A smart nut for accurate real-time monitoring of bolt preload

Abstract Bolted connections are indispensable in various engineering structures, and their reliable performance critically depends on maintaining adequate bolt preload. Preload loss due to dynamic loads, fatigue, vibration, and temperature fluctuations can lead to catastrophic structural failures. Existing preload monitoring methods, however, are often complex, invasive, and susceptible to environmental interference, limiting their widespread application. This paper introduces a novel smart nut designed for accurate real-time and continuous monitoring of bolt preload, which integrates a high-precision strain sensing unit within an internal annular groove, enabling non-invasive measurement. Finite element simulations were conducted to validate the structural design and optimal placement of the strain gauges, demonstrating a linear elastic response and symmetrical strain distribution under typical preload conditions. A full-bridge circuit incorporating four strain gauges inside the nut was constructed to correlate output voltage with applied preload. Calibration experiments, including direct pressure testing and bolt pre-tensioning tests, were conducted to achieve practical nut measurement functionality and confirm its accuracy. Experimental results using the calibrated nut to measure bolt preload showed that the nut can accurately measure bolt preload with a maximum error of less than 3.06%. This innovative solution provides a practical, accurate, and easily implementable method for real-time and continuous preload assessment, significantly enhancing structural safety and facilitating timely maintenance in bolted joints.