# Bolt Loosening Detection Using Key-Point Detection Enhanced by Synthetic Datasets
## 1 Introduction  
Threaded fasteners are a standard and essential part of the industry, providing connection, fastening, and sealing functions. Because of their low cost, interchangeability, and ease of installation and removal, they are used in a wide range of applications in mechanical, aerospace, civil, and marine engineering. Although threaded fasteners are often small and even inconspicuous, they play an important role in ensuring structural integrity and reliability [1]. Typical forms of threaded connections include bolts, nuts, and clamping components. In a bolted connection, the bolt usually has a preload, which is provided by the stress between the clamping member and the nut [2]. When the bolt or nut is tightened, the bolt elongates, and a preload is generated. An adequate preload is essential to ensure joint and fastening performance and to improve the reliability of the product [3]. Loosening is a critical problem faced by threaded fasteners. In engineering applications, threaded fasteners are exposed to a variety of operating environments and to various types of external loads, which results in a risk of loosening that is often unavoidable. Loose bolts can directly lead to a reduction in preload and cause mechanical failures, e.g., water, oil, and gas leaks in various engineering applications are mostly due to loose bolts. In a strict sense, loosening can be defined as a process in which the inner and outer threads rotate in the opposite direction to the tightening direction, resulting in a reduction in the preload. This is also referred to as \"self-loosening\". The magnitude of the reversed rotation angle can be used as an indicator of the severity of loosening [4]. Engineering experience and scientific studies have shown that retaining the preload during service iscritical to maintaining the function of threaded fasteners and ensuring structural integrity and reliability. Therefore, the need to periodically check the tightness of the bolts is critical to avoid structural damage and accidents. The manual inspection method is the most traditional and the most common method applied in engineering. One of the simplest methods is the \"straight line marking method\", in which a marker is used to draw a marking line on the tightened bolt and nut. The inspector determines the tightness of the bolt by seeing if the marker line remains continuous. The other main method is to use a torque wrench. Using a wrench with a torque meter, the inspector checks the torque of the bolt at determined intervals and ensures that it has not dropped [5]. The manual method is cheaper and simpler compared to other inspection methods. However, the tension-to-torque ratio of bolt tightening is affected by many factors, leading to a high error rate in the torque wrench method, which limits the application of this method when the accuracy of the measurement is required. In addition, its performance depends on the level of the inspector and the regular inspection interval. Moreover, in some special cases, the inspector may not have access to the site for inspection. In bolted connections, the bolt usually has a preload, which is provided by the stress between the clamping member and the nut, and there is a corresponding relationship between the bolt loosening and the force. Therefore, methods based on stress measurements have been proposed earlier, and these methods use different techniques to measure the strain or stress on the bolt, the nut, and the clamping components to determine whether the bolt is loose or not. Ultrasonic measurements based on the acoustoleastic principle, contact dynamic method through active sensing, impedance method, vibration-based methods, and other methods were applied [6]. Strain gauges, ultrasonic sensors [7], fiber optic Bragg gratings (FBG) [8], and piezoelectric ceramic transducers (PZT) [9] that can be used to measure stress or strain are frequently used.