Abstract
A washer is a common structural element that is directly used along the loading path of a bolted connection. Pre-load on a bolted connection directly impacts its load bearing capacity and pre-load monitoring is an important aspect of structural health monitoring (SHM). With the change of the pre-load on a bolted connection, the loading force on the washer will change and, therefore, the outer diameter and outer circumferential length of the washer will change. Taking advantage of the high sensitivity and the small size of a Fiber Bragg Grating (FBG) sensor, we propose an innovative smart washer encircled by an FBG sensor that can directly measure the circumferential strain change and, therefore, the pre-load on the washer. For protection, the FBG is embedded in a pre-machined groove along the circumferential surface of the washer. A theoretical approach is used to derive the linear relationship between the applied load and the circumferential strain of the washer. To validate the functionality of the FBG-enabled smart sensor for in situ bolt pre-load monitoring, a simple but effective testing apparatus is designed and fabricated. The apparatus involves a bolt, the FBG-enabled washer, a metal plate, and a nut. The bolt has an embedded FBG along its axial direction for precise axial strain and, therefore, force measurement. With the calibrated axial force measuring bolt, in situ experiments on the FBG-enabled smart washers are conducted. Experimental results reveal the linear relationship between the pre-load and the wavelength of the FBG sensor encircling the washer. Both analytical and experimental results demonstrate that the proposed novel approach is sensitive to the bolt pre-load and can monitor in real time the bolt looseness in the entire loading range.
Highlights
As one of the most common types of structural connection, bolted joints are commonly used in civil structures, such as long-span bridges and high-rise buildings [1,2]
This paper developed a simple but effective Fiber Bragg Grating (FBG)-enabled smart washer to monitor the pre-load of a bolt connection and to detect bolt looseness during its entire loading range
The smart washer was formed by encircling the washer by an optical fiber with an FBG sensor that was embedded in a pre-machined groove along the outer surface of the washer
Summary
As one of the most common types of structural connection, bolted joints are commonly used in civil structures, such as long-span bridges and high-rise buildings [1,2]. A real-time damage detection method based on PZT impedance was investigated in pipes connected by bolted joints [24]. A bolt tension monitoring device was proposed by Khomenko et al [36], and it utilizes removable and reusable fiber Bragg grating sensor(s) embedded in a bolt shaft for pre-load and retained clamping force measurements. They found that, depending on the joint configuration, the pre-load reduction varied considerably, and an increase in initial pre-load tended to reduce the pre-load relaxation [37].
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
