Transcript 101-1111-00L Fundamentals and Applications of Acoustic

Short Course 101-1111-00L:
Fundamentals and Applications of
Acoustic Emission
November 1, 2007
5-1 Concrete – Diagnosis
5-2 Concrete – Damage and Fracture
Mechanics
5-3 Superstructure – Building and Bridge
5-1 Concrete - Diagnosis
November 1, 2007 (1/3)
• Introduction
• Damage Assessment
• Corrosion Monitoring in
Reinforced Concrete
Damage Assessment
• In concrete engineering, two types of failure
modes in the reinforced concrete beam are
known.
• One is a bending failure, as bending cracks
are nucleated in the bending span and the
beam results in final failure.
• The other is a shear (diagonal-shear) failure,
as diagonal-shear cracks are suddenly
generated in the shear span at the final
moment.
Results AE observation in the two types of
failure mode
AE behaviors of the two modes in
Reinforced Concrete Beams
• In the case of a under-reinforced beam, sliding
between reinforcement and concrete was observed
due to yielding of the reinforcement.
• As a result, AE count rate increases exponentially.
• In the case that the reinforcement withstood in an
over-reinforced specimen, AE events are observed
at constant rate until the final failure.
• Right before the final stage, diagonal cracks were
suddenly observed without any precursors.
Observation fact
• The results demonstrate a potential for the
prediction of failure mode of reinforced concrete
beams by AE observation.
• It is confirmed that AE activities are sensitive to
local instability of the structure.
Recommended practice for in situ
monitoring of concrete structures
by AE [NDIS-2421 2000]
• In order to assess the damage levels of reinforced
concrete beams, one criterion to qualify the damage
levels is proposed on the basis of two ratios associated
with the Kaiser effect.
Damage Qualification of RC Beams
• (a) Ratio of load at the onset of AE activity to
previous load:
• Load ratio = load at the onset of AE activity in the
subsequent loading / the previous load.
• (b) Ratio of cumulative AE activity during the
unloading process to that of the last maximum
loading cycle:
• Calm ratio = the number of cumulative AE
activity during the unloading process/ total AE
activity during the last whole loading cycle.
Experiment
• Two reinforced concrete beams of 3.2 m
length were tested. AE sensor:150 kHz
resonance frequency
• Frequency range :10 kHz to 1 MHz
• Total amplification : 80 dB gain.
• Crack-mouth opening displacement (CMOD)
recorded
Bending Tests of RC Beams
Corrosion Monitoring in Reinforced
Concrete
Continuous AE Monitoring
AE observation under salt attack onshore
Mostly observed AE events were generated due to
rainfall. After three year exposure, in one beam, AE
activities following raindrops were observed.
Corrosion Process in Reinforced Concrete
• Continuous monitoring of AE events were conducted in
an accelerated corrosion test and a cyclic wet-dry test.
• An RC slab of dimensions 10 cm x 25 cm x 40 cm
• In a cyclic wet-dry test, the slab was soaked in the same
tank without electric charge for a week and then taken
out to get dry for a week. This cycle was repeated.
• AE sensors : 50 kHz resonance (RA5)
• Amplification : 40 dB gain in total
• Frequency range : from10 kHz to 1 MHz
Corrosion Monitoring by AE Activities
：Half-cell potential
：AE hits
-400
100
Corrosion standard, -350mV
80
-300
60
-200
40
-100
20
AE hits
Half-cell potential (mV)
-500
The second stage
0
0
The first stage
50
100
Time, day
0
150
Phenomenological model for corrosion loss
• At the 1st phase, corrosion
initiates in reinforcement.
• The rate of the corrosion loss
decreases at the 2nd phase
under aerobic conditions.
• At the 3rd phase of anaerobic
corrosion, expansion of rebar
due to corrosion products
nucleates concrete cracking.
Corrosion Monitoring by
AE Parameters
AE parameter analysis
RA = the rise time / the maximum
amplitude
Average Frequency (Fa)
= AE ringdown-count / the duration
time
Modified JCMS-III B5706
Crack
Classification due
to Corrosion
Phenomenological AE observation
1st stage: small other-type AE events
→ Onset of corrosion in rebar
2nd stage: fairly large tensile-type
AE events
→ Nucleation of corrosion-induced
cracking in concrete
(SEM)
Deterioration process due to salt attack,
prescribed in the codes
• The presence of two stages:
5-2 Concrete – Damage Mechanics and
Fracture Mechanics
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November 1, 2007 (2/3)
Introduction
Damage Evaluation
Application to Fracture Mechanics
Remarks
Introduction
• Relations between any AE parameters and other
physical parameters are quantitatively analyzed
and then applied to the diagnosis of concrete
structures.
• This is because the change of AE activity could
be related with the rate of the deterioration
process.
• Since AE techniques directly deal with the
occurrence of micro-cracks, applications to
damage mechanics and fracture mechanics are
straightforward.
Damage Evaluation
• The rate process analysis was introduced to
evaluate quantitatively the change of the
activity [Ohtsu & Watanabe 2001].
• When concrete contains a number of critical
micro-cracks, active AE occurrence is
expected under loading due to crack
propagation from existing defects or microcracks.
• In contrast, AE activity in sound concrete is
known to be stable and low up to final failure.
Damage evaluation of core-concrete sample in
compression test
Procedure of damage evaluation
• For damage evaluation, AE behavior of concrete
under compression could be analyzed, applying the
rate process theory.
• Based on Loland's model in damage mechanics, a
relation between AE rate and the damage
parameter is correlated.
• By quantifying intact modulus E* of elasticity in
concrete from the database, relative damages,
Eo/E* of concrete in existing structures are
successfully estimated.
Compression test of core-drilled samples
• In usual cases,
cylindrical samples of 10
cm in diameter and
20cm in height are
tested.
• AE sensor of wide-band
type (UT-1000,PAC):
resonance frequency:
approx. 1MHz)
• Frequency range : from
60kHz to 1MHz.
Rate Process Analysis
• AE occurrence from stress level V(%) to
V+dV (%) is represented as a function of
the incremental number of AE events, dN,
•
f(V) dV = dN/N,
• where N is the accumulated number of AE
events up to stress level V(%), which is
normalized by the compressive strength.
Approximation of probability fn. f(V)
• To discriminate AE activities at low
stress level whether high or low, the
function f(V) is approximated as the
following hyperbolic function,
f(V) = a/V +b
• where a and b are empirical constants.
Histograms on the value f(V) directly
estimated as dN/(N･dV)
at each increment dV in concrete
Damaged Core Samples
• Core samples were taken from an outlet
of cooling water in a nuclear power
plant
Results of the pore volumes over 0. 5 mm
radius, the rate a, and the strength
AE events versus stress
•
From two equations; f(V) dV = dN/N
and
f(V) = a/V +b,
a relationship between the number of
total AE events N and stress level V(%) is
obtained,
N = C V a exp (bV).
• Here, C is the integration constant.
A relationship between the number of total
AE events N and stress level V(%) :
N = C Va exp (bV)
Damage mechanics
• According to the continuum damage
mechanics, the state of damage is
represented by the scalar damage
parameter W from the modulus of
elasticity, E, of a damaged material is
expressed as,
E = E*(1 - W),
• where the modulus E* is that of an
intact material.
Introduction of Loland’s Model

  (E0  E A0 )
*
、
Damage evolution
Estimation of intact modulus: E*
Wc - Wo = (Eo - Ec)/E*.
• A linear correlation between loge(Eo－Ec) and
the rate ‘a ‘ value is proposed as,
loge(Eo－Ec) = Da + c.
• Then, it is assumed that Eo = E* when a = 0.
This allows us to estimate Young's modulus of
intact concrete E* from,
E* = Ec + exp(c).
Database
Relative damage is estimated as Eo/E*
Reinforced concrete pier and
anchor
Relative damages estimated in core samples.
Application to Fracture Mechanics
• The fracture process zone is
created ahead of a notch
(crack) in concrete, without
revealing the notch sensitivity.
• Nucleation of micro-cracks in
the fracture process zone is
clarified, which was ideally
introduced in order to explain
the tension-softening behavior.
Relation between the area of the zone and
the size of aggregate
• With the increase of the size of aggregate, the
fracture process zone grows broadly.
In the expansion test, which simulates
crack propagation due to corrosion of
reinforcing steel-bar, the moment tensor
analysis was performed.
Remarks
• AE techniques have been applied to concrete
engineering for more than four decades. A
variety of practical applications are achieved and
further going to be standardized.
• Based on these research activities, RILEM
technical committee : TC212-ACD ( Acoustic
Emission and Related NDE Techniques for Crack
Detection and Damage Evaluation in Concrete)
was established in 2004 to 2009.
• Recommendation practices are under preparation.
Thus, the world-wide standards are to be
established in concrete engineering.
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concrete
Introduction
Building
Concrete Bridge
Steel Bridge
Steel-Concrete Composite Slab
Remarks
Re-bar
Stud
Transverse stiffener
Lower steel plate
• An enormous number of infrastructures, in
particular, buildings and bridges have been
constructed. However, many of them are
currently known to have been aged and
deteriorated after long-year service.
• In concrete structures, damages due to
deteriorated or aged materials including
corrosion of reinforcement and poorworkmanship responsible for initial cracking are
often reported.
• Steel structures are deteriorated mostly by
fatigue, resulting fro increasing span length and
overloading of traffic vehicles.
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step 1: Inspection,
step 2: Evaluation of inspected results,
step 3: Prediction by deterioration model, and
step 4: Counter-measures for maintenance and
management.
• Thus, inspection procedures for the maintenance and
management of structures are of fundamental
importance prior to making a prediction model for
deterioration and deciding counter-measures for repair
and retrofit.
• Visual inspection is most extensively employed.
• Since defects, deterioration, and damage normally
grow inside structure, a safety assessment cannot be
based solely on the visual observation of cracks and
signs of damages in structural elements.
• The technique should enable to provide definitive and
quantitative evaluation in a short time.
• To inspect concrete and steel of superstructures,
applications of AE techniques are in progress.
• The AE method is expected to become a very
useful technique for evaluating the soundness
or for detecting damages of the superstructure.
• This is because the measurement can be
carried out without stopping traffic in a bridge
or without evacuation of residents in a
building.
• Some successful results on the applications to
buildings and bridges are stated.
• For AE monitoring of existing concrete
structures, it is essential to confirm that any AE
signals responsible for the deterioration are not
observed under service conditions.
• In the case that AE signals not of noises but due
to deterioration process are detected, the
monitoring and the analyses shall be conducted.
• The monitoring is performed continuously or
routinely, and sometimes temporarily after the
disasters.
• In this case, the selection and classification of
AE signals which is closely associated with the
deterioration are necessary.
• In a historical masonry building, the efficiency of AE
technique to monitor cracking is reported [Carpinteri
& Lacidogna 2006].
• Comparing crack traces and AE activity, it is shown
that AE measurement can be used to dynamically
measure structures in service.
• Since the structure was old and not in service, the
audible noises were readily observed due to the traffic
load.
• At five locations circled, continuous monitoring was
conducted for a week.
• Based on the code [JCMS-III B5706 2003],
the parameters are determined as the moving
average of more than 50 events.
• Locations of the observation are labeled by
the numbers from north to south and by the
capital letter denoted from the left to the right,
and then the floor number is appended.
• As an example, 1A1 denotes the north-the left
corner of the building at the fist floor.
Then internal defects
such as a cavity or a
honeycomb were found
at several locations,
which are indicated by
the letter “D”.
A reinforced concrete (RC) girder bridge with a
simple T-shape beam served for about 45 years.
• All girders were dismounted for replacement,
and AE generating behavior under a repetitive
bending load was measured, applying a large
6970 device. 6970
loading
13300
1080
1ch
2ch
3ch
P
4ch
5ch
6ch
270
2330 3@1650=4950 2330
Broadband AE
： Sensor
： Displacement Gauge
• Six broadband-type AE sensors (UT1000, PAC) were
attached on one side at the web of the girder. Output
signals from AE sensors were amplified by 40 dB
gain and the detection threshold was set to 38 dB
referring to the sensor output.
• Under a cyclic-incremental loading, the gradual
increase of total number of AE hits was observed.
• The Kaiser effect can be observed until the loading
cycle of 150 kN. Once the maximum cyclic load
exceeded 200 kN, AE activity was observed even
below the maximum previous load and increased in
unloading stages.
• The measurement was conducted in the bridge
on a general highway in an urban area.
• In the measurement, AE parameters were
recorded at the sensor output ratio of 42 dB as
the threshold by using an AE signal waveform
analyzer.
• 6-channel analog signal processing system was
employed with six broadband AE sensors and
40 dB preamplifiers.
Between piers P1 and P2 of the four-span bridge, AE sensors
were arranged at the transverse beam. The sensors were
arranged at interval of 1350 mm along the bridge axis at the
center between the floor slab supports. The line selected for
sensor arrangement corresponded to the line where the right-side
wheels of vehicles run.
AE energy detected at channel 1 is remarkably large,
probably because the sensor of channel 1 is close to the joint
of pier P1 where wheels of vehicles could convey impact
loads on the slab crossing over the joint.
300000
40
200000
20
100000
0
0
-20
1
•
2
3
4
Truck
5 (ch)
ENERGY 80
STRAIN
400000
60
300000
40
200000
20
100000
0
0
-20
1
2
3
4
Crane
5 (ch)
STRAIN ()
60
ENERGY (mV)
STRAIN 80
400000
STRAIN ()
ENERGY (mV)
ENERGY
Truck
Crane
• It is clarified that the running speed of vehicles should
be taken into consideration in the measurement.
• Otherwise, the damaged zone might not be correctly
identified, because the impact loading could generate
high AE activity at the joints.
• In other cases, vibration modes due to moving loads
may affect AE activities at local areas.
• In a large bridge of steel box-girders, cracks
were visually found.
• Consequently, the activities of cracks were
estimated by the AE method [Shigeishi 2004].
• Measurement, the bridge was in service,
carrying traffic.
A fatigue crack from a weld line with a lateral beam
through U-shaped rib stiffener.
Crack
• The b-value is remarkably small at AE sensor at
channel 6.
• It suggests an orientation of probable crack
propagation, because generation of large-scale
AE events is identified from the b-value observed
at channel 6.
ULIB3 b値
2.00
1.95
b値
b-value
1.90
1.85
1.80
1.75
1.70
1.65
1
2
3
4
CH番号
Channel
5
6
• AE measurement has been applied to the
superstructures to evaluate the deterioration or the
damage.
• In situ measurement under service or traffic loads is
desirable. The care for environmental noises is
essential.
• This is because the measuring conditions at the site
are varied one by one.
• It is quite important to conduct preliminary tests prior
to actual tests on equipment setting and measuring
conditions.
[Homework Nr. 5] reply to e-mail: [email protected]
5-1 After reading a paper “ACM2007-Ohtsu.pdf”,
summarize the corrosion process of reinforcement
in concrete based on AE activities and parameters.
5-2 Discuss AE applications to materials and
structures of your interest.
•
Optional (5-3) If possible, detect AE hits and
waveforms from a material on which you are
doing research.