Radio-frequency sensor for non-destructive evaluation of composite materials

The growing use of advanced composites in critical applications demands reliable, non-destructive testing. Traditional techniques often detect only large defects, missing subtle local property changes that can precede major failures. Microwave spectroscopy offers a promising alternative, probing both structural and dielectric properties with high sensitivity. This paper presents a novel dual-probe radio-frequency sensor (RFlect) using open-ended coaxial probes at 2.4 GHz. Compared to our earlier single-probe design, the dual configuration reduces scanning time and enhances detection of surface and subsurface defects, including minor cavities and inhomogeneities. It employs a cost-effective system comprising a signal generator, directional coupler, and RF power measurement setup to improve noise filtering without relying on expensive vector network analyzers. Integrated with a fused deposition modeling 3D printer's extruder, the sensor scans samples at millimeter resolution. Experiments confirm its ability to detect both visible and hidden subsurface defects by identifying changes in conductivity and dielectric constant. This work underscores the potential of dual-probe microwave spectroscopy as a cost-effective, high-performance non-destructive evaluation technique for various applications. Future efforts will focus on characterizing high-loss materials, such as biological tissues, thereby expanding the sensor's use in the biomedical field.