Composite Structures 87 (2009) 175–181

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Composite Structures

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Damage detection in repairs using frequency response techniques

Caleb White a,*, Henry C.H. Li a, Brendan Whittingham b, Israel Herszberg c, Adrian P. Mouritz a

a School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Australiab Department of Mechanical Engineering, Monash University, Melbourne, Australiac Cooperative Research Centre for Advanced Composite Structures Limited, Melbourne, Australia

a r t i c l e i n f o

Article history:Available online 23 May 2008

Keywords:AdhesiveFrequency responsePiezoelectricRepairScarfStructural health monitoring

0263-8223/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.compstruct.2008.05.010

* Corresponding author. Tel.: +61 3 9646 3524; faxE-mail address: c.white@crc-acs.com.au (C. White)

a b s t r a c t

Structural health monitoring (SHM) technology may be applied to composite bonded repairs to enablethe continuous through-life assessment of the repair’s efficacy. Adhesively bonded joints are an idealstarting point for real-time, in situ monitoring due to known mechanisms and locations of failure. Sim-ilarly, the ability to accurately monitor the health of a joint has potential to aid acceptance of adhesivebonding. This paper describes the development of an SHM technique for the detection of debonding incomposite bonded patches based on frequency response. Two commonly used repair schemes, the exter-nal doubler repair and the scarf repair, are examined. The paper outlines an experimental investigationon the frequency response of the repairs with and without defects under different boundary conditions.It was found that damage could be readily detected through changes in frequency response for both typesof repair. The results are discussed with implications for the development of a technology to monitor theintegrity of composite bonded repairs.

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1. Introduction

Adhesive bonding is ideally suited to joining advanced compos-ite structures. Bonded joints show greater fatigue resistance, cansustain higher loads and are lighter than mechanical interfaces[1]. Unfortunately, these advantages are often offset by inspectiondifficulties [2]. Consequently, quality assurance is commonlyachieved through careful management of the bonding process[3]. Nevertheless, sound process management alone is unable topredict or prevent bond failure due to aging or poor environmentaldurability.

The most common aerospace application of adhesive bonding isrepair. Traditionally, an inability to monitor bond condition hasforced conservative repair design. For a component to qualify forrepair it must be able to carry limit load without the patch. A repaircan only be used to increase the residual strength of a componentfrom above limit load to the design load [4]. The development of astructural health monitoring (SHM) technology that is able to pro-vide accurate and reliable information on the integrity of a bondedrepair in real-time could reduce maintenance effort and allow therepair of more significant damage. SHM technologies are a systemconsisting of sensors and supporting infrastructure including con-nections, power supplies and processing equipment coupled withdiagnostic and prognostic methodologies [5]. The majority of

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SHM techniques currently under development seek to monitorsubtle changes in strain or vibration as a result of damage.

Damage detection by measuring natural frequency reductionshas been shown to be an effective technique for the assessmentof structural health. One of the most widely used and cost effectivevibration based non-destructive testing techniques is the coin taptest. Unlike the ‘wheel tap’ technique applied in the rail industrywhere the health of a large steel wheel is ascertained by tappinganywhere on the wheel and listening to its ‘ring’, the coin tap testis highly localized [6]. Unless a defect is immediately below the re-gion struck it will not be detected by the coin tap test. Fortunately,in the case of a bonded repair, the region requiring inspection issmall and can be easily identified. The coin tap test combined withaccurate frequency response measurement could potentially forma simple SHM system for bonded repairs and joints. As aerospacestructures are typically thin any debonding would significantly de-grade stiffness lowering natural frequencies by a detectableamount. It is feasible that a SHM system could be developed usingpiezoelectric devices to produce and monitor precise ‘taps’.

This paper details a study conducted to assess the frequency re-sponse technique for the detection of debonds in typical aerospacestructural repairs. Two different types of repair schemes com-monly used for aerospace structures are examined. They are theexternal doubler repair and the scarf repair. The latter is usedwhere surface flushness is required for aerodynamic and stealthconsiderations. Piezoelectric transducers were used to excite thestructures and measure their vibration response for the purposeof damage identification.

176 C. White et al. / Composite Structures 87 (2009) 175–181

2. Specimen manufacture

As careful control of boundary conditions was required, the testspecimens were designed to be mounted with bolts to a rigid alloysupporting frame with dimensions of 290 mm � 290 mm. Boltswere selected rather than rivets to make it simple to mount and re-move the specimens from the frame as well as allowing differentboundary conditions by altering bolt torque.

2.1. External doubler repair

The external doubler specimen geometry was based on a singlesided doubler-type repair scheme as it was not intended to loadthe specimens. Two specimens, a control specimen and a damagedspecimen were produced. Both the panel and patch were carbon/epoxy fabricated from Cycom 970/T300 prepreg tape with quasi-isotropic symmetric stacking sequences of [�4504590]s and[450�4590]s, respectively. Nylon peel ply was co-cured on bothbonding surfaces. The parent panel measured 290 mm by290 mm with a nominal thickness of 1.6 mm. Removal of damagedmaterial was simulated with a 40 mm diameter hole located at thecentre of the panel cut with a diamond coated hole-saw. Mountingholes were drilled with a 9 mm laminate drill. The patch measured120 mm by 120 mm with a nominal thickness of 1.6 mm. Edges ofthe patch were tapered by laying increasingly smaller plies (3 mmfrom all previous edges) to minimise local stress concentrations.

The nylon peel ply was removed from the panel and patchimmediately prior to bonding, with no further surface preparation.

Fig. 1. External doubler parent structure with mandrel and peel ply in positionprior to patch bonding.

Release Film

OverplyAdhesive

a

Fig. 2. Specimens: (a) schematic of debond, (b) spe

A moderate temperature cure film adhesive, Cytec FM 73 (420gsm) was used to bond the patches to the parent panels. A sili-cone-rubber mandrel was used to limit squeeze out of adhesiveinto the centre hole. This also provided a flat surface for attach-ment of the piezoelectric devices. An image of one of the two spec-imens immediately prior to bonding is presented in Fig. 1. Themounting holes were drilled with a 9.5 mm laminate drill.

Airtech A5000 release film was placed between the parentpanel and film adhesive to create an artificial debond of approxi-mately 40 mm length from one edge of the patch. The release filmremained in place after cure. The film adhesive was cured at 120 �Cfor 60 min under a 1 atmosphere vacuum.

2.2. Scarf Repair

The scarf repair specimens were based on a typical scarf-over-ply repair scheme used for aerospace applications. The scarf anglewas 5�. The specimens were fabricated from Cycom 970/T300 car-bon/epoxy prepreg tape with a quasi-isotropic symmetric stackingsequence of [450�4590]2s. The overply consisted of two pre-curedorthogonal 45� plies of the same material. Each specimen wasmanufactured from a single panel measuring approximately600 mm by 200 mm. Once cured, this panel was cut into two smal-ler panels. Scarf edges were then dry ground on alternative edges(ensuring fibre alignment) with a CNC router and 12.7 mm ball-nose diamond tool. Cytec FM 73 (420 gsm) film adhesive was usedto join panels and co-cure overplies. Debonding damage was sim-ulated with the inclusion of Airtech A5000 release film betweenone edge of the overply and half the width of the scarf providinga debond of approximately 35 mm for the length of the scarf, asshown in Fig. 2. Specimens were then wet trimmed square(290 mm by 290 mm) and mounting holes drilled with a 9 mmlaminate drill.

2.3. Selection and mounting of piezoelectric devices

Lead zirconate titanate (PZT) piezoelectric disks of 12.7 mmnominal diameter (type 5A4E, thickness 0.191 mm) were obtainedfrom Piezo Systems Inc. A thin brass ribbon (0.040 mm) was usedto create an electrically conductive connection with the lower sur-face of the piezoelectric element. Sensors were assembled andbonded concurrently. Specimen bonding surfaces were degreasedwith isopropyl alcohol (IPA), abraded with 320 grit silicon–carbidepaper and then re-cleaned with IPA. Selleys� Araldite� epoxyadhesive was pasted onto the surface of the specimen then thebrass ribbon was then lowered onto the epoxy layer. A smallamount (droplet approximately 2 mm diameter) of conductiveink (Ciruitworks� CW2200MTP) was applied to the top of the brassribbon and the remaining brass coated with epoxy. The PZT diskwas gently placed over the conductive ink and secured with mylar

b

cimen with PZT devices (right-most not used).

Fig. 3. Bonded PZT disks without upper electrical connection.

0.000

0.005

0.010

0.015

0 2000 4000Frequ

Mag

nitu

de

Control

Damaged

Fig. 4. Transfer function for external doubler r

0.000

0.005

0.010

0.015

0 2000 4000Frequ

Mag

nitu

de

Control

Damaged

Fig. 5. Transfer function for external doubler re

C. White et al. / Composite Structures 87 (2009) 175–181 177

strain gauge tape as the adhesive cured. After curing, an additionalbrass ribbon was adhered to the upper surface of the PZT diskswith the conductive epoxy to form the upper electrical connection.

For the external repair specimens, the actuating PZT disk waslocated on the internal surface of the patch while the sensor wasmounted on the internal surface adjacent to the tapered/debondedregion of the patch (see Fig. 3). For the scarf repair specimens, thetransducers were mounted either side of the scarf tip with a sepa-rating distance of 50 mm, as shown in Fig. 2b.

3. Testing

Two different types of boundary conditions were examined inthe study, which were free–free and fixed–fixed. To achieve thefree–free boundary conditions, specimens were suspended by astring from a retort stand. Five tests of both the control and dam-aged specimen were made; each test consisted of ten taps whichwere averaged to develop a transfer function. Testing alternatedbetween the two specimens (damaged and undamaged) in each re-pair configuration to test repeatability. For the investigation of

6000 8000 10000ency (Hz)

epair with free–free boundary conditions.

6000 8000 10000ency (Hz)

pair with fixed–fixed boundary conditions.

0.000

0.005

0.010

0 200 400 600 800 1000Frequency (Hz)

Mag

nitu

deControl

Damaged

Fig. 6. Lower range of transfer function for external doubler repair with free–free boundary conditions.

0.000

0.005

0.010

0 500 1000 1500 2000Frequency (Hz)

Mag

nitu

de

Control

Damaged

Fig. 7. Lower range of transfer function for external doubler repair with fixed–fixed boundary conditions.

Table 1Median natural frequencies for external doubler repair

Control naturalfrequency (Hz)

Damaged naturalfrequency (Hz)

Reduction(Hz)

Free–free 156 144 12 (7.7%)319 300 16 (6.0%)506 494 12 (2.4%)763 719 44 (5.8%)956 925 31 (3.2%)

Fixed–fixed 1025 988 37 (3.6%)1269 1213 56 (4.4%)

178 C. White et al. / Composite Structures 87 (2009) 175–181

fixed–fixed boundary conditions specimens were mounted alter-natively on the mounting frame. A torque wrench was used toachieve a consistent 4 Nm of torque. Bolts were tightened in thesame order. Each specimen was mounted for each of the fives testsand then removed to investigate repeatability. To provide the ‘tap’input, electrical impulses of 0.1 ms duration were applied to theactuator. A PZT amplifier was used to drive the actuator with anominal final peak voltage of 70 V. A total of 10 tap inputs wererecorded at a frequency of 1 Hz for each test.

4. Results and discussion

4.1. External doubler repair

Transfer functions were generated based on the arbitrarysquare wave input signal as no significant difference was identifiedbetween input impulses. Transfer functions for free–free andfixed–fixed boundary conditions are presented in Figs. 4 and 5,respectively, for each of the five tests. Each transfer function wasaveraged from ten recorded taps.

For both boundary conditions investigated the presence ofdebonding appears to cause changes in frequency response over

the frequency range sampled (up to 10 kHz). Peaks in frequency re-sponse correspond to natural vibration modes of the specimen. Thelarge peaks at a frequency of around 50 Hz can be attributed toelectrical interference from the mains electricity supply by whichthe test equipment was powered. Repeatability of results wasfound to be excellent for the free–free condition and very goodthe fixed–fixed condition; this is most likely due to the relativecomplexity and greater variation in technique of the bolted re-straint. For both cases, the lower portion of the frequency spectrum

C. White et al. / Composite Structures 87 (2009) 175–181 179

appears to be the most useful for damage detection due to simplefrequency peak correlation. This lower portion of the transfer func-tion is presented for the free–free and fixed–fixed case in Figs. 6

0

0.005

0.01

0 2000 4000Frequ

Mag

nit

ud

e

Fig. 8. Transfer functions for scarf repair w

0

0.005

0.01

0 2000 4000Frequ

Mag

nitu

de

Fig. 9. Transfer functions for scarf repair wi

0

0.005

0.01

0 2000

Freq

Mag

nit

ud

e

Fig. 10. Applicable domain of transfer function for s

and 7, respectively. Matching corresponding natural frequencypeaks between control and damaged specimens becomes increas-ingly difficult at higher frequencies due to non-linear damping

6000 8000 10000ency (Hz)

Control

Damaged

ith free–free boundary condition case.

6000 8000 10000ency (Hz)

Control

Damaged

th fixed–fixed boundary condition case.

4000

uency (Hz)

Control

Damaged

carf repair with free–free boundary conditions.

0

0.005

0.01

0 2000 4000Frequency (Hz)

Mag

nitu

de

Control

Damaged

Fig. 11. Applicable domain of transfer function for scarf repair with fixed–fixed boundary conditions.

180 C. White et al. / Composite Structures 87 (2009) 175–181

due to frictional rubbing of de-bonded surfaces which appears totruncate frequency peaks. This effect is particularly noticeable forthe free–free boundary condition situation (Fig. 4). While the use-ful range will be unique for each monitoring configuration it is sug-gested that analysis would generally be confined to frequencieslower than 2000 Hz. In addition, lower frequencies also allowgreater signal propagation for large area coverage.

The median natural frequency from each of the five test sets forthe control and damaged specimen is presented in Table 1. Naturalfrequencies appear to reduce by a similar amount over the rangeanalysed for both types of boundary conditions. Reduction of nat-ural frequencies appears logical as the local reduction in structuralstiffness caused by the debond, reduces natural the frequency. Thisreduction is significant and measurable which is important if thetechnique is going to be a viable damage detection tool.

4.2. Scarf repair

Transfer functions were generated based on the square wave in-put signal. Functions for free–free and fixed–fixed boundary condi-tions are presented in Figs. 8 and 9, respectively, for each of thefive tests. Each transfer function was averaged from ten recordedtaps.

The presence of debonding causes changes in frequency re-sponse signature over the frequency range sampled (up to10 kHz) for both boundary conditions investigated. Clearly, the sig-nature is also sensitive to boundary condition variations. The largepeaks at a frequency of around 50 Hz can be attributed to mainselectrical interference. Similar to the external repair specimens,

Table 2Median natural frequencies for scarf repair

Control naturalfrequency (Hz)

Damaged naturalfrequency (Hz)

Reduction(Hz)

Free–free 613 594 19 (3.1%)994 975 19 (1.9%)

1725 1594 131 (7.6%)2281 2144 137 (6.0%)4888 4844 44 (0.9%)

Fixed–fixed 450 444 6 (1.3%)606 588 18 (3.0%)956 913 43 (4.5%)

1894 1838 56 (3.0%)2000 1900 100 (5.0%)2388 2288 100 (4.2%)3263 3113 150 (4.6%)

the lower portion of the frequency spectrum appears to be themost useful from a damage detection perspective. Appropriatetransfer function domains are shown in Figs. 10 and 11. Again, cor-relation is further complicated by non-linear damping due to fric-tional rubbing of de-bonded surfaces which appears to truncatefrequency peaks. It is anticipated that finite element modelling willprovide some insight to this phenomenon.

Measurable reductions of natural frequencies occurred in therange of 0–5 kHz for both boundary conditions, as shown in Table 2.As would be expected, the natural frequencies of the damagedspecimen were lower than that of the control because of the localreduction in joint structural stiffness caused by the debond.

5. Conclusions

An experimental investigation was conducted into the use offrequency response techniques for the detection of damage inadhesively bonded composite to composite repairs. Both the exter-nal doubler repair and the scarf repair were examined. Piezoelec-tric transducers were used to actuate and then measure thefrequency response of the test specimens, which consisted of acontrol specimen and one with significant debonding damage foreach repair configuration. Two types of boundary conditions wereexamined, which were free–free and fixed–fixed. The measurablereduction of natural frequency with the presence of a debond ishighly encouraging for both types of repairs. Excellent repeatabil-ity was also found in the test results.

Further research will include an investigation of signaturerepeatability and effects of other boundary conditions, as well asthe effects of temperature and moisture content, which are knownto vary significantly for aerospace vehicles. Future analysis couldprofit from the development of a pattern recognition algorithmcapable of identifying natural frequency peaks and spread due todamage. Such a code would be essential for the development of arobust automated structural health monitoring system. The abilityto accurately monitor the health of a joint has potential to aidacceptance of adhesive bonding as a reliable means of structuralconnection.

References

[1] Davis M, Bond D. Principles and practices of adhesive bonded structural jointsand repairs. Int J Adhes Adhes 1999;19:91–105.

[2] Adams RD, Cawley P. A review of defect types and non-destructive testingtechniques for composites and bonded joints. Bonding and repair ofcomposites. Butterworth Scientific; 1989. p. 1–15.

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[3] Hart-Smith LJ. How to get the best value for each dollar spent inspectingcomposite and bonded aircraft structures. Advanced materials: performancethrough technology insertion, vol. 38. SAMPE: Anaheim; 1993. p. 226–38.

[4] Davis M, Bond D. Certification of adhesive bonds for construction and repair. In:Proceedings of the 4th joint DoD/FAA/NASA conference on aging aircraft. St.Louis; 2000.

[5] Kessler SS, Amaratunga K, Wardle BL. An assessment of durability requirementsfor aircraft structural health monitoring sensors. In: Chang F-K, editor.Proceedings of the 5th international workshop of structural healthmonitoring. Stanford: DEStech Publications; 2005. p. 812–9.

[6] Cawley P, Adams RD. The mechanics of the coin-tap method of non-destructivetesting. J Sound Vib 1988;122(2):299–316.