoundly impact the structural integrity and overall product quality. In a multitude of industries, ranging from aerospace and automotive to the nuclear sector and manufacturing, even surface discontinuities can pose significant risks to component reliability. This paper presents a planar microwave sensor for non-destructive testing (NDT) to quantify the electromagnetic response to controlled crack-like discontinuities in polylactic acid (PLA) materials. The sensor comprises a host coplanar waveguide (CPW) positioned at the base of an RO3210 substrate and a multiple split-ring resonator (MSRR) on the surface, creating a compact device measuring 30 mm × 50 mm × 1.27 mm. When a discontinuity-free PLA sample-under-test (SUT) is placed above the sensor, the transmission coefficient exhibits a resonance at 1.780 GHz. As the width of the groove-based discontinuity increases, a systematic blue shift in the resonant frequency is observed. The relationship between resonant frequency shift and discontinuity width is established through empirical calibration for both surface and subsurface configurations. The results demonstrate the feasibility of the proposed sensor for calibrated detection and sensitivity-based discrimination of millimeter-scale crack-like discontinuities in PLA within the tested dimensional range.