Synthetic aperture focussing Technique Presentation for testing concrete block

DETECTION OF SUBSURFACE DISCONTINUITY BY SYNTHETIC APERTURE FOCUSING TECHNIQUE

DUAL DEGREE THESIS – STAGE IUnder the guidance of Dr. Sauvik

Banerjee

Anishu Rahman06D04021

Importance of Non-Destructive Testing

NDT methods for concrete structures provide the information about fundamental parameters like density,

elastic modulus, strength, surface hardness and surface absorption

The location, size and position of reinforcement from the surface

The location, size and shape of the sub-surface defects The input data for fracture mechanics calculation of defect s

criticality Information about the quality of construction material and

workmanship

Different Methods of NDT Testing

Searching TechniquesExamine the complete volume of the component under investigationHigh inspection speed with high reliability in finding indications

above some registration levelAnalyzing Techniques

Used to check whether the indications found by searching techniques are really some defect or not

No need to have high inspection speed since used to inspect those areas where some anomalies had been found by the searching techniques

Must be able to position and size of the defect and distinguish between defect and form-echoes

Problems arising in testing concrete by Ultrasonic methods

Concrete is a non-homogenous , anisotropic and inconsistent material Concrete is a mixture of cement, sand, aggregates, reinforcements and

chemical admixtures All components have different elastic properties which causes sound

wave to travel at different velocities Concrete having not the same composition everywhere causes

ultrasonic waves to travel at different speeds Aggregates present in the concrete having comparable size of the

wavelength of the ultrasonic input wave used causes scattering and attenuation of signal pulses

Water pores and air voids (having different acoustical impedances) present in the concrete also cause ultrasonic pulses to reflect back

If high wavelength pulse are sent to check the defects, then the penetration capacity of wave gets reduced and if low wavelength pulses are sent then they fiercely interact with aggregates to cause signal noise/structural noise

Various Ultrasonic Testing Methods1. Pulse Transmission Method-

Source and receiver are put in opposite direction velocity and attenuation of the pulse are used for empirical

correlation with strength and other characteristic parameters of the concrete

Lacks sensitivity

2. Impact Echo Method- Only one sided access is required Transient stress pulse is introduced by mechanical impact (Ball

impact) Input pulse is having half sine pulse shape Waves are reflected by internal cracks/voids and also by external

boundaries

Impact -Echo Method-Continued The reflected waves are again

reflected back inside the system causing transient resonance conditions

Displacement is created at surface when these waves return to surface which is converted into voltage time signal

FFT is done to get the frequency spectrum of the signal

Dominant frequencies (f)are then used to calculate the distance of the reflecting surface (D) by formula

D= V/2fV being the velocity of the P- wave

in the concrete.

Range and Limitations of Impact-Echo testing

oMinimum depth of detectable target = λ/2oDuration of impact determines the frequency

content of the input stress pulseShort impact time cause high frequency

pulse which can detect smaller defects but up to shallow depths

Large impact time produces low frequency pulse that can penetrate larger depths but has little sensitivity

SAFT(Synthetic Aperture Focusing Technique)

SAFT is a signal processing (post) tool that improves the accuracy of signal thus leading to better detecting capabilities

Utilizes the output response i.e. Z- Displacement vs. Time characteristics of Impulse-Echo measurement

A synthetic aperture imitates a large transducer by sampling its area at many points

The SAFT algorithm focuses the received signals to any point of the reconstructed image by coherent superposition

Line-SAFT produces B-scan images(2D) and surface scan gives 3D imaging (D scan)

SAFT Reconstruction Algorithm Signals received by pulse echo measurements at various receivers

are post processed To determine the image location I(r’) of the reflector point, first the

distance between the source and the reflector point and then the distance between reflector point and receiver is calculated

The sum of the distances is then divided by the velocity of P –wave to get the time taken to traverse from source to the image point and again from image point to the receivers

Half the impulse time is added to this time Z- Displacement associated with this time is drawn from the pulse

echo measurement of same receiver point Same is done for all receiver points and Z- displacements are

summed up

SAFT Reconstruction Algorithm-Continued

This displacement is then divided by no of receivers (n) in the aperture to get the location of the image point

Therefore if the point r’ is associated with a reflector or defect, a coherent summation will occur resulting in a large value of I; if this point is not associated with such a reflector no coherent summation will occur, resulting in a small value for I

Lateral resolution Achieved = Half the ultrasonic wavelength Axial resolution depends on pulse length transmitted, in most

cases about 5 periods of the ultrasonic frequency used

Modeling of Defected concrete block in LS-DYNA

Specimen taken is concrete block of M20 grade with size as 300mm*300mm*50mm (x, y, z)

Various parameters is set for modeling like 1 mm messing is taken in all the three directions Model is supposed to be simply supported with rotation about z axis

constrained Assumption of Hooke’s law of elasticity. Material properties is defined as Mass density (2400 kg/m3), Young's

modulus of elasticity (2.236e10 pa) and Poisson’s ratio (.2) Based on this velocity of the P- wave is found out to be 3200 m/s For the type of integration process, a fully integrated quadratic 8 node

element (solid element) with nodal rotation is taken Impulse signal (Input) is taken to be half sine pulse with impact time of

10 µs

Continued The position of the node where the impact pulse is to be

applied is defined, here it is exactly at the middle of the top surface of the concrete block

Impulse load previously defined is applied at this point The termination time of the calculation is provided to be 200

µs Node number of all the receiver points are entered in the

model Here we have taken 10 receivers on the left of the source node,

having gap of 2 mm extending up to 20 mm from source The model is then solved in LS-DYNA to get the Z- displacement

vs. time for all the receiver points This constitutes the output set of Impulse-Echo measurement

Cross- section of the concrete block with defect situated at 35 mm from the top surface and messing of 1 mm

Response at the receivers

The time vs. Z- displacement characteristics of the receiver node located 20 mm (Series1) and 2 mm (Series2) from the impact source point of concrete slab (with defect at 35 mm depth)

concrete block model with defect size of 40 mm

Reconstruction Algorithm Result The node points in the grid (21*50) are given integer values ranging

from 1 to 1050 These values are assigned along rows i.e. the leftmost and topmost

node point is assigned node number 1 and the one just right of it number 2.

MATLAB algorithm gives the plot of the node point vs. superimposed Z- displacement values

The node numbers for which the Z- displacement is maximum is from 480 to 710 (from the plot)

average of the two values gives the average node number of the defect checking the depth of node number numbered 595 we get the depth

of the defect as 28.33 mm (node number/21 gives the z- coordinate of the node)

Comparing with the actual defect position which is at 35 mm from the top surface the result is satisfactory

Continued..

Plot of reconstructed Z- displacement vs. the node number

Result

The depth of the defect found out by MATLAB algorithm came out to be 28.33 mm as against to actual 35 mm

Error of 19% is recorded in the analysis

References

[1] Mary Sansalone. Impact Echo: The complete story. ACI Structural journal: Title no.94-S71, November-December 1997.

[2] M. Schickert, Progress in ultrasonic imaging of concrete, Materials and Structures 38 (November 2005), pp. 807-815, 6 April 2005

[3] Meng-Lin Li,Wei-Jung Guan, Improved Synthetic Aperture Focusing Technique with Applications in High-Frequency Ultrasound Imaging, IEEE transactions on ultrasonics, ferroelectrics, and frequency control, vol. 51, no. 1, january 2004

  [4] Martin Schickert, Wolfgang Hillger, Automated ultrasonic scanning

and imaging system for application at concrete structures.

[5] Jorgen Arendt Jensen, Svetoslav Ivanov Nikolov, Kim Lokke Gammelmark,

Morten Hogholm Pedersen, Synthetic aperture ultrasound imaging, Ultrasonics 44 (2006) e5–e15

References Continued.. [6] J. Opretzka, M. Vogt and H. Ermert, A Model-Based Synthetic

Aperture Image Reconstruction Technique for High-Frequency Ultrasound, 10.1109/ULTSYM.2009.0094

  [7] S.F. Burch and J.T. Burton, Ultrasonic synthetic aperture focusing

using planar-pulse-echo transducers, Ultrasonics, November 1984

[8] W. Muller, V. Schmitz and G. Schafer, Reconstruction by synthetic aperture focusing technique (SAFT), Nuclear Engineering and Design 94 (1986) 393-404 393 North-Holland, Amsterdam, pp. 393-404

[8] Martin Schickert, Martin Krause; and Wolfgang Muller, Ultrasonic Imaging of Concrete Elements Using Reconstruction by Synthetic Aperture Focusing Technique, JOURNAL OF MATERIALS IN CIVIL ENGINEERING, ASCE / MAY/JUNE 2003, pp. 235-246

[9] C. Cheng, M. Sansalone, The impact-echo response of concrete plates containing delaminations: numerical, experimental and field studies, Materials and Structures, 1993, 26, pp. 274-285

[10] www.lstc.com

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