#2017 The Acoustical Society of Japan

Acoust. Sci. & Tech. 38, 4 (2017)

Non-contact alignment-free soundness evaluation of adhesive anchors
by exciting/detecting longitudinal bolt vibrations
using electromagnetic acoustic waves
Kazuhiko Hasebe, Yosuke Mizuno and Kentaro NakamuraInstitute of Innovative Research, Tokyo Institute of Technology,
4259, Nagatsuta-cho, Midori-ku, Yokohama, 226–8503 Japan(Received 15 December 2016, Accepted for publication 8 February 2017)
Keywords: Non-contact evaluation, Electromagnetic acoustic waves, Adhesive anchors, Longitudinal bolt vibrations
PACS number: 43.35.Zc [doi:10.1250/ast.38.225]
1.

Introduction
Adhesive anchors are widely used in civil engineering
applications, particularly to anchor the bolts of illuminatorsand ventilators in tunnels. The evaluation of the soundness ofthese anchors is essential to structural safety. Conventionally,
their soundness has been evaluated based on visual inspection
and hammering tests performed by skilled workers. Meanwhile, certain non-destructive methods of testing adhesive
anchors have also been developed. In one method, ﬂexuraland longitudinal vibrations of bolts are excited with the useof an electromagnetic acoustic transducer (EMAT) and thenmeasured with a laser Doppler vibrometer (LDV) [1]. In
another method, the leakage vibrations at the concrete surface
are measured by means of an acoustic emission (AE) sensor[2]. The AE induced by damage progression has also been
detected by AE sensors [3]. Furthermore, echoes of the guided
waves directed to bolts excited with a piezoelectric transducer
have been shown to be useful in evaluating the anchor
soundness [4–6]. However, in all these previous methods,
either contact with the target under test or precise positionalignment of the evaluation system is indispensable.
The use of an EMAT [7,8] enables not only non-contact
excitation but also non-contact detection of vibrations [9,10].
When a dynamic magnetic ﬁeld is applied to a conductive
material placed under a static magnetic ﬁeld, the material
undergoes vibrations due to the Lorentzian force experienced
by eddy currents induced on the material surface. Therefore,
the application of a periodical dynamic magnetic ﬁeld can beutilized to excite the vibration of the material. The detectionof vibrations can be performed through the reverse process,namely, when a vibrating material is placed under a static
magnetic ﬁeld, eddy currents are generated on its surface,
thereby inducing a detectable dynamic magnetic ﬁeld.
To date, various conﬁgurations of EMAT-based noncontact ultrasonic inspection systems have been implemented[1,9–14], because they do not require coupling agents (whichlead to contamination) or smoothing fabrication; further, such
systems can operate at high temperatures. A major drawbackof EMAT-based systems is their low signal intensity. The
signal-to-noise ratio in such systems has been improved bye-mail: knakamur@sonic.pi.titech.ac.jp

sweeping the exciting frequency and thus exploiting acoustic
resonance [15].
In this work, based on this resonance-based technique,we implement a new EMAT-based system for non-contact
evaluation of the soundness of adhesive anchors, wherein
we use the EMAT to both excite and detect longitudinal
vibrations of bolts. This conﬁguration does not require precise
position alignment of the EMAT system, which will be ofgreat use in practical applications. We investigate theinﬂuence of the amount and degradation of adhesive on theresonance frequency and the quality factor.
2.

Experimental setup
Figure 1(a) schematically shows the cross-sectional viewof the EMAT used in the experiment. The EMAT (diameter:40 mm, length: 17 mm) consists of three ring-shaped neodymium magnets, a steel yoke, an exciting coil (30 turns, selfinductance: 12 mH), a receiving coil (30 turns, self-inductance:11 mH), and an acrylic bobbin. The self-resonant frequencies
of both the coils are over 4 MHz, below which no impedance
peaks or dips exist. The interval between the bolt and the coils
is approximately 0.5 mm. The steel stud bolt touches neither
the yoke nor the coils; it touches the acrylic bobbin, whichappears to have almost no inﬂuence on the measurement
results. As the EMAT is ﬁxed to the bolt using a magneticforce, precise alignment is not required. When an alternating
current is applied to the exciting coil, an alternating magnetic
ﬁeld is generated, thereby inducing an eddy current on thesurface of the bolt. Due to the Lorentzian force arising fromthe interaction between the static magnetic ﬁeld and the eddy
current, longitudinal vibrations of the bolt are excited, and as
regards detection, an eddy current is generated on the surfaceof the bolt as it vibrates under the static ﬁeld. The resulting
change in the magnetic ﬁeld caused by the eddy current isdetected with the use of the receiving coil.
Four M8 steel stud bolts (length: 100 mm) were ﬁxed
using adhesive (solidiﬁed; employed at the bottoms of theholes; EA-500S, Asahi Kasei) in the holes (diameter: 9 mm,depth: 70 mm) of four concrete blocks (diameter: 100 mm,height: 250 mm). The amount of the adhesive utilized for thefour samples was varied as 1.15, 2.30, 4.50, and 4.50 g (4.50 gcorresponds to completely ﬁlled holes). The adhesion of one

225Acoust. Sci. & Tech. 38, 4 (2017)

(a)

(a)

(b)

Fig. 2 (a) Frequency dependences of the received
electromagnetic acoustic transducer (EMAT) signal
amplitude and the laser Doppler vibrometer (LDV)measured vibration velocity. (b) Resonance frequencies of the EMAT-based and LDV-based resultsplotted as functions of the amount of adhesive.

(b)Fig. 1 (a) Schematic cross-sectional view of EMAT
utilized for exciting and detecting the longitudinal
vibrations of a steel stud bolt. (b) Experimental setup
for measuring (i) frequency dependences and (ii)
damping characteristics of the bolt vibrations. AWG,
arbitrary waveform generator; EMAT, electromagnetic
acoustic transducer; FRA, frequency response analyzer; LDV, laser Doppler vibrometer.

of the 4.50-g samples was artiﬁcially degraded by means of
the following procedure: 1) after the bolt was ﬁxed, it wasinductively heated and maintained at 220 C for 2 h, 2) thebolt was cooled by air to room temperature, and 3) processes
1) and 2) were repeated again (See Appendix for details)[16–19].
The measurement setup is depicted in Fig. 1(b). First,
with the use of conﬁguration (i), we measured the frequency
characteristics of the longitudinal vibrations of the bolt anddetermined the ﬁrst-order resonance frequency. Continuouswaves output from a frequency response analyzer (FRA;FRA5097, NF) were ampliﬁed by 26 dB (peak-to-peakvoltage: 8 V) and input to the EMAT. The received signal
from the EMAT was ampliﬁed by 70 dB, ﬁltered with abandpass ﬁlter (BPF, 1–100 kHz), and observed with the same
FRA. The frequency of the continuous waves was swept from8 kHz to 28 kHz with a step of 32 Hz over a 20-s period(which can be set to values less than even 1 s in practicalapplications). To conﬁrm the validity of this measurement,we also directly measured the vibrations of the bolt using anLDV and a BPF (1–100 kHz). Subsequently, using conﬁguration (ii), we derived the damped oscillations of thelongitudinal vibrations of the bolt at the ﬁrst-order resonancefrequency and calculated the quality factor. The burst waves
output from an arbitrary waveform generator (AWG) were226

ampliﬁed by 26 dB (peak-to-peak voltage: 8 V) and input tothe EMAT. The received signal was ampliﬁed by 70 dB,ﬁltered using a BPF (10–100 kHz), and observed with an
oscilloscope (OSC). An LDV-based measurement was also
performed for veriﬁcation.
3.

Experimental results
The frequency dependences of the received EMAT signal
amplitude and the LDV-measured vibration velocity are
shown in Fig. 2(a). The EMAT signal was divided by thefrequency-dependent gain of the ampliﬁer to ‘‘ﬂatten’’ thedata. With increasing adhesive amount, the signal amplitudeof the resonance peak in the EMAT decreased, which was thesame trend as the vibration velocity of the LDV-measured
resonance peak. The obtained resonance frequencies are valid
because the ﬁrst-order resonance frequency of a 100-mm-longsteel bar (longitudinal sound velocity: 5,120 m/s [20]) undera ﬁxed-free boundary condition is calculated to be approximately 10.3 kHz. Figure 2(b) shows the resonance frequencies of the EMAT-based and LDV-based results (calculated
as the frequencies at which maximal peaks were observed inFig. 2(a)) plotted as functions of the amount of adhesive.
With increasing amount of adhesive, the resonance frequency
increased. Further, heat-induced degradation was found toreduce the resonance frequency. The measurement error
between the EMAT-based and LDV-based results was 1.68%
at maximum, which indicates that the EMAT-based measurement is highly reliable.
Figure 3(a) shows an example (for an adhesive amount of
1.15 g) of the temporal variations in the received EMAT
signal amplitude s and the LDV-measured vibration velocity v
when the excitation of the vibration was abruptly stopped. Thevibration was stopped by setting the voltage applied to theexciting coil to 0 V, and the corresponding instant of time wasdeﬁned as 0 s. After 0 s, the amplitude of the EMAT signal
decayed exponentially, which agreed with the behavior of thevibration velocity. The received EMAT signal was found to
be inﬂuenced by the current that continues ﬂowing in theexciting coil shortly after stopping the vibration. In order toremove this inﬂuence, we calculated the exponential ﬁts of theK. HASEBE et al.: NON-CONTACT ADHESIVE ANCHOR EVALUATION USING EMAT
References(a)(b)

Fig. 3 (a) Temporal variations in the received EMAT
signal amplitude and the LDV-measured vibration
velocity when the excitation of the vibration wasabruptly stopped. The adhesive amount was 1.15 g.
(b) Quality factors of the received EMAT signal
amplitude and the LDV-measured vibration velocity
plotted as functions of the amount of adhesive.

maximal points of the measured results (while the amplitude
became 50% to 3% of that during excitation). Subsequently,
the quality factor Q was calculated as [21,22]
Q¼

!0;
2að1Þ

where a represents the coeﬃcient of t (time) in theexponential ﬁts (876 for the EMAT and 891 for theLDV in Fig. 3(a)) and !0 the angular resonance frequency.
Figure 3(b) shows the quality factors of the received EMAT
signal amplitude and the LDV-measured vibration velocity
plotted as functions of the amount of adhesive; the quality
factor is nearly constant irrespective of the amount of theadhesive, but the heat-induced degradation lowers the quality
factor. The EMAT-based and LDV-based results were in good
agreement with each other with an error of <3:72%. Our
results suggest that the shortage of adhesive arising fromshoddy workmanship and the aging degradation of adhesive
can be potentially evaluated using the resonance frequency
and the quality factor, and that these parameters can bedetected by a non-contact electromagnetic method without
using an LDV. Note that some other samples with similar
structures were also tested, and the measured results exhibited
almost the same trends, which supports the reliability of theconclusion.
4.

Conclusion
We developed a new non-contact method for evaluating
the soundness of adhesive anchors. The longitudinal vibrations of bolts were both excited and detected with the use of
an EMAT. As the amount of adhesive increased, theresonance frequency also increased, but the quality factor
remained nearly unchanged. Meanwhile, heat-induced degradation of the adhesive lowered both the resonance frequency
and the quality factor. As our conﬁguration does not requireprecise position alignment, we believe that this method will
be of great potential use in evaluating shoddy construction
and the aging degradation of adhesive anchors in the nearfuture.

[1] H. Maekawa, S. Ushida, T. Kawada and H. Miyata, ‘‘Evaluation methods for soundness of adhesive portion around postinstalled anchor bolts in concrete based on electromagneticpulse,’’ Proc. Jpn. Concr. Inst., 37, 1693–1698 (2015) (inJapanese).
[2] T. Kimura, S. Uchida, H. Miyata and T. Kamada, ‘‘Nondestructive evaluation methods for soundness of adhesive
anchoring portion in concrete by electromagnetic pulse,’’ Proc.
Jpn. Concr. Inst., 36, 2116–2121 (2014) (in Japanese).
[3] P. Rizzo, A. Spada, S. Degala and G. Giambanco, ‘‘Acoustic
emission monitoring of chemically bonded anchors,’’ J. Nondestruct. Eval., 29, 49–61 (2010).
[4] M. D. Beard and M. J. S. Lowe, ‘‘Non-destructive testing ofrock bolts using guided ultrasonic waves,’’ Int. J. Rock Mech.
Mining Sci., 40, 527–536 (2003).
[5] M. D. Beard, M. J. S. Lowe and P. Cawley, ‘‘Ultrasonic guided
waves for inspection of grouted tendons and bolts,’’ J. Mater.
Civ. Eng., 15, 212–218 (2003).
[6] J. Yu, M. Bae, I. Lee and J. Lee, ‘‘Nongrouted ratio evaluationof rock bolts by reﬂection of guided ultrasonic waves,’’ J.
Geotech. Geoenviron. Eng., 139, 298–307 (2013).
[7] P. K. Larsen and K. Saermark, ‘‘Helicon excitation of acousticwaves in aluminium,’’ Phys. Lett., 24A, 374–375 (1967).
[8] E. R. Dobbs, ‘‘Electromagnetic generation of ultrasonic waves
in metals,’’ J. Phys. Chem. Solids, 31, 1657–1667 (1970).
[9] R. B. Thompson, G. A. Alers and M. A. Tennison, ‘‘Application of direct electromagnetic Lamb wave generation to gas
pipeline inspection,’’ Proc. IEEE Ultrason. Symp., pp. 91–94(1972).
[10] W. D. Wallace, J. R. Houck, R. Bowers, B. W. Maxﬁeld andM. R. Gaerttner, ‘‘Transducerless method for ultrasonic ﬂaw
testing in metals,’’ Rev. Sci. Instrum., 39, 1863–1864 (1968).
[11] M. Hirao and H. Ogi, EMATs for Science and Industry:
Noncontacting Ultrasonic Measurements (Kluwer Academic
Publishers, Chicago, 2003), p. 1.
[12] K. Nakamura, Ultrasonic Transducers: Materials and Designfor Sensors, Actuators and Medical Applications (WoodheadPublishing, Cambridge, 2012), p. 392.
[13] F. Hernandez-Valle and S. Dixon, ‘‘Initial tests for designing ahigh temperature EMAT with pulsed electromagnet,’’ NDT&EInt., 43, 171–175 (2010).
[14] S. E. Burrows, Y. Fan and S. Dixon, ‘‘High temperature
thickness measurements of stainless steel and low carbon steel
using electromagnetic acoustic transducers,’’ NDT&E Int., 68,73–77 (2014).
[15] M. Hirao, H. Ogi and H. Fukuoka, ‘‘Resonance EMAT system
for acoustoelastic stress measurement in sheet metals,’’ Rev.
Sci. Instrum., 64, 3198–3205 (1993).
[16] S. R. Hartshorn, Structural Adhesives: Chemistry and Technology (Springer-Verlag, New York, 2012), p. 91.
[17] M. Yamashita and K. Suganuma, ‘‘Degradation mechanism ofAg-epoxy conductive adhesive/Sn-Pb plating interface by heat
exposure,’’ J. Electron. Mater., 31, 551–556 (2002).
[18] X. Buch and M. E. R. Shanahan, ‘‘Thermal and thermooxidative ageing of an epoxy adhesive,’’ Polym. Degrad.
Stabil., 68, 403–411 (2000).
[19] J. K. Nelson, Dielectric Polymer Nanocomposites (Springer
Science & Business Media, New York, 2009), p. 172.
[20] National Astronomical Observatory of Japan, Chronological
Scientiﬁc Tables (Maruzen Publishing Co., Ltd., Tokyo, 2013),
p. 435 (in Japanese).
[21] M. Kutz, Handbook of Measurement in Science and Engineering, Vol. 1 (John Wiley & Sons, Hoboken, 2013), p. 370.

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Acoust. Sci. & Tech. 38, 4 (2017)[22] L. Cai, Fundamentals of Mechanical Vibrations (John Wiley &Sons, Chichester, 2016), p. 114.

Appendix
The heat-induced degradation of the adhesive appears tooriginate from the degradation of the material itself [16–18]
as well as from cracks/chips caused by the mismatch between
the thermal expansion coeﬃcients of the adhesive and thesteel [19]. Here, we examine how the adhesive used inexperiment was degraded by heating after bolt ﬁxation. We
prepared two 48-mm-long hexagon bolt (M12) samples.
Adhesive (EA-500S) was attached to each sample on its
end side to a ‘‘depth’’ of 30 mm. One of the two samples washeated at approximately 200 C for 1 h and then air-cooled toroom temperature. Subsequently, the adhesives of the twosamples were broken in half (Fig. A.1(a)), and the boundaries
between the adhesives and the bolts were examined with amicroscope. Unlike the case of the non-heated sample(Fig. A.1(b)), a number of chips were observed in the heatedsample (Fig. A.1(c)), which can be interpreted as heatinduced degradation.

228

(a)

(b)

(c)

Fig. A.1 (a) Photograph of the bolt and adhesives. (b, c)
Micrographs of the boundaries between the adhesives
and bolts of the non-heated and heated samples,
respectively.