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Rapid NDI of Concrete Structures using Impact Echo Technique
Widely used for detection of defects in concrete structures
Impact-Echo: Basics
fC
d p
2
The value of the peak frequency (f) in the amplitude spectrum can be used to determine the depth (d) of the reflecting interface
A point test method
Spring loaded steel ball used as impactor to generate stress wave in the range of 20-80 kHz
P-wave arrives at the test surface after echoing off from the reflector. The recorded signal exhibits periodic pattern
R-Wave Reflected P-Wave S-Wave
Stress Wave propagation in finite element model
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The presence of a flaw within a structural elementdisrupts the propagation and reflection path of theimpact generated stress waves.
(a) (b)
60mm thick concrete slab modelA planar void of dimension40mx40mmx2mmPlanar void at a depth 35mmSteel Ball dia ~ 5mm
Modelling of Stress Wave Propagation
Impactor
60mm35mm
40mm
200 mm
Receiver
20mm
Harisha, G.M, M. Tech Dissertation, IITB, 2011.
-3E-08
-2E-08
-1E-08
-7.6E-22
1E-08
0 0.00002 0.00004 0.00006 0.00008 0.0001 0.00012 0.00014
Disp
lace
men
t
Time (s)
Without Void
With Void @35mm depth
Typical Response and Frequency Spectra
Void frequency
44kHz
Thickness frequency
25kHz
Rayleigh Wave
Harisha, G.M, M. Tech Dissertation, IITB, 2011.
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Impact Echo Array (In collaboration with IGCAR)
Distributed Actuator/Receiver Network
Automated large area inspection
– can be programmed for automatic
data collection Diagnostic imaging tool (SAFT algorithm) for
defect reconstruction– multiple echoes from a single reflector
at different receivers
Advantages:
Improved defect resolution capability
Rapid inspection, faster turnaround time
Reduced maintenance cost
ReceiverImpactor
Reflector
P(x)
Reflector
ImpactorReceiver
Concrete Specimen
Receiver
1D Array
2D Array
Concrete Specimen
Banerjee, et al, 21st International Conference on Structural Mechanics in Reactor Technology (SMiRT 21), November 6-11, New Delhi, India, paper ID 597, 2011.
L1L2 R1 R2
P(x,z)
Damage Detection Algorithm
Image reconstruction of the object from the wavelet transformed data
fTC tRI(P(x,z))= W
I = Damage intensity at any point P
R = number of receivers in the array tf = time of flight of the wave corresponding to the total path distance (S to P and P to R)WTC = wavelet transform coefficient
x
z60mm
200 mm
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7
Damage Intensity Maps
20 40 60 80 100 120 140 160 180 200
10
20
30
40
50
60
X-Coordinate
Dep
th in
mm
0.5
1
1.5
2
2.5
50 100 150 200
10
20
30
40
50
60
X-Coordinate
Dept
h in
mm
0.5
1
1.5
2
2.5
3
Planar void of size 20mm at 35mm depth Circular void of size 20mm at 35mm depth
In case of planar void, back wall is also reconstructed with a high intensity
Banerjee, et al, 21st International Conference on Structural Mechanics in Reactor Technology (SMiRT 21), November 6-11, New Delhi, India, paper ID 597, 2011.
Damage Identification in Composite Structures UsingUltrasonic Guided Waves
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Guided Wave (Plate Wave/Lamb Wave):
Guided Wave Propagation in Solids
Multimodal and dispersive
Flexural
Extensional /compressional
Transmitter Receiver
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Multimodal and dispersive: the particle motion (symmetric or extensional and antisymmetric or flexural) and velocity of each mode depends on the thickness and material properties of the plate, as well as the frequency of the excitation of the wave.
May propagate a large distance without significant distortion or decay in their shapes
Extremely sensitive to the presence of discontinuities in their path, and carry information on certain properties of the flaws as they propagate away from the flaws.
Properties of Guided Waves
- Distortion of the shape of the waveform as the wave propagates away from source
2H
y
x
)()( tkxieyfu
General form of guided wave motion
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Dispersion Relation
Dispersion curve for an aluminum plate of 1 mm thickness.
0),,( 21 kkg 0),,( kg
sin;cos 21 kkkk
ck c = phase velocity
Dispersion Equation
21
2
222
2
1
2
42
)tan()tan(
kkk
HH
For isotropic plate of thickness 2H
21
2
222
2
2
1
42
)tan()tan(
kkk
HH
22
22
21
21 ; kkkk
22
11 ;
ck
ck
0),Re( 21
Antisymmetric
SymmetricdkdVgroup
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Dispersion curve for a unidirectional graphite/epoxy composite plate of 1 mm thickness along fiber direction (left). Dispersion curve is dependent on propagation direction (right)
Dispersion Relation: Composite Plate
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Transmitter (source) Receiver
PZT patch polarization direction
Guided Wave Propagation in a Thin Aluminum Plate: Experiment
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 0.00002 0.00004 0.00006 0.00008 0.0001 0.00012 0.00014
50mm
100mm
200mm
Time (sec)
Nor
mal
ized
Apl
itude
(V)
-1.50E+00
-1.00E+00
-5.00E-01
0.00E+00
5.00E-01
1.00E+00
1.50E+00
0.00E+00 5.00E-05 1.00E-04 1.50E-04
Source
Time (sec)
Input source: 5 cycle sine pulse in a Hanning window with central frequency 200kHz
Response at the PZT receivers at various distances from the source showing presence of both S0 and A0 modes. S0 and A0 modes get separated at large distances from the source
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-1.50E+00
-1.00E+00
-5.00E-01
0.00E+00
5.00E-01
1.00E+00
1.50E+00
0.00E+00 5.00E-05 1.00E-04 1.50E-04
Experiment Exact Theory LS DYNA
Time (sec)
S0 A0 Reflection in LSDYNA simulation
Modeling of PZT Driven Surface Motion
Input Output100 mm
Input Signal: In-plane point forceOutput Signal: In-plane surface displacement
PZT
Plate
offset
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Wave Propagation in Sandwich Structures
Honeycomb Composite Sandwich structures inwhich thin metallic / composite skins are bondedwith adhesives to the faces of extremely light weightand relatively thick metallic /compositehoneycombs.
Extensively used in modern day applications. Advantages:
High strength to weight ratio. High energy absorption capacity. Increased payload capacity. Increased service life.
Disadvantages: Bond between Honeycomb and skin degrades
with age. Continuous and varied intensity loads lead to
separation of the skin from thehoneycomb, thereby compromising the safetyof the structure.
Typical Honeycomb Sandwich Structure
Defects in Honeycomb Sandwich Structure
Piezo: C3D8E (An 8-node linear
piezoelectric brick); Electrode: C3D8R (An 8-node linear
brick, reduced integration, hourglas
s control)
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Effect of disbond in Honeycomb Composite Sandwich Structure
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Detection of disbond in HCSS:
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Detection of disbond:
DI map correspond to 1-2, 3-4, 5-6 & 7-8 actuator/sensor path
DI map correspond to 1-2, 3-4, 5-6, 7-8, 1-8 & 2-7 actuator/sensor path
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