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Engineering Structures 26 (2004) 895906www.elsevier.com/locate/engstruct

Detection of bolt load loss in hybrid composite/metalbolted connections

Vincent Caccese a, Richard Mewer b, Senthil S. Vel a,

a Department of Mechanical Engineering, University of Maine, 5711 Boardman Hall, Room 214, Orono, ME 04469-5711, USAb Naval Undersea Warfare Center, Newport, RI 02841-1708, USA

Received 15 August 2003; received in revised form 17 November 2003; accepted 17 February 2004

Abstract

Hybrid composite/metal connections are susceptible to bolt load loss due to viscoelastic creep and/or environmental effects.Accordingly, the focus of this research is on experimentally quantifying changes in bolt load of composite/metal hybrid connec-tions. A proof-of-concept model was created consisting of a fiber reinforced composite panel bolted to a steel frame. A piezo-electric actuator bonded to the center of the composite panel was used to provide controlled vibration input. The response of theplate was measured using either shear accelerometers or dynamic strain sensors located at the four corners of the compositepanel. The load on an instrumented bolt was decreased and three different monitoring techniques were used to detect bolt loadloss, including (a) low frequency modal analysis, (b) high frequency transfer functions between the actuator and sensors and (c)high frequency transmittance functions between pairs of sensors. Experiments demonstrate that the transmittance functionapproach shows the most promise and was able to reliably detect a single bolt loosening. A damage index based on change intransmittance function is very sensitive to changes in bolt load.# 2004 Elsevier Ltd. All rights reserved.

Keywords:Bolted connection; Composite joint; Hybrid connection; Bolt load loss; Structural health monitoring

1. Introduction

This paper describes the development of a system todetect bolt load loss in hybrid composite/metal con-nections. Determination of bolt load is critical to theevaluation of integrity of bolted connections. A fullytightened bolted joint can withstand orders of magni-tude more load cycles than an untightened joint due tothe mechanics of how the bolted joint withstands an

applied load. Accordingly, estimation of bolt load lossis a vital information in assessing the condition of arepetitively loaded composite/metal bolted connection.Even so, little work has been done to develop real timemethods of detecting loss of bolt force. This workrepresents the development and evaluation of monitor-ing methods for bolted composite panels using readilyavailable equipment and recently developed monitoring

techniques with a goal to detect small changes in bolt

load.The development of structural health monitoring

and damage detection systems is the focus of much

current research. Monitoring systems can be used to

substantially reduce maintenance costs or to prevent

catastrophes. In developing a damage detection system,

the objectives need to be clearly defined so that an

optimum and robust detection scheme is the outcome.An accurate assessment of structural integrity depends

on a proper evaluation of both the global structural

response and the condition of the connections and

interfaces. Accordingly, real time monitoring of con-

nection integrity is an important challenge. Global

damage typically produces large structural changes that

usually can be detected by analyzing the lower

vibration frequencies. This is often done in conjunction

with numerical models of the structure. Localized dam-

age mechanisms such as delamination, bolt loosening

and cracking may be difficult to detect using the lower

Corresponding author. Tel.: +1-207-581-2777; fax: +1-207-581-2379.

E-mail address:[email protected] (S.S. Vel).

0141-0296/$ - see front matter # 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.engstruct.2004.02.008


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structural modes. Techniques that excite and analyzethe higher frequencies are typically more robust whenthe detection of local structural changes or damage isthe goal.

Composite materials are used increasingly in engin-eering applications because of their high specific

strength. A synergistic effect and economical design isoften achieved when composites are used in combi-nation with metal superstructure. Accordingly, com-bined composite/metal structural systems requirehybrid connections. More often than not structuralfailures occur at connections and interfaces, therebymaking it difficult to assess integrity using techniquesthat appraise only the global structure. Hybrid com-posite/metal connections in particular are susceptibleto metal fatigue, bolt loosening primarily due to viscoe-lastic creep of the composite, temperature effects dueto coefficient of thermal expansion mismatch andmoisture absorption causing differential strain between

the metal and composite.The motivation for developing a system to detect

bolt load loss came about through a project to designan efficient structural system for underwater bodies tobe used on ship hulls. This is a part of a joint effortbetween the University of Maine, the Navatek Ltd ofHonolulu, HI, and Applied Thermal Sciences (ATS) ofSanford, Maine called Modular Advanced CompositeHull-forms (MACH). The project is performed in con-junction with the Navy Surface Warfare Center at Car-derock, MD (NSWCCD). The mission of the MACH

program is to develop fast efficient surface vessels thatuse additional underwater bodies attached to a moretraditionalhull-form. The goal is to deploy ships where more pay-load and/or higher speeds can be achieved at little orno additional power consumption and with excellent

sea-keeping ability. Fig. 1 shows one example vesselcalled MIDFOIL where a hydrofoil and a paraboliclifting body shape are combined with a catamaran hullto achieve additional buoyancy and dynamic lift whichgreatly improves the performance and sea-keeping ofthe vessel. Relatively, inexpensive pilot tests onMIDFOIL and similar vessels have shown that thismethod has great advantage for fast military supportcraft and commercial vessels such as ferries. Recentstudies have shown that the addition of underwaterbodies can dramatically improve speed, reduce fuelconsumption and increase payload. These efforts havealso demonstrated that composite material construc-

tion can bring about increased structural efficiency.The method proposed for construction of a com-

posite/metal hybrid version of the underwater liftingbody is to use a skin made of composite materialsattached to a metal framework. Fig. 2 shows a sche-matic of this concept. This type of system will allow forease of maintenance of equipment housed inside thelifting body and it will provide a metal skeletonto facilitate attachment of propulsion equipment. Theskin designed is of monocoque, stiffened or sandwichconstruction depending upon structural requirements.

Fig. 1. MIDFOIL craft with underwater lifting body (photo courtesy of Navatek Ltd).

896 V. Caccese et al. / Engineering Structures 26 (2004) 895906


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It is subsequently attached to the metal structure usinga hybrid connection. Detailing this connection to haveadequate strength and watertight integrity is imperative.

Often, bolted connections are critical to the functionof the structure and their failure may have high associa-ted repair costs, or may endanger lives. It is essentialthat a minimum torque be maintained for a hybrid bol-ted composite/metal connection to respond properly,especially if watertight integrity is to be insured. Realtime detection of bolt load loss is a major benefit inevaluating the performance of such connections. How-ever, there has been little research on the detection ofbolt load loss in hybrid connections. Accordingly, theeffort summarized in this paper focuses upon real timedetection of bolt load loss. To the authors knowledge,

a real time method for detection of bolt load loss due tostress relaxation has not been published to date. Parket al. [1] have developed an impedance-based healthmonitoring technique using piezoelectric wafers bondedto a structure to assess damage in bolted pipeline struc-tures. However, their method is not very sensitive tostress relaxation of a single bolt and one or more boltshave to be completely loosened in order to affect a sig-nificant change in the impedance signature. In addition,under the high frequency ranges used in this impe-dance-based method, the sensing region of the PZT islocalized to a region close to the sensor/actuator.

The results presented in this paper represent a

potential robust methodology based existing methodsthat were originally developed for other purposes. Toaddress this issue, a proof-of-concept test bed wasdeveloped that consists of a square plate made ofE-glass/vinyl ester composite. The hybrid connectionwas formed by bolting this plate to a steel frame usingsteel bolts, including an instrumented bolt. Severalinterrogation techniques were employed and comparedincluding: (1) low frequency modal analysis; (2) highfrequency transfer functions and (3) high frequencytransmittance functions. Each of these techniques useda piezoelectric actuator bonded to the panel to deliver

a characterized disturbance in a controlled manner.The technique using transmittance functions to evalu-ate changes in bolt tensioning level shows the mostpromise and was able to detect small changes in boltload.

2. Methods of damage detection

The fundamental premise behind vibration baseddamage detection techniques is that changes in thephysical properties will alter a systems modal proper-ties. Thus changes in mass, damping, and stiffness ofa system should lead to measurable changes in thesystems dynamic properties, such as the naturalfrequencies, mode shapes and damping. The vibrationbased structural health monitoring techniques dis-cussed herein are methods using frequency domaincalculations including transfer functions and transmit-

tance functions.Comprehensive literature reviews on the subject of

structural health monitoring can be found in refs.[24].The use of statistical analysis procedure and statisticalpattern recognition was applied to a vibration baseddamage detection scheme by Fugate et al. [5]. A pass-ive control technique using piezoelectric materials wasused to detect damage by Lew and Juang [6]. In thismethod, the natural frequencies of a system are ident-ified to detect damage in a closed loop system. A sys-tems damping almost always increases when a virtualpassive controller is added. Todd et al. [7]used chaotic

input signatures and a state-space method for damagedetection. A novel feature called the local attractorvariance ratio was developed using chaos theory. Theyshowed how a properly tuned chaotic excitation couldbe used to robustly detect structural changes.

Techniques based on neural networks require amodel to train the system to detect damage [8].Zubaydi et al. [9]investigated the damage detection ofcomposite ship hulls using neural networks. Theydeveloped a finite element model for a stiffened plate tosimulate dynamic response of the structure with andwithout damage. They were successful in identifyingcrack length and location on a faceplate. Very small

damages in composite materials, such as cracks, weresuccessfully found using wavelet analysis [10]. A finiteelement model was used in conjunction with micro-mechanics theory of composite damage. A crack size assmall as 0.06% of the total plate area can be efficientlydetected using this technique. Localized flexibilitymatrices properties were the focus of the model-basedstructural damage detection investigated by Park et al.[11]. The three flexibility methods investigated were:a free-free sub-structural flexibility method, a defor-mation-based flexibility method, and a strain-basedflexibility method. The structural damage detection

Fig. 2. Hybrid structure constructed using GRP composite skinover metal substructure.

V. Caccese et al. / Engineering Structures 26 (2004) 895906 897


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methods were based on the relative changes in localizedflexibility. Often damage detection schemes are firstproven theoretically using numerical models. Kim andStubbs[12] presented one such paper, where they useda finite element analysis to evaluate a two-span con-tinuous beam with modeled damage. A derived algor-ithm was used to predict the locations and severities ofdamage using changes in modal characteristics [12].Banks and Emeric [13] used a Galerkin method toapproximate the dynamic response of structures withpiezoelectric patches acting as sensors and actuators.Non-symmetrical damage such as a cut that extendedpart of the way into a beam was investigated. The ana-lytical results compared well with experimental resultsin the range of investigation up to 1000 Hz. Ganguli[14] used a fuzzy logic system to locate damage onhelicopter rotor blades. A fuzzy logic system canbe expressed as a linear combination of fuzzy basisfunction and is a universal function approximator [14].

The purpose of this study was to determine theapproximate location of the damage and then allow forother more intrusive techniques to pinpoint the actualdamage location.

Salawu[15] presented a review of various investiga-tions on the effects of structural damage on naturalfrequencies. Many damage location methods use chan-ges in resonant frequencies because frequency measure-ments can be quickly conducted and are often reliable.However, changes in ambient conditions such as tem-perature and moisture can cause significant frequencychanges in composite materials, and findings suggest

that detection of damage using frequency measure-ments might be unreliable when the damage is locatedat regions of low stress [15]. Similar results werepresented by Kessler et al. [16] where the frequencyresponse method was found to be reliable for detectingdamage in simple composite structures, but infor-mation about damage type, size, location and orien-tation could not be obtained. Another investigation byZak et al. [17]showed good agreement between experi-mental and numerical calculations of the first threebending natural frequencies of a delaminated com-posite beam. Kuo and Jayasuriya [18] used transferfunctions to determine the extent of joint loosening in

automobile vehicle frames with high mileage. Themethod was successful as presented in the paper, butdid not give specifics for frequency ranges investigatedand what type of frequency response functions (FRFs)were utilized.

3. Time-frequency methods for structural evaluation

Several popular methods used to quantify propertiesof dynamical systems use the dynamical systemsapproach where the input and output data for the sys-

tem is evaluated. In so doing, the system characteristicscan be quantified digitally by measurements of thetime-histories of the input, x(t), and the output, y(t)and converting these signatures to the frequencydomain using a fast Fourier transform (FFT).Fig. 3(a)shows an ideal single input-single output (SISO) systemwhere there are no extraneous inputs or outputs existdue to such phenomenon as the environment or electri-cal noise. The system is characterized, in this type ofmodel, by the FRF H(f). FRFs are measured dynami-cally by initiating a system response with a quantifiablewide-band forcing function and measuring the responseof the resulting system output. Forcing functions typi-cally used are of the swept sine type where a wide rangeof frequencies are swept at constant input amplitude.

By using the Fourier transform of the governingequations, we obtain the following system relationship:

Yf Hf Xf 1

Thus the convolution integral reduces to a simplealgebraic expression, where X(f) and Y(f) are theFourier transforms of the input, x(t) and output, y(t),respectively. In real systems, the goal is to obtain anunbiased estimate of H(f) and the influence ofunwanted noise on system quantification must be con-sidered.

4. Experimental measurement of transfer function

Experimental measurement of the transfer function

of continuous systems is not as simple as taking theFourier transform of its equation of motion. The mass,stiffness, and damping matrices can be estimated, butnot calculated exactly for most real continuous sys-tems. Thus, the responses measured by the transducersare discretized in order to take advantage of existingcomputational power.

Real systems inherently have multi-degrees offreedom. Therefore, characterization of a typical sys-tem necessitates multiple sensing and/or actuationlocations. The system equation must then be written inmatrix form where M, C and K become the mass,

Fig. 3. (a) Ideal single input single output system and (b) systemwith input and output noise.



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damping and stiffness matrices, respectively, F(t) i s aforce vector and C is a matrix defining the directioncosines of each force.

Md2yt

dt2 C

dyt

dt Kyt CFt 2

Transforming this expression to the frequency

domain results in annnreceptance FRF matrix,Hnn,

Hnnjx Mjx2 Cjx K

h i13

wheren is the number of degrees of freedom analyzed.It is possible to estimate spectral density functions in

terms of the Fourier transforms of the input and out-put signatures. In general, given two signals a(t) andb(t) and their Fourier transforms Af;T and Bf;Ttaken over a period DT, a one sided autospectral den-sity function, Gaa and cross-spectral, Gab can be com-puted from real data as follows:

Gaaf 2

ndDT

Xndi1

Af;T Af;T f>0 4

Gabf 2

ndDT

Xndi1

Af;T Bf;T f>0 5

The spectral densities are a function of frequencyand can be averaged across nd distinct sub-records ofduration DT. The in the above expressions indicatesthe complex conjugate.

In analyzing real systems, input and output noisemust be considered. Sources of noise in damage detec-tion systems come from phenomenon such as the elec-trical background environment, temperature variations,moisture fluctuations and ambient vibrations. Theseeffects may yield misleading results and their influenceneeds to be minimized. Fig. 3(b) shows a system withnoise on both the input and output. In an active sens-ing system the known inputs, x(t), are a predeterminedforcing function intentionally sent as a signal to theactuator. The input noise, m(t), and its associatedFourier transform, Mf;T, is due to the naturalvibration of the structure caused by ambient loadingconditions. The natural vibration can be thought of asan additional input signal which is often difficult to

quantify and likely uncorrelated with x(t). The trueoutput signals for the system are u(t) andv(t) and theirsum will be affected by noise on the output, n(t).Accordingly, the input and output noise will influencethe measured output y(t). This represents the casewhere the input noise passes through the system.

The normal method of estimating the FRFs usingthe signals x(t) and y(t) will result in a biased estimateand is not appropriate for this system. Unbiased FRFestimates can be accomplished by first writing thesystem equations in the frequency domain for eachoutput, yi(t) responding to n input pairs (Xkand Mk),

as follows:

Yif;T Xnk1

Hyikf Mkf;T Xkf;T Nif;T

6

This results in a frequency response matrix that has

the row order equal to the number of responses andcolumn order equal to the number of inputs. Writingthe expression for any single input xk(t) and multiply-ing each term by the complex conjugate, Xkf;T, anddropping the i and ksubscript for simplicity results inthe expression:

Gyxf Hyxf Gmxf Gxxf Gnxf 7

If the noise terms are uncorrelated with each otherand the true signals then the cross-spectral densityterms between the true signals (u(t), v(t)) and the noisesignals (m(t), n(t)) will equal zero. This condition isexpressed mathematically as:

Gmxf Gmnf Gnxf 0 8

This leads to an unbiased estimate for H(f) withoutthe need of measuring the natural input m(x) of:

Gyxf Hyxf Gxxf 9

This is a practical expression which can be used toobtain the estimate for the FRF matrix H.

5. Transmittance function theory

Transmittance functions are similar to the FRFs andare derived as the complex ratio between Fourier trans-forms of a response point and a reference point on astructure. The transmittance function between tworesponse points,a and b, can be written as:

Tabf Gabf

Gbbf 10

The motivation for using transmittance functions isthat excitation does not need to be measured; there-fore, changes in the structure due to the environmentaleffects (temperature and moisture) are partly cancelled[19]. Also, the cross-spectral density used in transmit-

tance functions is a measure of the linearity betweentwo response points on the structure and can detectlocal damage by propagation changes (phase delay andamplitude modulation) in the structural response.Since, the cross-spectral density function is the Fouriertransform of the cross-correlation function, it repre-sents the frequency domain characterization of thesimilarity of the magnitude and phase of two signals,e.g. of two nearby response points on the structure.Hence, if used in the correct frequency range, it canaccurately detect damage over small distances on astructure. Furthermore, measured transmittance data



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inherit certain advantages over modal data. Firstly,

transmittance functions have few sources for comput-

ing error, except the minimal error from the numerical

FFT. Secondly, they carry complete information on

the dynamic behavior of the test structure, in terms of

both the vibration modes and the damping, at many

frequency points, including those away from theresponse of the structure[20].

The transmittance function does not depend on whe-

ther the receptance, mobility or inertance spectral den-

sities are measured, since it is a ratio of the FRFs.

Therefore, different sensor types can be used to mea-

sure the vibration response. When damage occurs, the

peaks and valleys of the transmittance function mis-

align. This misalignment caused by a change between

the healthy and damaged system can be quantified.

Generally, the sensitivity of the technique to detect

small damage increases as the actuator and sensor

move close to the damage, and as the frequency ofexcitation increases [21]. The damage values for some

cases are nearly four times larger at high frequencies

(1020 kHz) than at low frequencies (2001800 Hz)

[20]. However, the size of the PZT sensors plays a role

in the effective frequency range over which the sensor

can detect damage. Different sized sensors are more

tuned for certain vibration frequencies. A larger sensor

would not be as effective as a small sensor for detecting

high frequency vibration, because the sensor would be

much larger than the vibration wavelengths. Also, at

the higher frequencies there are additional hardware

concerns, because of the higher sampling rate and the

likelihood that additional sensors will be required.

Successful transmittance function testing for wind tur-

bine blade damage analysis was presented by Ghoshal

et al. [22] and by Schulz et al. [21] for beams and

plates. Additional tests on other types of structures and

damages are needed to confirm the characteristics of

this method.

6. Damage detection approaches using a damage

index

A damage index can be used to quantify the change

in dynamic properties of a structure in a potentially

damaged state compared to a baseline healthy

state. The damage index, Dij, quantifies the aggregate

change of the FRF or transmittance functions across a

given frequency range in terms of a scalar value. This

scalar quantification eliminates the need to store the

complete time history and/or spectral data when

data storage limitations are of concern. The damage

index (Dij), defined over a frequency range off1to f2is

given as:

Dij

f2f1

Qhijf Qdijf

df

f2f1

Qhijf

df

11

Here Qhijf represents either the FRF, Hhijf, ofresponse point, i, relative to force, j, or the transmit-

tance function, Thijf, of response point, i, relative to

response point, j in a healthy state and Qdijf repre-

sents the damaged state. A matrix of damage indices

are computed for the system whose size is equal to the

number of response points measured. This matrix can

be interpreted based upon the phenomenon being mea-

sured. In the case of bolt loosening on a structural sys-

tem, the desired outcome is to quantify the change in

damage index relative to bolt torque. Of course other

phenomenon may also change the structure, which canbias the estimate. The study presented in this paper is

an assessment of the feasibility of several methods in

detection of bolt load loss.

7. Experimental procedure

An experimental platform was created to investigate

three different health monitoring schemes that show

potential for evaluation of bolt stress relaxation in

hybrid type composite/metal systems. These methods

included: (1) evaluation of changes in fundamentalmodal properties; (2) evaluation of transfer functions

with the structure subjected to a high frequency input;

and (3) evaluation of transmittance functions with the

structure subjected to a high frequency input.

7.1. Specimen preparation

As a test bed for the study, a composite plate and

steel frame apparatus were designed and fabricated.The test article consists of a 622 mm (24.5-in.) square

E-glass/vinyl ester composite plate drilled at itsperimeter and bolted to a steel frame with sixteen

12.7 mm (1=2-in:) diameter steel bolts. The composite

plate was a 6.35 mm (1=4 in:) thick and consisted of

eight layers of Brunswick Technologies Inc. (BTI) 0/90

E-glass knit fabric (BTI CM-2408) in a symmetric

lay-up, [(0/90)4]S. The matrix material is a Dow Dera-

kane 8084 resin infused at the University of Maine

Crosby Laboratory, using a Vacuum Assisted Resin

Transfer Molding (VARTM) process. Sixteen 14.3 mm

(9/16 in.) holes were drilled around the plate perimeter

to match the bolt pattern of the steel picture frame.



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7.2. Instrumentation

A piezoelectric actuator (ACX QP10W), shown inFig. 4, was bonded to the center of the plate using anepoxy adhesive. Accelerometers and dynamic strainsensors were used to measure the dynamic response ofthe plate to an excitation produced from the actuator.Both sensors types were supplied by PCB Piezotronicsof Depew, New York. Ceramic shear ICP accel-erometers, model 352B10, were chosen for their abilityto measure low amplitude vibration, minimal mass,and ability to operate in a frequency range of up to25 kHz. This sensor has internal signal conditioning

circuitry that minimizes noise and improves sensoraccuracy. The dynamic strain sensor, model 740B02,was chosen for its high frequency response, low profile,and low cost. Because of cost considerations, strainsensors are an attractive option for large-scale healthmonitoring schemes. For the lower frequency dynamicmeasurements, a model 352A24 PCB accelerometerwas used. The excitation signal was generated using aSigLab dynamic signal acquisition and processinghardware. The SigLab hardware has two input and twooutput channels and has a bandwidth of 20 kHz.SigLab is used as the function generator, data acqui-

sition system, and data analyzer in the experiments.A.L. Design Inc. of Buffalo, New York supplied the 16bolts used to fasten the composite plate to the steelframe including one instrumented bolt. The load sens-ing bolt, model ALD-BOLT-1/2-2, was the same as theother bolts, except for the internal strain gauges. Thefactory calibration for the instrumented bolt is used todetermine the bolt load. All bolts were lubricated usingLocktite anti-seize and the applied torque was mea-sured using a micro-meter torque wrench. It was foundthat the anti-seize lubricant results in a much morereliable load-torque correlation.

8. Results and discussion

8.1. Detection of bolt loosening using fundamentalfrequency

As a baseline, an attempt was made to detect boltloosening using fundamental frequency information.This study was conducted two ways: namely (1) withthe torque in only one bolt being reduced with theother bolts left at full level (720 in. lb); and (2) with thetorque in all bolts reduced by the same amount.

The response of the plate was sensed by the lowfrequency accelerometer mounted 19 mm (0.75 in.)from the edge of the ACX actuator. Data from threetrials records consisting of 4096 data points wererecorded and averaged. The system excitation wasthrough the PZT actuator using a two volts root meansquare (VRMS) chirp function amplified at a gain of100 over a frequency range of 0 to 1000 Hz. Estimationof the transfer function was performed with the virtualnetwork analyzer module (VNA) of the SigLab inter-face program. The fundamental frequency was esti-mated from the transfer function. Another experimentwith a narrower bandwidth of 100 Hz, centered aroundthe fundamental frequency, was performed to moreprecisely determine the fundamental frequency. Trans-

fer function, fundamental frequency and damping esti-mates were recorded at applied torque valuesincrementally from 0 to 81. N-m (0 to 7 0 n.-lbs).

The results of the study of a single loosened bolt arepresented inFig. 5. It is observed that a change in thetension of one bolt around the perimeter of theplate does not significantly change the fundamentalfrequency of the plate, except for a slight decrease infrequency when the bolt is completely loose. The effectof bolt torque on the damping coefficient for the fun-damental frequency was also investigated. The damp-ing coefficients were computed using the half-power

Fig. 4. Test panel showing sensors and actuators.



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bandwidth method. Fig. 6 is a plot of the dampingcoefficient f as a function of applied bolt torque of thebolt that was loosened. The figure shows that there isno significant change in the level of damping when thetorque on only bolt is altered. Results of the moredramatic case where all of the bolts are loosened to thesame load level is presented in Fig. 7. Again, it isobserved that a change in the tension of all the boltsdoes not significantly change the fundamental fre-quency of the plate, expect for the case when all of thebolts are completely loose.

8.2. Detection using the transfer function method

The potential for using transfer function informationto detect bolt loosening is presented in this section.Various bands of input frequency, with a 4 kHz band-

width, were analyzed for their effectiveness. Duringthese tests, all of the bolts were torqued to a level of81.3 N m (720 in. lb) and a change in torque wasapplied to the instrumented bolt only. One sensor wasplaced directly next to the instrumented bolt andanother sensor was placed directly on the other side ofthe plate away from the loosened bolt. Both the ICPaccelerometers and dynamic strain sensors wereemployed to determine which sensor produced betterresults. The Siglab hardware generated the 4 kHzbandwidth chirp excitation signal for the ACX actu-ator. It was also used to measure the two transfer func-tions for the sensor near the bolt and away from thebolt relative to the actuator. The bolt torque was incre-mentally varied, for the instrumented bolt, in the samemanner as described in the previous section. It wasobserved that the chirp signal in the band from 13 to17 kHz resulted in the most sensitive response.

Figs. 8 and 9 show example FRFs using the shearaccelerometer signals for the sensors next to and awayform the loosened bolt, respectively. The plots showthe response measured on the decibel scale relative tothe PZT actuator input signal as a function of both theloosened bolt load and frequency range investigated.

The figures show that the transfer functions, in theband of 1317 kHz, change significantly as the boltload is reduced. Plots of the damage index for thesetwo cases is shown in Fig. 10. Fig. 11 portrays thedamage index when a strain sensor is used in place ofthe accelerometers. Inspection of the damage indexplots show an increasing change in the damage indexas the bolt is loosened. The damage index plots do notshow significant dependence on the location of the sen-sors relative to the loosened bolt. The transfer functionplots change as the bolt load varies, and thus thedamage index changes. The results do show that there

Fig. 5. Fundamental frequency dependency with single bolt loosening.

Fig. 6. Fundamental frequency damping coefficient with single boltloosening.

Fig. 7. Fundamental frequency dependency with all perimeter boltsloosened.



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is a change in the dynamic characteristics of the systemwith a bolt loosening, and that the sensitivity to thechanges are different depending on the input frequencyrange and what type of sensor is used. The damageindex plots for dynamic strain sensors shows that thereare greater changes for the sensor away from the bolt,than the sensor next to the loosened bolt as seen inFig. 11. The damage index plot using the accel-erometers (Fig. 10) shows a larger damage indexresponse for the sensor located closer to the loosenedbolt, and a smaller damage index response for the sen-sor placed on the opposite side of the plate.

8.3. Detection using the transmittance function method

Transmittance testing procedures are similar to theprocedures using transfer functions. For the transmit-tance tests, the response in the frequency domain of

two sensors is compared with each other, in contrast toestimating transfer functions where the response of thesensor is compared to the excitation signal. Thus, forthe transmittance testing, sensor A was connected toinput channel 1 and sensor B was connected to refer-ence channel 2 of the Siglab hardware, thus thetransmittance function A with respect to B (TAB)was calculated using this configuration according toEq. (10). Tests were conduced by incrementally loosen-ing a single bolt as described previously. Switch boxeswere employed to facilitate a quick change of sensors

so that multiple transmittance functions could berecorded easily.

Transmittance function estimates were made in thefrequency range from 0 Hz to 20 kHz, which wasdivided into a series of subranges with a 2 kHz band-

Fig. 8. Frequency response functions using accelerometer next tothe bolt.

Fig. 9. Frequency response functions using accelerometer awayfrom the bolt.

Fig. 10. Loosening indicator for frequency response function testwith accelerometers.

Fig. 11. Loosening indicator for frequency response function testwith strain sensors.



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width. The various analysis bandwidths were investi-

gated so to determine the best range in which to detectbolt loosening for this particular structure. The use of

smaller bandwidths allows for a higher frequency resol-

ution than would be possible if the damage estimateswere taken over the entire 020 kHz range. Responses

of strain sensors were compared to accelerometers

and larger damage indices were found using the accel-eration as the measurand. The frequency band from 7

to 9 kHz was observed to produce the most sensitivity

of the damage index to bolt loosening. A sampletransmittance function, T23, for this frequency range is

presented in Fig. 12 based upon the instrumentation

scheme inFig. 4. The healthy state is represented by abolt torque of 81.3 N m (720 in. lb) and loosened state

plotted correspond to torques of 61.0, 33.9 and 6.8 Nm (540, 300 and 60 in. lb).

The damage index was computed for the loosenedstates and an increasing trend with bolt loosening isobserved as shown in Fig. 13. This figure includes acomplete set of transmittance functions for input in the79 kHz range noting that transmittance TAB is vir-tually equal to transmittance TBA due to the symmetryof the system and sensor placement. The horizontalaxis in this figure is the decrease in torque of the instru-

mented bolt from its fully torqued level of 81.3 N m(720 in. lb). This set of parameters resulted in a worstcase damage index of 1.4 when the bolt was loosenedby 74.6 (660 in. lb) to 6.8 N m (60 in. lb). As a com-parison, the frequency range from 18 to 20 kHz resultsin a damage index of 1.0 for the T23 response when thebolt is loosened to 6.8 N m (60 in. lb). The use ofstrain sensors in the range of 79 kHz resulted in adamage index of 0.6 for the same bolt looseningparameters. The damage levels for the transmittancefunction T23 were higher than all other reported trans-mittance functions in the 7 to 9 kHz frequency range

when using accelerometers as sensors. Also demon-strated inFig. 13is that the damage index of sensors 2and 3 was in all cases the largest of the four measureddamage indices. It is noted that the bolt loosened liesbetween sensor 2 and 3.

8.4. Repeatability of transmittance function data

The repeatability of the transmittance testing resultswas also investigated in ten separate trials as portrayedin Fig. 14. The damage indices were calculated usingaccelerometers for the 7 to 9 kHz frequency range for a

Fig. 12. Transmittance function with accelerometers over 79 kHz range.

Fig. 13. Damage index using transmittance function over 7-9 kHzrange.



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bolt torque decrease of 54.2 N m (480 in. lb). The testsshowed damage indices calculated from the transmit-

tance functions had some variation. The D23 damageindices had a minimum of 0.677 and a maximum of0.758, +/0.041, which is approximately 6% of thesmaller value. The other damage indices results exhib-ited lesser variation than D23.

9. Summary and conclusions

Structural monitoring systems can be used to sub-stantially reduce maintenance costs and to preventpotentially catastrophic structural failures. Implemen-tation of robust systems must include keeping themonitoring objectives clearly in mind while developingthe system. The research summarized in this paperfocuses on the use of several different methods to assesschanges in bolt load in hybrid metal/composite con-nections. The bolted connection is one of the mostwidely used in practice. Loss of bolt load can occurdue to viscoelastic creep of the composite material,thermal effects, moisture effects and vibrations. Often-times, maintaining adequate bolt load is critical toproper functioning of the structure, especially in fric-tion type joints and will typically increase fatigue life.

Methods attempted to monitor bolt load loss are

under the umbrella of structural vibration analysis.Three different schemes were used which cover a largerange of input vibration frequencies. The methodsinclude: (1) assessing the change in fundamental modeproperties; (2) assessing the change in transfer func-tions; and (3) assessing the change in transmittancefunctions. A proof-of-concept model was created con-sisting of a steel frame and an E-glass/vinyl ester com-posite panel that was bolted to the frame using 16 bolts.Controlled vibration input was provided to the panelusing a piezoelectric actuator bonded to the compositepanel at its center.

The technique using fundamental mode properties

was only able to detect changes in bolt load when all of

the 16 bolts on the panel were loose. It is also observedthat a change in the tension of a single bolt does not

significantly change the fundamental frequency of theplate, expect for the case when it is completely loose.

Use of transfer functions to detect bolt looseningshows some promise. Various bands of input frequency

from 0 to 20 kHz with a bandwidth of 4 Hz were ana-lyzed for their effectiveness. It was observed that the

band from 13 to 17 kHz was the most sensitive. Chan-

ges in FRF were quantified using a damage index thatcompares the current state to a healthy baseline state.

Observed in testing was an increasing change in the

damage index as the bolt is loosened. The damageindex plots do not show significant dependence on the

location of the sensors relative to the loosened bolt.The results do show that there is a change in the

dynamic characteristics of the system with a bolt loos-ening, and that the sensitivity to the changes are differ-

ent depending on the input frequency range and whattype of sensor is used.

Transmittance function estimates were made in the

frequency range from 0 Hz to 20 kHz, which was div-ided into a series of subranges with a 2 kHz band-

width. The frequency band from 7 to 9 kHz was

observed to produce the most sensitivity of the damageindex to bolt loosening. A bolt load loss of only 8%

was reliably detected and resulted in a damage index of0.16 using accelerometers. An increasing trend in dam-

age index with bolt loosening is observed with theworst case being 1.4 with the bolt loosing 92% of its

original preload. Strain sensors resulted in a lowerdamage index of 0.6 for the same bolt loosening para-

meters. The damage levels for the transmittance func-

tion T23 were higher than all other reportedtransmittance functions in the 7 to 9 kHz frequency

range when using accelerometers as sensors. Alsodemonstrated is that the damage index of sensors 2 and

3 was in all cases the largest of the four measured dam-

age indices and the bolt loosened lies between sensor 2and 3. In summary, the transfer function method

shows much promise for detecting bolt loosening.

Acknowledgements

The authors gratefully acknowledge funding for thisproject through the Office of Naval Research under

grant number N00014-01-1-0916. Dr. Roshdy G.S.Barsoum of ONR is the cognizant program officer.

Assistance of support personnel in the Mechanical

Engineering department is acknowledged, includingKeith Berube who fabricated the test specimen.

Fig. 14. Repeatability of computing damage index using transmit-tance functions.



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