acoustic emission monitoring for concrete structures:...

Applications of Acoustic Emission for Reinforced Concrete:

Acoustic Emission Monitoring for Concrete

Structures: Qualitative vs. Quantitative Methods

Thomas Schumacher (Presenter), Assistant Professor and

Lassaad Mhamdi, PhD Student

Tuesday, October 23, 2012

Civil & Environmental Engineering

• WHAT ARE ACOUSTIC EMISSIONS (AE)?

• SOURCES FROM CONCRETE STRUCTURES

• MEASUREMENT PROCESS

• ANALYSIS METHODS

• STRENGTHS AND

LIMITATIONS • CONCLUSIONS

OVERVIEW

• 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 • Passive technique (vs. UT) • Source parameters unknown

• 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

Medium

Sensor

to DAQ

Source

External load

WHAT ARE AE?

• Primary sources

SOURCES FROM CONCRETE STRUCTURES

Rebar debonding (slip, pull-out)

Wire debonding/fracture Concrete crushing

Inclined cracks

Flexural cracks

Crack surface interaction

Rebar fracture

• Secondary sources

• Noise: impacts on road surface, electrical

MEASUREMENT PROCESS

( ) ( ) ( ) ( ) ( )= ∗ ∗ ∗M S DAQR t tf t tf t tf t S t

1

3

2

4 5

n sensors n signals

( )S t

( )R t

( )DAQtf t( )Stf t

( )Mtf t

Qualitative AE Analysis

Quantitative AE Analysis

OVERVIEW ANALYSIS METHODS

Event forming

Qualitative Quantitative

Estimation of source parameters (time,

location)

Estimation of source mechanisms Waveform analysis

Parameter extraction

Data analysis

Stored AE waveforms, R(t)

Sim

ple

forw

ard

proc

edur

es

Non

-line

ar in

vers

e pr

oced

ures

OVERVIEW ANALYSIS METHODS

Event forming



location)



Statistical analysis


• Kaiser Effect (Kaiser, 1950): Previous stress level needs to be exceeded in order for new AE to be produced

• Felicity Ratio (Fowler, 1986):

Source: ASTM E602 (1982)

QUALITATIVE METHODS

7 Source: Koeppel, 2002

• NDIS-2421 (Ohtsu et al., 2002):

• b-Value Analysis

(Gutenberg & Richter, 1952):

QUALITATIVE METHODS (CONT.)

8

Source: Ohtsu et al., 2002 • Historic-Severity Index (Fowler, 1989):

Source: Golaski et al., 2002

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 H

its) [

-]

Frequency distribution of hit amplitudesEstimated b-value (slope of this line)± one standard deviation of dataData mean value

Sour

ce:

Schu

mac

her,

2008

• Others: AE Rate Process Analysis (Ohtsu et al., 2004)

• Waveform analysis (Grosse, 1996):

QUALITATIVE METHODS (CONT.)

Sour

ce: G

ross

e, 19

96

Magnitude Squared Coherence

Signal 1

Signal 2

• Data analysis of many extracted parameters/waveforms • Do not try to explain source mechanisms • Can be performed with as few as 1 sensors • Often implemented in commercial AE systems

• Relative measure, only

comparable for same conditions • Depend on selected acquisition

and threshold criteria:

SUMMARY QUALITATIVE METHODS

Source: Schumacher, 2008

OVERVIEW METHODS

Event forming



location)



Statistical analysis


Motivation of using quantitative methods: • Earthquakes studied for a long time, a variety of analysis tools exist • Earthquakes = AE, different scales

QUANTITATIVE METHODS

Adapted from: earthquakesandplates.wordpress.com

Source


QUANTITATIVE METHODS (CONT.)


• Estimation of source locations (Geiger, 1910):


Moment Tensor Inversion (Aki & Richards, 1980): Seismic sources (and AE) can be represented by a model of equivalent forces.

• Radiation pattern inferred through measured surface displacements of p-wave:

• Simplification: Result: • Fracture type • Orientation of fracture planes • Moment magnitude

Source: Finck, 2005


=

Solve for using LSQ

Green’s functions

Source: Grosse & Ohtsu, 2008

• Example 1: explosive/implosive sources

• Example 2: pure vertical shear sources


Source: Grosse & Ohtsu, 2008


Absolute MTI

Methods

Relative MTI

Methods

Hybrid MTI

Methods

Calculation of Green`s functions for each single event

Limitations:

Accurate estimation of Green`s functions can be difficult Applications:

SiGMA-AE by M. Ohtsu

Calculation of Green’s functions not required for each event, but for cluster of events with common ray path

Limitations:

Dependence of the solution on the a-priori knowledge of the reference event

Applications:

Rel. MTI by Dahm & Grosse

Capitalize on the strengths of both the absolute and relative MTI methods Weighing scheme to

compensate for different types of systematic errors

More reliable and robust estimate of the moment tensor

Applications:

MTI Toolbox by L. Linzer

• MTI Methods developed for AE

• Visualization: Stereograph–focal mechanism diagram–Beach ball


Source: Wikipedia

Adapted from Suetsuga, 1994

N

E

N

E

D

x

y

z x

y

x

y

z x

y y

z

2D focal mechanism diagram

y

x

explosion implosion shear

• Others: Full waveform inversion (the ‘king discipline’)


Complete source-time function

Source: Grosse et al., 2001 Source: Katsaga et al., 2008

Possible in concrete?

• Try to relate observations with source mechanisms

• Require a network of sensors • Difficult to apply non-linear inverse procedures • Require reliable high-fidelity data with high signal-to-noise ratio • Challenging to apply in cracked media

SUMMARY QUANTITATIVE METHODS

Shigeishi et al., 2003

STRENGTHS AND LIMITATIONS


• Strengths

• Limitations

– Simple procedures – Few sensors needed – Work even after concrete

cracks

– What do they physically represent?

– Relative result – Conditions need to be the

same

– Physically meaningful parameters that relate to source mechanism

– Absolute result

– Complicated procedures – Many sensors required – Difficult to apply in large

structures and once severe cracking has occurred

• Real-time feedback on fracture processes • Applied during testing/normal operation – no disturbance

• Local method (vs. global) – Use in targeted areas of concern

• Potential applications for monitoring of concrete structures: – Fracture of rebars and prestressing wires – Crack propagation monitoring – Location of mechanical noise during operation – Fracture monitoring during experiments

• Use of high-fidelity sensors crucial!

• Needs: long-term experience, standards

CONCLUSIONS


Source: www.thestar.com

Civil & Environmental Engineering

acoustic emission monitoring for concrete structures:...

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