The relationship between the degree of stress concentration and the H _p(y) signals of metal magnetic memory was investigated by tension–tension fatigue tests. Sheet specimens of 45CrNiMoVA steel were machined and two types of notches were cut into them to obtain two stress concentration factors. During the testing process, H _p(y) signals were collected from three measured lines on the specimen surface. In the absence of a precut notch, the H _p(y) signal curve of a measured line at the central position was linear. However, the H _p(y) signal curves of measured lines near a precut notch or across a notch showed a distribution of abnormal magnetic peaks. The magnetic gradient K of the abnormal magnetic peaks was found to increase as the stress concentration factor increased. This feature could potentially be used to characterise the degree of stress concentration of ferromagnetic materials.

The generation and propagation of cracks are critical factors that affect the performance and life of large structures. Therefore, in order to minimize maintenance costs and ensure personal safety, it is necessary to monitor key structures. The sensor based on ultra-high frequency radio frequency identification (UHF RFID) antenna has the advantages of passive wireless, low cost, and great potential in the field of metallic structure health monitoring. In this paper, aimed at the key parts of a metallic structure, a dual-tag system is used for crack monitoring. In conjunction with mode analysis, the principles of the sensing tag and the coupling principles of the dual-tag are analyzed. Considering that the dual-tag is placed in different methods, the effect of mutual coupling on the sensing performance of the tag is studied. The results show that the frequency of the sensing tag can be tuned by adding the interference tag, and the dual-tag sensor system has reasonable sensitivity. The results also provide potential guidance for the optimal placement of multiple tags in the near-field region.

Wireless monitoring of cracks is a critical task in evaluating the state of metal structure. Radio-frequency identification (RFID) antenna sensors have been intensively investigated for crack detection. Nevertheless, the development of RFID antenna sensors satisfying the requirements of high sensitivity and coverage remains a challenge. In this paper, an ultrahigh frequency (UHF) RFID antenna sensor with coupled ring resonators has been proposed, where the outer ring is designed for radiation and sensing, and the inner ring is utilized for impedance matching. The sensor is directly placed on metallic surface, forming an electromagnetically induced transparency-like (EIT-like) effect to enhance sensing capabilities. Due to a relatively uniform electromagnetic field constrained under the outer ring, surface cracks can be detected in different positions and directions. The sensor is sensitive to both depth and width of crack, with a sensitivity of -5.46 MHz/mm in crack depth and -10 MHz/mm in crack width detection. According to the results of this investigation, a potential sensing technique for practical applications is demonstrated.

Purpose: Radio frequency identification tags for passive sensing have attracted wide attention in the area of Internet of Things (IoT). Among them, some tags can sense the property change of objects without an integrated sensor, which is a new trend of passive sensing based on tag. The purpose of this paper is to review recent research on passive self-sensing tags (PSSTs). Design/methodology/approach: The PSSTs reported in the past decade are classified in terms of sensing mode, composition and the ways of power supply. This paper presents operation principles of PSSTs and analyzes the characteristics of them. Moreover, the paper focuses on summarizing the latest sensing parameters of PSSTs and their matching equipment. Finally, some potential applications and challenges faced by this emerging technique are discussed. Findings: PSST is suitable for long-term and large-scale monitoring compared to conventional sensors because it gets rid of the limitation of battery and has relatively low cost. Also, the static information of objects stored in different PSSTs can be identified by a single reader without touch. Originality/value: This paper provides a detailed and timely review of the rapidly growing research in PSST.