E-Journal of Advanced Maintenance Vol. 11 No. 1 (2019) 27-33
Japan Society of Maintenology

Social infrastructural diagnosis by hammering inspection with AE sensor
Takashi MATSUNAGA1, Ryota OGAWA1, Mitsuyuki SAGISAKA1, and Hiroaki
FUJIYOSHI1, Yoshihiro ISOBE1*
1

Nuclear Fuel Industries, Ltd., 950 Asashiro-Nishi, 1-Chome, Kumatori-Cho, Sennan-Gun, Osaka
590-0481, Japan

ABSTRACTDigital hammering inspection system with AE (Acoustic Emission) sensor has been developed and the effectsof state, stiffness, etc. on the natural frequency of metal and concrete objects to be inspected wereevaluated in terms of improper installations and age-related degradations. The system has been widely
applied to social infrastructural diagnosis including expressways (West Nippon Expressway Company Limited,NEXCO-WEST), nuclear power plants (ex. Chubu Electric Power Co., Inc.), reprocessing plant of spent nuclear
fuel (Japan Nuclear Fuel Limited) and a successful on-site trial of a new technology of i-Construction led byMLIT (the Ministry of Land, Infrastructure, Transport and Tourism).
KEYWORDS

ARTICLE INFORMATION

social infrastructure, hammering inspection, AE sensor, expressway,nuclear power plant, i-Construction

Article history:
Received 24 Octorber 2018
Accepted 15 May 20191.

Introduction

On existing expressways, sections whose service period has exceeded 30 years are increasing.
Therefore, the safety of road structures (such as roads, tunnels, bridges, and their auxiliary structures)is attracting attention, and the importance of securing their integrity through periodic inspections is
gaining renewed interest. Nuclear industries face the similar situation where the service period ofsome nuclear power plants is over 30 years and even 40 years. In such a context, we have developed
the digital hammering inspection system with AE sensor for social infrastructural diagnosis. Although
the conventional hammering inspection is widely used in various industries, inspection results dependon the skill of inspector. On the other hand, the developed system is objective and recordable. In
addition the system is portable and easy to handle.

2.

Development of Hammering Inspection with AE sensor

2.1. Outline of hammering inspection with AE sensor
In the digital hammering inspection with AE sensor (Fig. 1), an object to be inspected is struckwith a hammer to induce vibration. We extracted a natural vibration peak from the frequency
distribution of an acquired waveform and used its frequency as an evaluation index.
The theory of natural vibration of a simple harmonic motion of spring is shown by eq.1f 12km(1)where f is natural frequency, k is spring constant and m is mass.

*E-mail: isobe@nfi.co.jp

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

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Y. Isobe, et al./Social infrastructural diagnosis by hammering inspection with AE sensor

Due to eq.1, the effects of state, stiffness, etc. on natural frequency can be summarized in Table 1.
An example of the diagnosis of chemical anchors based on these effects is shown in Fig. 2 [1]. When
there exist improper installations and age-related degradations, the peak evaluation frequency declines
from the sound reference level.
Table 1. Effects on natural frequency.
State(mass, configuration)
Natural frequency
Stiffness,
restraint from surroundings
Natural frequency

large > small
low > highlow > highlow > highFig. 1. Hammering inspection system with AE sensor
and an example of signal analysis

Fig. 2. Relationship between various construction defects / aged deterioration and natural frequency

2.2. Examples of on-site inspections
An example of on-site inspection results of mechanical anchors of JF (Jet Fan) in an expressway
tunnel is shown in Fig. 3 [2]. A tentative diagnostic criterion of 2200 Hz was adopted based on theresults of average peak evaluation frequency of Fig. 3, and five JF anchors that were below thecriterion were selected due to the time limitation under traffic regulation to check their tightening
torque (bolts A to E shown left to right by red circles in the figure). All the five anchors indicated adecrease in torque from the design tightening torque (240 N•m). Since the jet fan is secured by a totalof 16 JF anchors (four anchors in each of the four attachments), the integrity of the jet fan is not likely
to be impaired immediately. Then, the vibration was measured again after retightening the bolts up tothe design tightening torque of 240 N•m, and their evaluation peak frequency were as shown in Fig. 4.
In the on-site JF anchor testing, the evaluation peak frequency shifted to the high-frequency side dueto the constraint increase accompanied by retightening.

Fig. 3. On-site JF anchor inspection in expressway tunnels andthe summary of the inspection in terms of evaluation peak frequency

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Fig. 4. Changes in evaluation peakfrequency with retightening

Y. Isobe, et al./Social infrastructural diagnosis by hammering inspection with AE sensor

Another example of on-site inspection is the inspection of foundation bolts (Fig.5). Fig. 6 shows
the result of sorting the evaluation peak frequency of 36 sound foundation bolts by their surplus
length, namely, the extra-length above the nut indicating that thepeak frequency tends to shift toward the low frequency side as thesurplus length increases. This is because the natural frequency
decreases as the vibration length increases. Therefore, bycorrecting the effect of surplus length, it is possible to capture thechange due to the deterioration of foundation bolts. In this time,from the results of these 36 bolts, the primary regression line wascalculated and correction was carried out using the slope -37.9 Hz/ mm.
Figure 7 shows inspection results of foundation bolts ofexpressway traffic direction poles considering a correction of 37.9
Hz every +1 mm with bolt surplus length 30 mm as a reference.
The evaluation peak frequency corrected for the measured
fundamental bolts was about 3318 Hz in average and about 282
Hz in standard deviation. Also, two foundation bolts thatexceeded the standard deviation significantly and became low Fig. 5. Foundation bolt inspection
frequency side values were confirmed which might be due to low torque, corrosion of the bolts andthe degradation of concrete foundation [1,4,5]. The inspection time is approximately 90 seconds perbolt without any surface preparation, which is an important factor as an actual field screening
inspection.

Fig. 6. Evaluation peak frequency as a function ofsurplus length of sound M36 foundation bolts

Fig. 7. Inspection results of foundation bolts ofexpressway traffic direction poles

The ground anchor has been used as one of stabilizing technologies for cut slopes as shown in
Fig. 8. The measurement of residual tensile force of the ground anchor has been conventionallycarried out by lift-off testing, however the development of a simple alternative tension measuringmethod is required in terms of cost and time [3].
The bending vibration of beam is show by eq.2,

1 2
EIf  2
2 L
A(2)

where f is frequency, A is cross section of beam, I is second
moment of area, E is longitudinal elastic modulus, λ is constantof constraint condition of beam, ρ is mass density and L is thelength of beam [6]. Since eq.2 has no stress variable, it isnecessary to take the tension force of the anchor into account incase of the bending vibration of ground anchor. Thus, weselected a multiple regression analysis which includes the tensileforce.
Fig. 8. Ground anchor inspection29

Y. Isobe, et al./Social infrastructural diagnosis by hammering inspection with AE sensor

As shown in eq.3, the tensile force estimation formula was examined by the multiple regression
analysis with evaluation peak frequency and head length as variables (S: estimated tensile force (kN),f; evaluation peak frequency (Hz), ℓ; head length (m), α ꞏ β ꞏ γ; coefficient).
𝑆=α∙𝑓+β/ℓ2+γ(3)

The results of the analysis are summarized in Table 2, indicating that the multiple correlation
coefficients of Type II and Type IV were considerably strong. However, the multiple correlation
coefficient was R = 0.39 to 0.43 for Type I due to the variation in the evaluation peak frequency andType III due to the small range in the tensile force, respectively. Figure 9 summarizes the relationshipbetween the actual tension measured by the lift-off testing and the estimated tension by equation (2)and Table 2 for total 144 ground anchor installed along the expressways. It is shown that the ratio of
the estimated tension to the actual tension is in the range of 89% to 120%, and for the anchor of 89%
out of 144 anchors, it is possible to estimate the tension with an accuracy of ± 5%.
Table 2 Multiple regression analysis for the ground anchor inspection results
Product No. ofSE CorporationAnchor head
diameter (mm)

TypeI

F30TA

400.390.045-0.446201.3

TypeIIF60UA

480.870.235-5.413

296.3

Anchor typeMultiple correlation
coefficient : R

coefficient : αcoefficient : βcoefficient : γ

TypeIII

F70UA

480.430.029-0.196397.0

TypeIVF40UA

400.720.033-0.223

74.3

Finger joints (Fig. 10) suffer fatigue cracking
during their service period and AE hammering
inspection has been applied to detect fatigue cracking asa fast screening inspection, since UT needs relatively
long inspection time to detect fatigue cracking [7].
Figure 10 shows an FEM analytical model andanalytical results of the finger portion of expansion
device of 540mm in length. There are four kinds ofmodels with a crack of 1 mm in width in the center partand 0, 25, 50, 75 % in cross section deficient rate andeigenvalue analysis was performed in a cantilever statein which the material was SS 400 with complete Fig. 9. Relation between estimated andconstraint condition at the finger root.
actual tensile force
The natural frequency of the frequency domain ofinterest was 1521 Hz in the sound state, and it tended to decrease with the increase of the cross section
deficient rate. Figure 11 shows the inspection results of finger joints Type 1 (540x100x40 mm) andType 2 (750x100x40 mm) with displaying the lines of the average value ± standard deviation. FromFig. 11, the obtained evaluation peak frequency was about 1350 Hz for Type 1 and about 1900 Hz forType 2. In addition, the standard deviation for each type falls below 100 Hz. Therefore, it can beapplied to the fast screening inspection of abnormality based on the change in the evaluation peakfrequency obtained by FEM analysis (about 500 Hz at the cross section deficient rate of 50%).
Regarding finger joints the value of which were significantly out of the average value, there could bea chance of fatigue cracks, corrosion and structural deficiency in ribs, webs, end portions of the faceplate, etc.

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Y. Isobe, et al./Social infrastructural diagnosis by hammering inspection with AE sensor

Type 2Type 1

Fig. 10 FEM analysis of the effect of cracking infinger joints on the natural frequency for Type 1Fig. 11 Field inspection results of finger joints

Guardrail supports suffer corrosion at theground level after long service period especiallywhen snow melting agents are used. Mockup tests
were carried out to apply AE hammering inspection
to detect the corrosion as shown in Fig. 12 [8]. It
can be seen that the evaluation peak frequency
tends to shift to the lower frequency side as thedegree of simulated degradations by machining
increases (Fig. 13). From this result, the inspection
showed possibility to detect deterioration such as
thinning and crack of the guardrail support. An
example of field AE hammering inspection isshown in Fig. 14.
Field inspection results of guardrail supports
are shown in Fig. 15, indicating significant
decreases in the evaluation peak frequency for Fig. 12 Mockup test conditions for simulated
supports 1 to 3 where significant corrosion occurred defects by machining the guardrail support
compared with others. Therefore, AE hammering
inspection is useful as a screening inspection of guardrail supports associated with corrosion.

Fig. 14 Changes in the evaluation peak frequency
as a function of defect levels

Fig. 13 Frequency distributions of the mockup test
conditions

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Y. Isobe, et al./Social infrastructural diagnosis by hammering inspection with AE sensor

Fig. 15 Examples of field inspection of guardrail supports

Deterioration of concrete and corrosion of reinforcing bars trigger degradation of reinforced
concrete structures. They occur in relation to each other. Typical degradation is caused by salt damage,ASR (Alkali Silica Reaction), etc. As a result, concrete might crack and or crumble off. The
degradation weakens the concrete structures and may result in collapse. The digital AE hammering
inspection system has been deployed to concrete inspections.
Figure 16 shows the grid of inspection positions and the color contour of evaluation peakfrequency of the concrete slab of an expressway bridge. In this figure, two areas indicated by the redcircles show the area which was judged as "peeled" by the conventional hammering inspection
conducted beforehand. Since the peak frequency in each area is 2 to 3 kHz, it can be seen that it islower than the peak frequency in the region [9].
As part of i-Construction new technology field trial led by MLIT, the effectiveness of AE
hammering inspection on the quality evaluation of the concrete surface of concrete structures at theconstruction stage was examined at the Ohune Ohashi Bridge in Hakodate City. Figure 17 shows thesnapshot of inspection (A1), site appearance (A2), inspection results visualized by evaluation peakfrequency color contour (A3). The concrete placement conditions are "regular" in the area of Part IIand "carefully placed" in the areas of Part III and IV. The inspection was carried out at equal intervalsof 50 cm lattice. The state of concrete surface can be seen almost uniform by the contour. On the otherhand, the average value of the evaluation peak frequency in the "carefully placed" Part III and IV areseveral tens to 100 Hz higher than the regularly placed Part II. This can be explained by the higher
stiffness of Part III and IV due to the high denseness of careful placement of concrete [10].
(A1)

(A3)

(A2)

Fig. 16 Examples of field inspection of concrete
bridge slab

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Fig. 17 Example of quality evaluation for concrete
construction

Y. Isobe, et al./Social infrastructural diagnosis by hammering inspection with AE sensor

3.

Discussions

Digital hammering inspection system with AE sensor has been developed and the effects of state,
stiffness, etc. on the natural frequency of various metal and concrete objects were evaluated in termsof improper installations and age-related degradations. Because the system is portable and easy tohandle, the results were successfully obtained by field inspections as well as laboratory experiments
as shown in the previous section.
The digital hammering inspection like the conventional hammering inspection will be deployed
as a screening inspection in social infrastructures especially when the number of the objects is so hugeand most of the objects are in a sound condition. Also, an approach is more practical to conductdetailed inspections using, for example, ultrasonic and magnetic techniques in case some abnormalindications are identified through the screening inspection,
Although the conventional hammering inspection can be replaced and sophisticated by the digital
hammering inspection, detailed diagnostic criteria of various metal and concrete objects should beestablished in accordance with safety and operational rules of industries. For that purpose, database of
the detectability for various improper installations and age-related degradations are necessary in thefuture.

4.

Conclusion

Social infrastructural diagnosis using digital hammering inspection system with AE sensor has
been developed as a fast screening inspection and applied to various fields including expressways,
nuclear power plants, reprocessing plant of spent nuclear fuels, construction fields, etc.

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