Research On Bolt Distributed Force Sensing Technology Based On Fiber Bragg Grating

Bolts are widely-used and important fasteners that play a pivotal role in industrial applications due to their excellent structural connection properties.To ensure a safe and reliable connection between the connected parts,it is necessary to conduct real-time online monitoring of the stress state of the bolts during pretensioning and service,especially for bolts used in some large equipment and facilities.For this reason,it is important to develop various sensors based on the bolt structure and working principle to accurately measure and monitor the force state of the bolt.At present,there are some problems with electromagnetic sensors for real-time online measurement and monitoring of bolt stress state,such as poor anti-electromagnetic interference,complicated lines and inability to long-distance signals transmission,etc.Optical sensors based on fiber Bragg grating(FBG)sensing technology have the advantages of anti-electromagnetic interference,corrosion resistance,quasi-distributed or distributed measurement based on wavelength division multiplexing technology due to the use of light as the medium of information transfer,which can meet the requirements of long-term real-time monitoring of bolt stress state.However,there are some shortcomings in the reported FBG sensors for bolt force monitoring,which are simply embedded or attached to the bolt body without specific analysis of the bolt stress distribution and force form,especially for long bolts,whose stress conditions are more complex.Therefore,a novel FBG bolt force sensor with performances of distributed measurement and anti-torque interference is designed according to the demand of force monitoring of bolts in engineering practice in this paper.The main work is as follows.(1)The research status on bolt force monitoring technology is investigated.According to the shortcomings of related research based on electromagnetic sensing principle,a bolt force monitoring program based on fiber grating sensing technology was proposed.The current research status of existing fiber grating bolt force sensors is outlined,the problems are pointed out,and preliminary ideas for subsequent research work are given.(2)The structural design and prototyping of a distributed FBG force sensor were completed: an FBG force sensor with three strain measurement units was designed,while the influence of external torque on the strain measurement units was reduced by using miniature bearings.Through theoretical derivation,the connection between the load applied to the bolt and the wavelength shift of the fiber grating was established.A prototype sensor was fabricated according to the designed structure,and the fabricated sensor was embedded in the bolt center embedding hole to obtain a smart bolt with strain and temperature sensing capability.(3)A comprehensive performance tests of the developed distributed FBG force sensor was conducted,including temperature compensation characteristics test,axial load calibration test,creep performance test,distributed measurement performance test,torsion resistance test and dynamic force detection test.The temperature sensitivity and strain sensitivity of the distributed FBG force sensor were obtained through the temperature compensation characteristics test and axial load calibration experiment,respectively;the creep performance test showed that the sensor could meet the requirements of long-term force monitoring of the bolt;the distributed measurement structure could sense the difference of artificial loading force;compared with the measurement unit without torsional structure,the designed torsional structure based on miniature rolling bearing has a lower The distributed FBG force sensor has an operating bandwidth of 0-606.8Hz,which meets the requirements of its dynamic force measurement.Comprehensive analysis of the performance indexes of the sensor shows that it can meet the requirements of long-term bolt distributed force state monitoring.