Review of Bolted Connection Monitoring

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Hindawi Publishing Corporation International Journal of Distributed Sensor Networks Volume 2013, Article ID 871213, 8 pages http://dx.doi.org/10.1155/2013/871213

Review Article Review of Bolted Connection Monitoring

Tao Wang,1,2 Gangbing Song,1,2 Shaopeng Liu,1 Yourong Li,1 and Han Xiao1

1 College of Mechanical Engineering and Automation, Wuhan University of Science and Technology, Wuhan 430081, China 2 Smart Material and Structure Laboratory, Department of Mechanical Engineering, University of Houston, Houston, TX 77204, USA

Correspondence should be addressed to Tao Wang; [email protected]

Received 5 August 2013; Revised 18 November 2013; Accepted 19 November 2013

Academic Editor: Ting-Hua Yi

Copyright © 2013 Tao Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This paper reviews the research of monitoring technologies for bolted structural connections. The acoustoelastic effect based method, the piezoelectric active sensing method, and the piezoelectric impedance method are the three commonly used to monitor bolted connections. The basic principle and the applications of these three methods are discussed in detail in this paper. In addition, this paper presents a comparison of these methods and discusses their suitability for in situ or real-time bolt connection monitoring.

1. Introduction This method results in an accurate bolt-strain measurement. Another method, which is used when both sides of the bolt Bolts are important elements for detachable assembly of are accessible, involves a caliper to directly measure bolt elon- components in structures and machines. Bolts provide the gation. However, in many instances, this is impossible since required axial or preload forces on the components. A bolt’s the bolt elongation is so small that it is difficult to measure axial load or preload has to be carefully controlled to ensure it accurately. Please note that this method is not suitable for the safety and reliability of structures. Insufficient preload in situ monitoring. or excessive preload is a frequent cause of bolt joint failure An ultrasonic method is considered as a valuable method [1, 2]. Monitoring the preload of high-tension bolts is critical tomeasureboltaxialload,whichisbasedontheacoustoelas- in maintaining the strength and safety of bolted joints in tic effect that the velocity of the ultrasonic wave propagated engineering structures. There are many ways to indirectly along the bolt depends on the axial stress. measure bolt tension. Current practice relies on the torque wrench technique. Interface contact characteristics change with the preload. However, with this technique, the torque load dissipates due With the active piezoelectric sensing technique, the variation to the friction between the bolt threads and the nut, which of the interface contact characteristics can be monitored by prevents accurate measurements of the axial load. The torque the ultrasonic signal generated by the piezoelectric trans- wrench method was reported to show large intrinsic errors ducer; thus, the bolt connection status can be monitored. This in bolt tension measurement by up to 50%. This typically active sensing method has a great potential for in situ/real- occurs since the torque applied to fastened bolts is consumed time monitoring. mostly by friction—only 10–15% of the torque is converted Vibration-based damage assessment of bolted structures to the tension in the bolts. Friction consumes about 90% has also been accomplished using the transmittance function of the torque, with 50% dissipated in the bolt head and method as described by Caccese et al. [4]. The vibration- 35% in the bolt threads. Additionally, the friction varies so based health monitoring strategy has been developed by Razi widely from bolt to bolt that errors of 50% are common, et al. to detect the loosening of bolts in a pipeline’s bolted even with a perfect torque control. This complication leads to flangejoint,andtheempiricalmodedecompositionisapplied overconservative designs for safety, which in turn increases to establish an energy-based damage index and several dam- both weight and cost of bolted connections [3]. age scenarios simulated by loosening bolts through adjusting An effective way to measure the bolt strain is using a stan- the applied torque were investigated [5]. Zadoks and Yu dard resistance strain gauge mounted to the body of the bolt. [6] investigated the self-loosening behavior of bolts under 2 International Journal of Distributed Sensor Networks transverse vibration. Pai and Hess [7]studiedtheloosening of threaded fasteners due to dynamic shear loads with this Le? Se traditional dynamic method. Vibration-based damage assessment of bolted structures is a feasible method, and common L vibration test equipment can fulfill the purpose [8]. However, i for the bolt loosing detection, the subsequent treatment of Figure 1: Model of an axially loaded bolt. vibration signal is complex and for the complicated bolted structures, it is difficult to detect the bolt looseness status and loosened bolt position based on this method [9–11]. made to determine the acoustoelastic constant for certain This paper reviews the measurement technologies for bolt kindsofsteelboltsrevealedthattheacoustoelasticconstant connectionstatus.Threemainmethodsforboltedstructure is independent of any effects that heat treatment may have on monitoring are discussed in this paper. These three methods the bolts [14]. are acoustoelastic effect based method, the piezoelectric For a bolt subjected to an axial load, certain parts of the active sensing method, and the piezoelectric impedance total length are partially stressed or unstressed. To take this method. This paper presents the basic principle and the fact into account, assume that the initial bolt length is the sum applications of these three methods and also discusses their oftheeffectivelength𝐿𝑒 and the unstressed portion 𝐿0 (see suitability for the in situ bolt connection monitoring. Figure 1): 𝐿 =𝐿 +𝐿 . 2. Acoustoelastic Effect Based Methods 𝑖 0 𝑒 (2) 𝐿 Abolt’sacoustoelasticresponsechangeswithitsaxialstress. Suppose that 𝑒 is subjected to a uniform uniaxial stress 𝜎=𝐹/𝑆 𝐹 𝑆 The following three properties associated with the acous- 𝑒,where is the axial load and 𝑒 is the effective toelastic response can be used to compute the axial stress cross-sectional area. From (1a)and(1b), the pulse-echo time- level: (1) the transit time ultrasound along the bolt (portable of-flight related to the ultrasonic wave is given by instruments are commercially available), (2) the transit time 2𝐿 2𝐿 𝑡= 𝜎 + 0 , ratio between longitudinal and shear waves propagating 𝑉 𝑉 (3) along the bolt (or mode-converted shear wave at the lateral 0 surface), and (3) the mechanical resonance in the bolt. In −1 where 𝐿𝜎 =𝐿𝑒(1 + 𝐸 𝜎) is the stressed length and 𝐸 is this section, three different methods, which are, respectively, Young’s modulus of the bolt material. basedonthesethreeproperties,arereviewed. Combining (1a), (1b), and (3) results in

−1 2.1. Time-of-Flight Method. The time-of-flight (TOF) in the 2𝐿𝑒 (1 + 𝐸 𝜎) 2𝐿 𝑡= + 0 . bolt, measured by the sing-around technique with an ultra- (4) 𝑉0 (1+𝐴𝜎) 𝑉0 sonic probe set on the bolt head, is obtained from the time difference between the reflecting waves in the first round trip The first-order development of(4)gives and the second round trip. 𝐿 The propagation of ultrasonic waves is sensitive to both 𝑒 −1 𝑡≅𝑡0 [ (𝐸 +𝐴) 𝜎+1] , (5) residual and applied stresses in materials. The elastic wave 𝐿𝑖 propagation depends on the direction and polarization of the 𝑡 =2𝐿/𝑉 waves as well as the direction of the applied stress. Because where 0 𝑖 0 is the pulse-echo time-of-flight corresponding to longitudinal and transverse waves in the of these effects, the velocities of ultrasonic waves depend on 𝐿 different stress states in the material. unstressed initial length 𝑖.Equation(5) shows that the time- In the case of plane waves in a homogeneous and isotropic of-flight under stress is a linear function of the applied stress. material, the velocities of longitudinal and transverse waves, From (5), it can be seen that the bolt axial stress or which propagate in the same direction as the applied stress, forcecanbedeterminedoncethetime-of-flightofultrasonic can be written in their first-order approximation as12 [ , 13] waves under stressed and unstressed states is measured. Once the relationship between 𝐹 and Δ𝑡 (the change of time-of- 𝑉𝐿 =𝑉𝐿0 (1+𝐴𝐿𝜎) , (1a) flight due to axial load) is calibrated, the load value can be determined from the precalibration curve based on the Δ𝑡 𝑉𝑇 =𝑉𝑇0 (1 + 𝐴𝑇𝜎) , (1b) measurement of . Yasui and Kawashima [15]usedabroadbandnormal where 𝑉𝐿0 and 𝑉𝑇0 are the longitudinal and transverse wave incidence transducer in the measurement of the bolt load, velocities in the unstressed state and 𝐴𝐿 and 𝐴𝑇 are the which excites and receives simultaneously a 10 MHz longi- acoustoelastic constants of the longitudinal and transverse tudinal wave and a 5 MHz transverse wave. Hirao et al. [16] waves, respectively. (Hereafter, subscripts 𝐿 and 𝑇 will be used a noncontacting shear-wave electromagnetic acoustic omitted if the equations for the longitudinal wave and the transducer (EMAT) which generates and detects the shear transverse wave are identical.) The acoustoelastic coefficients wave propagating in the axial direction of the bolts. Both 𝐴𝐿 and 𝐴𝑇 are negative, which indicates that the wave experiments show a good linear relationship between the load velocity decreases, while the stress increases. Measurements and the TOF or the signal phase. International Journal of Distributed Sensor Networks 3

The variation of ultrasonic velocity in the range of actual employing mode-converted ultrasonic waves produced by a stress acting in the bolt is very small, and, as a result, 10 MHz transducer, it is observed from tensile test results that experiments require precise and accurate measurement of theTOFratiooftwodifferentlypolarizedacousticwavesis the ultrasonic velocity. The phase detection method [17]is linearly proportional to the axial stress in bolts as expected adoptedfortheprecisemeasurementofTOFinsteadofthe from theory. conventional pulse-echo technique that is sensitive to noise. 5 MHz ultrasonic transducers are located on the top and 2.3. Mechanical Resonance Frequency Shift Method. The sim- bottomofthesampletomeasuretheTOFandtheexperiment ple one-dimensional isolated resonator model applies to results show that the ultrasonic velocity decreases linearly ultrasonic waves propagating along a bolt. If the assumption corresponding to the stress increase. is made that complete reflection occurs at the flat and parallel SincemostoftheTOFmethodsuserelativelyhigh- ends of the bolt; then, it can reasonably be assumed that the frequency(about5MHzto10MHz)ultrasonicwaves,the acoustic resonant frequencies are given by excitation waveform of the emitter which corresponds to 𝑛V a230KHzfrequencysinesignaltruncatedbyahamming 𝑓𝑛 = , (7) window was used to measure stress level in the prestressed 𝐿 steel strand [18]. A guided ultrasonic wave for monitoring where 𝑛 is a harmonic integer, V is the velocity of acoustic the stress levels in seven-wire steel strands (15.7 mm in waves, and 𝐿/2 is the length of the bolt. diameter) was proposed and investigative experiment shows When a stress is applied to the sample, both 𝐿 and V the potential and the suitability for evaluating the service change, resulting in a change in the resonant frequency. From stress levels in the prestressed seven-wire steel strands. (7), the fractional change in frequency can be written as Because the time-of-flight in the bolt is only in the range of tens of nanoseconds, the data acquisition process requires Δ𝑓 ΔV Δ𝐿 𝑛 = − . a high sampling rate for the data acquisition system (1.0 ns 𝑓 V 𝐿 (8) for 1 GS/s in equivalent sampling rate) and thus increases 𝑛 the cost. Most references used oscilloscopes to capture the Note that Δ𝐿/𝐿=𝜀=𝜎/𝐸,where𝜀 isthestrainofthebolt. signals; however, such devices are not suitable for the in situ The calculation of ΔV/V as a function of stress is complex. bolt health monitoring. It can be shown that for an isotropic elastic medium, the acoustic wave velocity varies linearly with the stress according 2.2. Velocity Ratio Method. For fastened bolts in a structure, to (1a)and(1b). Equation (7)canbewrittenas it is impossible to loosen the bolt to measure the time-of- Δ𝑓 flight of an ultrasonic wave. The velocity ratio method uses 𝑛 1 =−(𝐴𝐿 + )𝜎. (9) the difference in the acoustoelastic coefficients of longitudinal 𝑓𝑛 𝐸 wave and transverse waves. Using (5), the ratio of the time- of-flight of a transverse wave to that of a longitudinal wave is This equation shows that the frequency deviation from the approximated by the following equation: resonance is a linear function of the applied stress. The negative sign on the right-hand side indicates that frequency 𝑡𝑇 𝑉𝐿0 𝐿𝑒 𝐹 will decrease for a positive (i.e., tensile) stress. Thus, the ≅ [1 − (𝐴𝑇 −𝐴𝐿) ]. (6) Δ𝑓 𝑡𝐿 𝑉𝑇0 𝐿𝑖 𝑆𝑒 measurement of 𝑛 can be used to determine the stress produced in the sample. Thismethodismorepracticalsincethevalueofaxialload Conradi et al. used the transmission oscillator ultrasonic is calculated from the ratio of time-of-flight in the stressed spectrometer (TOUS) [20] and later Jeyman alternatively state only, without the time-of-flight measurement in the used the reflection oscillator ultrasonic spectrometer (ROUS) unstressed state [13, 19]. [21]tomeasurechangesintheresonantfrequencyofboltsdue Mode conversion takes place at a solid/liquid interface to stress-induced elongation and change in sound velocity. when two orthogonally polarized components of shear waves The experiments used the lead zirconate titanate (PZT) piezo- convert to longitudinally polarized waves in a solid or electric transducer to generate 5 MHz ultrasonic waves to when two orthogonally polarized shear and longitudinal measure the bolt stress. The experimental results confirmed components are converted into a longitudinal wave in liquid. linearity between the frequency shift and the applied stress. In the process of conversion, the mechanical properties and Based on the pseudocontinuous wave technique, Joshi the stress state of the solid medium affect the polarization and Pathare [22] used the carrier phase detection technique and speed of waves after refraction. Kim and Hong [3]pro- to track the frequency shift of the mechanical resonance of the posed an ultrasonic technique employing mode-converted bolt. The experiment was carried out with frequency varying ultrasound and applied it to obtain axial stress in high- from 4891 KHz to 4953 KHz. The frequency changes linearly tension bolts. Simultaneous generation and measurement with the applied stress. of both longitudinal and shear waves in bolts are made The time-of-flight change in the bolt is only about tens using mode conversion of longitudinal waves to reduce the of nanoseconds, and the measurement result is susceptible experimental error. To measure the TOF of longitudinal to the environment interference especially in industrial sites. and shear waves, multiple mode-converted ultrasonic signals Additionally, most of the aforementioned methods require produced in the bolt are captured via an acoustic lens. By high sampling rate of several hundred kilo Hertz or even 4 International Journal of Distributed Sensor Networks

mega Hertz. The requirement of the high sampling rate and PZT2 PZT1 the associated high cost of the data acquisition systems are among the major obstacles to the practical adoption of in situ automated bolt structural health monitoring. In addition, the environmental noise as a main influencing factor for the precise measurement of the time-of-flight also hinders the application of the method in the field. (a) Test specimens

3. Piezoelectric Active Sensing Method Amplifier PZT1 All surfaces, including those machined, are rough to different PZT2 NI USB-6361 degrees, and, as a result, the contact between surfaces is restricted to discrete areas at the tips of the surface asperities. Inthisrespect,allboltedjointsalsodeveloppartialcontactat their imperfect interfaces. The applied torque on the bolt may (b) Schematic of bolt connection monitoring system change the interfacial characteristics such as stiffness, damp- Figure 2: Bolt connection status testing device. ing,andtruecontactarea.Oncetheinterfacialcharacteristics changes are obtained, the tightness of the bolted connections can be determined. 0.08 Yang and Chang [23] provide an attenuation-based diag- nostic method to identify the location of the loosened bolts, as

) 0.06

well as to predict the torque levels of those bolts accordingly. 2 The attenuation-based method is based on the damping phenomena of ultrasonic waves across the bolted joints. Incident waves are split into transmitted and lost waves at the 0.04 micro contact interface. Considering the fact that waves are power transmission, the energy dissipation phenomena at the energySignal (V 0.02 imperfect contact interface of bolted joints can be described by examining transmitted wave. Based on the fact that the wave propagation occurs mainly in the true contact area, the 0 0 50 100 amount of transmitted wave energy is proportional to the true contact area. According to the classical Hertz contact theory, Torque (N.m) within the limit of maximum Hertzian pressure, the true First time Fourth time contact area is proportional to the square root of the pressure Second time Average applied to the contact surface. The torque level in a fastener Third time affects the pressure of a contact surface at the bolted joint. Figure 3: Signal energy under different torque levels. Therefore, the torque level can be determined by measuring the transmitted wave energy. Basedontheabovetheory,Wangetal.[24]established the experiment apparatus with two PZT patches bonded to is captured by PZT2. The signal of PZT2 is converted to a different sides of a bolted connection, as shown in Figure 2 digitalsignalbyNIUSB-6361andsavedinthecomputerfor [24]. In the active sensing method, one PZT is used to further analysis. The received signal energy of PZT2 is then generate a stress wave that will propagate to across connecting calculated. surface, and the other PZT is used to detect the stress The experiment results are shown in Figure 3 [24]. It can wave signal. The preloading force determines the connection be seen that, when the torque is under 40 N ⋅ m, with increase condition and influences the wave propagation crossing the of the torque level, the received signal energy of the sensor interface. By monitoring sensor signal, we can indirectly (PZT2) also increases. However, when the applied torque is estimate the preloading force on the bolt. beyond 60 N⋅m,thereceivedenergyhaslittlechangesanditis As shown in Figure 2 [24], two steel rectangular/circular in a saturation state. That is, because of exceeding this torque plates are connected by a pair of M10 bolt and nut, and level, there is no more deformation space in the contact theboltedconnectionisfastenedbyatorquewrenchwith interfaceandthetruecontactareahasbasicallynochange different torque levels during the experiments. The computer thus; the received energy has little change. generates a five-peak Gaussian pulse with a center frequency The experimental results demonstrate the feasibility of of 100 KHz every 1 s, and then the signal is converted into this method for bolt looseness monitoring. Since this method analog signals by the NI multifunction DAQ device NI USB- is easy to implement and nondestructive in nature, it is 6361 to actuate PZT1. PZT1 produces an ultrasonic wave, suitable for in situ bolt monitoring for practical use. The which propagates from the top plate to the bottom plate piezoelectric active sensing method has a tunable and high- through the bolted interface, and then the transmitted wave frequency bandwidth. It is especially suitable for monitoring International Journal of Distributed Sensor Networks 5 bolted joints, which involve interfaces. In the active sensing sensing frequency range, the impedance method can cover method, the frequency for data acquisition can be much lower different sensing areas. For a large structure, the bolted con- than those used in the acoustoelastic effect based methods, nection status affects only the local dynamic characteristics thus lowering the hardware cost. ofthestructure.Theimpedancemethodisappropriateto With a piezoelectric wafer as an actuator and a sensor, study the local dynamics of the bolt joint with appropriate when the ultrasonic wave generated by the wafer is incident sensing frequency. at the interface in the bolted joints, it is transmitted through Utilizing the electromechanical coupling properties of theasperityjunctionsandreflectedbackattheairgaps.When piezoelectric materials and with the impedance method, the ultrasonic wavelength is long compared to the scale of the Park et al. [33, 34] performed the experiment on a civil air gaps in the interface, the interface as a whole behaves like pipeline structure connected by bolted joints. The experiment a reflector. Marshall et al. use the reflection coefficient ofthe demonstrated the capability of the impedance method to ultrasonic for measuring the contact pressure relaxation in track and monitor the integrity of the typical civil facility. bolted joints [25]. In most impedance methods, a damage index is used to PZT-induced Lamb wave-based methods are often used assess the degree of tightness of bolts and it is difficult to for bolt loosening detection. In general, this type of method quantitatively assess the bolt connection status. is also active sensing based. Doyle et al. experimentally A spectral element method (SEM), based on a wave investigate the bolted space structures with the acoustoelastic propagation approach, was used by Ritdumrongkul et al. [35] damage detection method and the results show a good linear to model the structure impedance where the piezoelectric relationship between the Lamb wave signal delay time in the ceramic patch is bonded. Through the measurement of the propagation path and the applied torque [26]. The support electrical impedance of a PZT actuator-sensor patch bonded vector machine (SVM) was used for damage detection of on a bolted structure, the change in structural properties bolted joints and the feature vectors were created by comput- due to damage simulated by loosening the bolts is detected. ing the Fourier amplitude at the excitation frequency using By comparing the simulated result of the proposed model running Fourier transformation [27]. The wavelet transform with the experimental results, the loosening of bolts is quan- and pattern recognition techniques, as probabilistic neural titatively identified as the change in stiffness and damping networks (PNNs) and support vector machines, are employed at the bolted joint. This method solves the problem of the for the damage detection for a specimen made of two steel quantitative detection of bolted connection status, but the plates jointed by eight steel bolts, in which the damages were SEM is a little complicated and not universal. introduced by removing several bolts from the joints [28, 29]. Combining the impedance method with other ultrasonic The power spectral density (PSD) of the recorded signal was methods is another effective bolted structure health moni- developed to assess the loosening state of joints by Amerini toring technique. Wait et al. [36] combined the impedance et al. [30]. In [31], the acoustic moment method, the second method with the Lamb wave propagation based approach to harmonic method, and the sidebands method were used to analyze bolted system’s structural integrity. An and Sohn [37] analyze the relationship between the tightening/loosening illustratedanintegratedimpedanceandguidedwave-based states indices and the torque applied. damage detection techniques by utilizing impedance and For the piezoelectric active sensing method, since lower guided wave signals simultaneously obtained from surface- frequencyoftheultrasoniccanbeused,thecostoftest mounted piezoelectric transducers to the detection of bolt equipment can be dramatically reduced. The signal of active loosening. For the combined impedance method, it can detect sensing method is relatively easy to analyze and has a better the loosened bolt position in bolts group joints. noise immunity; therefore, this method may be more suitable for the industrial application. A precision impedance analyzer has a high cost. Wireless impedance devices based on AD5933 impedance measurement chip have been designed and used for structure health monitoring with reduced cost and size [38–41]. However, the 4. Piezoelectric Impedance Method limited scanning and sampling frequency of the impedance The theoretical development of the application of impedance chip restricts its wide application [42]. measurements to structural health monitoring was first pro- For all above impedance methods, they determine the posed by Liang et al. [32]in1994.Inthistechnique,apiezo- bolted connection status by impedance signal analysis. An ceramic patch is surface-bonded to the monitored structure effective monitoring theory is not formed yet because of and excited by an alternating voltage sweep signal, typically in lack of research for bolted structural dynamic characteristics, the kilohertz range, by an impedance analyzer. The vibrating especially local high-frequency dynamics. The impedance patch transfers its vibrations to the host structure and method is very suitable for local dynamic characteristics simultaneously, the structure influences the electrical circuit study because of its high actuation frequency and tunable comprisingthebondedpatchandtheacsource.Aplotofcon- sensing area. Using the impedance method to investigate the ductance (real part of admittance) as a function of frequency dynamiccharacteristicsoftheboltedjoinmaybethefurther constitutes a unique vibration signature of the structure, research interests. Once understanding the effects of the reflecting structural characteristics such as inherent stiffness, bolt connection status on the local dynamic, the impedance damping, and mass distribution. In this manner, the same method has a good potential for in situ monitoring of bolted patchactsasanactuatoraswellasasensor.Byadjustingthe connections. 6 International Journal of Distributed Sensor Networks

Table 1: Comparison of the test method.

Acoustoelastic effect based methods Piezo active Piezo impedance Time-of-flight Velocity ratio Resonance sensing method method frequency shift Nature of signal Time Velocity ratio Frequency Voltage Impedance Depend on the scan Requirement on High High High Relatively Low frequency and sampling rate structure Relative cost of the High High High Relatively Low Low sensor/probe High for impedance Relative cost of entire analyzer; low for Relatively high Relatively high Relatively high Relatively cost system AD5933 chip-based circuit Suitability for No No No Yes Yes real-time monitoring

5. Conclusion characteristics. The impedance method has a good potential for in situ monitoring of bolted connections. Bolts are widely used to assemble components in mechanical The comparison of the methods discussed in this paper is and civil structures due to their high reliability, strong load listed in Table 1. bearing capacity, and easy maintenance. However, bolt connections are often subjected to external dynamic loads, such Though fiber optical sensors (FOSs), such as fiber bragg as impact, vibration, and thermal stresses, and bolt loosening gratings (FBGs), have found many applications in structural leads to the failure of engineering structures. Therefore, the health monitoring [43, 44] due to their advantages, including detection of bolt loosening, as a part of structure health mon- high accuracy, multiplexing capacity, and EMI (electro- itoring, attracts much attention, and the development of a magnetic immunity), use of FOSs for bolted connection nondestructive testing method is needed to evaluate the state monitoring is rarely reported. The authors think that FOSs, of bolt connection. This paper reviews three main methods especially FBGs, possess a great potential to monitor bolt for the bolted joints monitoring. looseness in real time, especially for applications involving For the acoustoelastic effect based method, including high electromagnetic interference. The multiplexing capacity time-of-flight method, velocity ratio method and mechanical of FOSs is another attraction for bolted connection moni- resonance frequency shift method can determinate the bolt toring in case of a large number. With sensor optimization axial stress. It is an ideal way for the bolted structure monitor- algorithms [45], a limited number of fiber cables can provide ing, but the stress-induced velocity changes are quite small; in situ monitoring of many bolted connections. thus the time-of-flight variation and frequency shift are very small, requiring a high sampling rate for the data acquisition Acknowledgments device and thereby increasing the cost. There are many factors, such as the material-microstructure effects, environ- This research was partially supported by the Open Fund mental noise, and the thickness of bonding layer, which can of the Key Laboratory for Metallurgical Equipment and affect the measurement accuracy of the flight time and make ControlofMinistryofEducationinWuhanUniversityof in situ measurement challenging. Science and Technology, the Natural Science Foundation of Bolt preload changes the dynamic characteristics of the Hubei Province under Grant no. 2011CDA121, and the Natural interface, such as stiffness and damping, and it affects the Science Foundation of China under Grant nos. 51375354 and ultrasonic wave, which is generated by the piezoelectric 51105284. transducer, passing through the interface. By monitoring and analyzing the received ultrasonic signal, the bolt connection status can be determined. Experimental results show that References the piezoelectric sensing method based on interfacial char- [1] S. Basava and D. P. Hess, “Bolted joint clamping force variation acteristicschangeisaneffectivemethod.Withfurtherin- due to axial vibration,” Journal of Sound and Vibration,vol.210, depth research in bolted contact interface properties, contact no. 2, pp. 255–265, 1998. interface dynamic characteristics, and local dynamics, this [2]S.Izumi,T.Yokoyama,A.Iwasaki,andS.Sakai,“Three- method has a great potential for in situ bolt health monitor- dimensional finite element analysis of tightening and loosening ing. mechanism of threaded fastener,” Engineering Failure Analysis, The piezoelectric impedance method has a high sensi- vol.12,no.4,pp.604–615,2005. tivity to the local structure damage and a large-frequency [3] N. Kim and M. Hong, “Measurement of axial stress using mode- bandwidth. It is specially fitted for monitoring bolted joints converted ultrasound,” NDT and E International,vol.42,no.3, which are dominated by local dynamics of high-frequency pp.164–169,2009. International Journal of Distributed Sensor Networks 7

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