acoustic emission monitoring - university of delaware
TRANSCRIPT
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The World Federation of NDE Centers
Acoustic Emission monitoring
Thomas SchumacherUniversity of Delaware
E-mail: [email protected]
Short course on NDE for the infrastructureBurlington Vermont, July 16th and 17th, 2011
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� Overview of fundamental basis
� Overview of technology
� Review of latest developments
� Strengths of method
� Limitations of method
� NDE application: case studies
� Summary and conclusions
Overview of presentation
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� Acoustic Emission (AE) is the term used for transient elastic waves generated by the release of energy within a material or by a process (EN, 2000).
� Irreversible process� Source time, location, and mechanism
unknown� Passive technique� Sensing via surface-mounted piezo-
electric transducers� Similarity to earthquakes, i.e. nano-
seismic activity� Frequency range of AE in concrete:
~10 to 500 kHz
Overview of fundamental basis
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Medium
Sensor
to DAQSource
External load
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� Primary sources� Micro-cracking (distributed)� Macro-cracking (localized)� Compression failure (crushing)� Yielding and fracture� De-bonding between materials
� Secondary sources� Sliding/friction between interfaces
� Artificial sources� Calibration sources (pencil lead break, ball drop, pulse)
� Noise� From bearings, supports� Background: ambient traffic, vibrations� From electrical circuit, cell phones
Overview of fundamental basis (cont.)
4
Schumacher, 2008
Angerinos et al., 1999
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� Elastic waves in finite media (non-dispersive)
� Reflected/diffracted waves� Guided waves in plate-like members (dispersive)
� Plate waves� Lamb waves
� Wave attenuation� Geometrical� Scattering� Internal friction
Overview of fundamental basis (cont.)
5
Compression wave (fastest) Shear wave Surface wave (slowest)
Frequency, f [kHz]
Nor
mal
ized
am
plitu
de [-
]
0 100 200 300 400 5000.0
0.2
0.4
0.6
0.8
1.076 mm (3 in.)
Frequency, f [kHz]0 100 200 300 400 500
152 mm (6 in.)
Frequency, f [kHz]0 100 200 300 400 500
305 mm (12 in.)
Frequency, f [kHz]0 100 200 300 400 500
1143 mm (45 in.)
Increasing travel distance
Adapted from Wood: http://www.geo.mtu.edu/
Schumacher, 2008
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� Measurement process
Overview of technology
6
Source: Ch. Grosse, TUM
b-Value
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� Model of the measurement process
Source signal, S(t)�
Stress wave�
Propagation�
Surface motion � voltage�
Amplification�
Filtering Response function�
Digitization/storage on PC�
Response signal, R(t)
Overview of technology (cont.)
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Pre-amplifier, tfR(ω)
Source, S(ω)
Stress wave front, p-wave
Sensor , tfS(ω)
Data acquisition system , tfR(ω)
Medium, tfG(ω)
( ) ( ) ( ) ( ) ( )G S RR S tf tf tfω ω ω ω ω= ⋅ ⋅ ⋅
( ) ( ) ( ) ( ) ( )G S RR t S t tf t tf t tf t= ∗ ∗ ∗⇕
Adapted from Schumacher, 2008
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� Sensors� Piezo-electric (PZT) devices � Voltage output proportional to surface motion� Resonant vs. broadband� Coupling
� Pre-amplifiers� Amplify small sensor output
� Transient recorder� 14 to 18-bit dynamic range typical� Recording rates ≤ 40 MHz (practical ≤ 10 MHz)� Analog filters� Parameter extraction� Full waveform storage� Independent recording using trigger criteria
Overview of technology (cont.)
8Source: Vallen Systeme GmbH
Schumacher, 2008
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1)Fowler et al., 1989, 2)Ohtsu et al., 2002, 3)Gutenberg & Richter, 1949,4)Grosse, 1996, 5)Geiger, 1910, 6)Aki & Richards, 1980
� Overview methods of analysis
Overview of technology (cont.)
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Stored AE signals, R(t)
AE event forming
Qualitative Quantitative
Source parameters5):- Location
- Time
AE parameters- Hit rates/energy/…
Waveform analysis:- Comparisons4)
Moment Tensor Inversion6)
- Historic-severity1)
- Load-Calm ratio2)
- b-Value analysis3)
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� Qualitative� Statistical analysis of AE parameters� Does not relate observations with physical parameters (source mechanisms)� Can be performed with as few as 1 sensor� Readily available and implemented in commercial AE systems� Relative measure, only comparable if exact same conditions� Depend on selected acquisition and threshold criteria
� Developed methods� Load-Calm ratio (Ohtsu, 2002)� Historic-Severity index (Fowler, 1989)� b-Value analysis (Gutenberg &
Richter, 1952)
Overview of technology (cont.)
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Source: ASTM E602 (1982)
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� Qualitative (cont.)� Kaiser Effect (Kaiser, 1950): In most metals, AE are not observed
during the reloading of a material until the stress exceeds its previous high value.
� Felicity Ratio (Fowler, 1986): Break down of Kaiser Effect due to material instability where AE start to occur before its previous high value is reached.
Overview of technology (cont.)
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Koeppel, 2002
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Overview of technology (cont.)
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� Qualitative (cont.)� NDIS-2421 (Ohtsu, 2002)
� Historic-Severity Index (Fowler, 1989)
� Problem: selection of triggerinfluences results!
Golaski et al., 2002
Ohtsu, 2002
Schumacher, 2008
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Overview of technology (cont.)
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� Qualitative (cont.)� b-Value analysis (Gutenberg & Richter, 1949)
� Waveform correlation (Grosse, 1996)
2 2.5 3 3.5 4 4.5
0
0.5
1
1.5
2
AE Magnitude [AdB/20]
log(
Cum
ulat
ive
AE
Hits
) [-
]
Frequency distribution of hit amplitudes
Estimated b-value (slope of this line)± one standard deviation of data
Data mean value
50 hits
Amax
Grosse, 1996
Magnitude-squared coherence
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� Quantitative� Relates observations with physical parameters (source mechanisms)� Requires a network of sensors (≥ 6 for moment tensor inversion)� Requires data with high signal-to-noise ratio� Difficult to apply (complicated procedures, still in research stage)
� Source Locations� Arrival time difference method (Geiger, 1910)� ≥ 4 sensors� Accuracy from outside sources low
Overview of technology (cont.)
14
81
23
4
5
6
7
p-wave front
1st hit sensor
AE source
600 650 700 750 800 850 900 950 1000-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
Sample # [-]
Sig
nal a
mpl
itude
[m
V]
/ A
IC f
unct
ion
valu
e [-
]
Original Signal
Filtered Signal
AIC Function (on Filtered SignalFloating Threshold Picker
AIC Picker
Schumacher, 2008
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� Moment Tensor Inversion (MTI) (Aki & Richards, 1980)� Source mechanism� Requires ≥ 6 sensors� Pre-requisite: accurate
locations (to computeGreen’s functions)
� Radiation pattern inferredthrough surface observations
� Problematic for crackedspecimens (high non-homogeneity)
� Knowledge of responsecharacteristics of systemcomponents required
� Need to use high-fidelity sensors
Overview of technology (cont.)
15
Grosse et al., 2003
Grosse et al., 2003Sansalone, 1997
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� Moment Tensor Inversion (MTI) (cont.)
Overview of technology (cont.)
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Grosse et al., 2001
Shigeishi et al., 2003
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� Development of high-fidelity sensors (e.g. Glaser-NIST)
� Sensitive, extreme broad-band, absolutely calibrated
� Wireless sensor networks� Array techniques� High accuracy outside sources
Review of latest developments
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Grosse et al., 2004McLaskey et al., 2007
Source: KRN Services
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� More robust hybrid Moment Tensor Inversion (Linzer, 2001)
� Combines absolute and relative MTI(relative: No need to compute Green’s functions)
Review of latest developments (cont.)
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Linzer, 2001
Linzer, 2001
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� Probability based source location algorithms (Schumacher, 2010)
� Use of seismology based methods for quantitative analyses� New location methods, MTI, moment magnitude, tomography
Review of latest developments (cont.)
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Schumacher, in review
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Strengths of method
Advantages:� Applied during testing/loading� No disturbance during application� Real-time feedback� Detection AND characterization of
internal fracture processes as theyoccur
� Covers volume (distributed sensing)
Useful for:� Monitor progression of existing damage (e.g. crack propagation)� Real-time detection of occurring overloads (alarm system)� Continuous (long-term) monitoring of critical components� Verification of retrofits and repairs (before/after)� Complimentary for in-service load testing (Acoustic Emission Testing)
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Katsaga et al., 2007
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Limitations of method
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� Very few standards available for infrastructure (NDIS-2421, RILEM)� Large variability in structures (type, geometry, material properties)� Complexity of structures and components � Changing boundary conditions (e.g. cracking or sensor coupling)� Tests not truly reproducible due to nature of AE
� Cannot tell current state such as existing cracks, only change in state
� Background noise can be significant = low signal-to-noise ratio� High variability of signal strengths
� Quantitative analyses often difficult to apply in real-world situations� No long-term monitoring experience with this method
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NDE application 1: pressure vessels
� Well established, confidence high� Large pool of samples – baseline data available� Well-defined problem (geometry, material properties)� Loading protocol established� Loading known –
applied pressurecan be easilycontrolled
� Analysis method:historic-severityindex (Fowler, 1989)
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Catty, 2010
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NDE application 2: laboratory RC beam
� Large-scale experiment on RC beam using quantitative analyses(Katsaga et al. 2008)
� Source parameters� Moment Tensor analysis� Insight into development
of fracture during loading� More shear type sources
in the later loading stages
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Katsaga et al., 2008
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NDE application 3: wire breaks on bridge
� Continuous monitoring of post-tensioned bridge (Fricker & Vogel, 2006)
� Monitoring for steel wire breaks� Verified by induced breaks
(after bridge decommissioned)
� Ideal application: sources ofinterest1) high energy comparedto other sources2) and noise
� Example of alarm system
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1) 2)
Fricker & Vogel, 2006
Fricker & Vogel, 2006
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� RC deck girder bridge in Cottage Grove, OR (Schumacher, in preparation)
NDE application 4: in-service load test
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Crack displ.
AE sensors
Strain gage
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� RC deck girder bridge in Cottage Grove, OR (cont.)
NDE application 4: cont.
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Qualitative Quantitative
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NDE application 5: retrofit of steel bridge� Noisy bearing of swing bridge in Reedsport, Oregon
� AE activity during operation before/after replacement of bearing
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Summary and conclusions� Passive method for monitoring of fracture processes� Applied during testing/normal operation – real-time feedback
� Useful for monitoring and as alarm system:� Prestressed concrete beams� Crack progression monitoring� Location of mechanical noise
during operation� Fracture monitoring during
experiments
� Promote use of principlesfrom seismology forquantitative AE
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Source: ITI, Northwestern University (website)