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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

4

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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9

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

7

13

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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