Monolithic Passive Dual-Resonator Smart Washer Fabricated by Multi-Material Jetting for Near-Field Monitoring

In passive resonant washers, load-induced frequency shifts provide the sensing mechanism, whereas the measured near-field signature also reflects the reader–sensor coupling conditions. This work presents a chipless near-field smart washer fabricated by inkjet-based multi-material jetting as a monolithic dual-resonator structure. The sensing resonator integrates a vertically interdigitated capacitive element within the mechanically active region of an M12 washer geometry, whereas a second resonator is positioned outside the main load path to provide a comparatively load-insensitive reference feature in the measured radio-frequency signature. The printed capacitive and inductive elements showed deviations below 12% from the design values, indicating the suitability of the adopted lumped-element design approach and fabrication process. Controlled tightening tests were combined with systematic reader-position mapping in the lateral and vertical directions to evaluate the response of the printed washer under practical near-field interrogation conditions. The sensing resonance exhibited a monotonic frequency downshift during tightening, while the reference resonance remained substantially load-insensitive. Four multivariate formulations based only on the extracted resonance frequencies and notch depths were evaluated through cross-validation across the mapped reader-position conditions. The reduced LC semi-empirical interaction model achieved the best overall interpolation performance, with a median absolute error (MdAE) of 1.57 Nm across 432 out-of-fold predictions. These results demonstrate the potential of multi-material jetting for integrated passive resonant washers able to support near-field monitoring of threaded joints.