{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,8,7]],"date-time":"2026-08-07T17:12:51Z","timestamp":1786122771391,"version":"build-2736575974"},"reference-count":25,"publisher":"MDPI AG","issue":"8","license":[{"start":{"date-parts":[[2026,8,7]],"date-time":"2026-08-07T00:00:00Z","timestamp":1786060800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"Shanxi Key Laboratory of Advanced Semiconductor Optoelectronic Devices and System Integration","award":["2024SZKF10"],"award-info":[{"award-number":["2024SZKF10"]}]},{"name":"Taiyuan University Horizontal Research Project","award":["H2025105"],"award-info":[{"award-number":["H2025105"]}]},{"name":"Taiyuan University\u2019s Horizontal Research and Development of AI-Integrated Intelligent Compliance Management System for Material Warehousing"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Micromachines"],"abstract":"<jats:p>Despite the aim of meeting the demand for long-term online monitoring of structural cracks in fields such as infrastructure, rail transit, aerospace and others, traditional detection methods fail to realize passive wireless, flexible conformal and non-contact measurement. This paper proposes a flexible wireless passive crack sensor based on resonant rings. Taking polyimide (PI) as the substrate, the sensor integrates a sensitive interdigital resonant ring structure. Variations in crack width disturb the electromagnetic field, which further leads to a resonant frequency shift to realize crack width detection. The sensing mechanism is elaborated based on microwave resonance and equivalent circuit theories. Structural optimization and crack width sensitivity analysis are carried out via electromagnetic simulation. Samples are fabricated by flexible printing technology, and a test platform is established. Experiments reveal that the sensor achieves excellent linearity within the crack width range of 0~2.5 mm, with the resonant frequency decreasing monotonically as crack width increases, and a sensitivity of 67.02 MHz\/mm. It can operate stably under varying distances, installation angles and bending conditions, demonstrating outstanding flexible conformability. Featuring no power supply requirement, a chip-free design, a simple structure and strong anti-interference capability, the sensor is suitable for long-term crack monitoring of metal structures. Compared with existing studies, the proposed sensor exhibits prominent advantages in flexible adaptability, wireless passive performance and engineering practicability and can provide a novel wireless passive solution for structural health monitoring.<\/jats:p>","DOI":"10.3390\/mi17080943","type":"journal-article","created":{"date-parts":[[2026,8,7]],"date-time":"2026-08-07T11:44:53Z","timestamp":1786103093000},"page":"943","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Flexible Wireless Passive Resonance Ring Sensor for Nondestructive Crack Monitoring of Metal Structures"],"prefix":"10.3390","volume":"17","author":[{"given":"Yingmin","family":"Wang","sequence":"first","affiliation":[{"name":"Department of Intelligent and Information Engineering, Taiyuan University, Taiyuan 030032, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2018-8664","authenticated-orcid":false,"given":"Xiaodong","family":"Huang","sequence":"additional","affiliation":[{"name":"Department of Intelligent and Information Engineering, Taiyuan University, Taiyuan 030032, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Pan","family":"Pei","sequence":"additional","affiliation":[{"name":"Key Laboratory of Instrumentation Science and Dynamic Measurement, North University of China, Taiyuan 030051, China"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,8,7]]},"reference":[{"key":"ref_1","first-page":"e23172","article-title":"Conformal metal crack detection sensor based on flexible graphene film antenna","volume":"32","author":"Tong","year":"2022","journal-title":"Int. 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