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Final report (1st stage)
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1
Chapter 1
Introduction
1.1 Motivation
There has been a tremendous increase in the usage of ultrasonic methods for inspection of
materials. Not all of these methods can be incorporated for the inspection of concrete
because of its inhomogeneous nature. The use of SAFT reconstruction technique for the
positioning and sizing of defect points is fairly new phenomenon in non-destructive testing
of concrete. It utilizes the results obtained from the Impact-echo testing of the concrete
sample. Its better resolution and improved results over other ultrasonic testing method is
used for many other applications including the use in photography for getting higher
resolution digital images by using array of digital cameras. Nowadays, this method is used
for medical ultrasound imaging to get better resolution image of internal organs of human
body. Its vast benefits have always prompted researchers to explore new areas of its
utilization.
1.2 Objectives
The objective of the study can be broadly categorized as-
Understanding the ultrasonic wave propagation in concrete
The importance, need and different methods of Non-destructive techniques
Effect of various subsurface discontinuities on the propagation of stress waves
Understanding the basic method of Impulse-echo technique and studying the
benefits of SAFT method over Impulse-echo method.
Researching the basic reconstruction technique of SAFT algorithm
LS-DYNA modeling of concrete slab with defect and response collection on various
receivers to finally backtrack the location and size of the defect by the use of
MATLAB algorithm.
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1.3 Importance and Need of Non-Destructive Testing
Regular and frequent inspections are essential to ensure the structural integrity of concrete
structures. Non-destructive testing (NDT) methods are well suited for this purpose because
they allow the detection of invisible defects at their early stages of development without
harming the structure itself. In many cases the high repair cost can be significantly reduced
if the damage is detected as early as possible. Several NDT techniques can be employed for
the health monitoring, the detection of cracks, tendon ducts and other built-in components
in the superstructure which generally require one-sided accessibility. This requirement
restricts the applicable NDT methods to ultrasonic testing, impact echo, impulse radar and
in some cases thermography.
It is often necessary to test concrete structures after the concrete has hardened to
determine whether the structure is suitable for its designed use. Ideally such testing should
be done without damaging the concrete. The tests available for testing concrete range from
the completely non-destructive, where there is no damage to the concrete, through those
where the concrete surface is slightly damaged, to partially destructive tests, such as core
tests and pullout and pull off tests, where the surface has to be repaired after the test. The
range of properties that can be assessed using non-destructive tests and partially
destructive tests is quite large and includes fundamental parameters such as density, elastic
modulus and strength as well as surface hardness and surface absorption. Reinforcement
location, size and distance from the surface are also assessed from these tests. In some
cases it is also possible to check the quality of workmanship and structural integrity by the
ability to detect voids, subsurface cracking and delaminations.
Non-destructive testing can be applied to both old and new structures. For new structures,
the principal applications are likely to be for quality control or the resolution of doubts
about the quality of materials or construction. The testing of existing structures is usually
related to an assessment of structural integrity or adequacy. In either case, if destructi ve
testing alone is used, for instance, by removing cores for compression testing, the cost of
coring and testing may only allow a relatively small number of tests to be carried out on a
large structure which may be misleading.
3
1.4 Different Methods of Non-Destructive Testing
Non-destructive testing methods may be broadly classified into two sections:
searching techniques
analyzing techniques
The searching techniques must be able to examine the complete volume of the component
under inspection or at least the welding areas. They must combine a high inspection speed
with a high reliability in finding and documenting indications above some registration levels.
The analyzing techniques are used to decide whether an indication found by a searching
technique is really a defect or some type of a form-echo. They need not to have very high
inspection-speeds because they only are used to inspect those parts of a component where
overstepping of the registration levels were found.
These analyzing techniques must be able to
differ between defect- and form-echoes
measure the defects position
classify in "crack-like" or "globular"
evaluate the defect’s size
Thus they must be able to give the input data for the fracture mechanics calculation of the
defects criticality. They should provide the exact location, size and shape of the subsurface
discontinuity as well as the attributes of the components inside the concrete block under
investigation.
1.5 Cracking and Subsurface discontinuities in Concrete
Cracking affects the appearance of concrete. In some cases it affects its structural adequacy
and durability. In reinforced concrete, cracking allows easier access to air and moisture
which can cause steel to rust and eventually weaken the concrete. A common problem in
repair and rehabilitation of concrete structures is to determine the extent of cracking within
a structure. In plate-like structures such as bridge decks, slabs and infill walls in frames,
cracking often occurs in the form of delaminations in the plane of the reinforcing bars. For
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example, in reinforced concrete bridge decks, chloride-induced corrosion of reinforcing bars
leads to bursting forces which produce cracks around the bars. These cracks propagate in
the plane of the bars due to the larger bursting forces caused by continued corrosion and
forces caused by expansion of water which penetrates the cracks and undergoes freezing
and thawing. In reinforced concrete infill (or shear) walls in frame structures, cracking
around bars leading to delaminations can be caused by cyclic loading in an earthquake.
5
Chapter 2
Various Ultrasonic Testing Methods
2.1 Transmission Technique (Basic ultrasonic testing)
Ultrasonic testing is an established non-destructive technique for the detection of defects
and the characterization of materials. While most applications of this method are centered
on the inspection of metals, the ultrasonic pulse transmission technique has also been used
for a long time for testing concrete elements. For measurement, a transmitting and a
receiving transducer are placed on opposite sides of the test object, and an ultrasonic pulse
is sent through the concrete. The velocity and attenuation of the pulse are then used for
empirical correlation with strength and other characteristic parameters of the concrete.
For applications such as the detection of tendon ducts or small flaws, the transmission
technique is usually not well suited because it lacks sensitivity. The reason for this is the
strongly inhomogeneous nature of concrete. Even at low frequencies the ultrasonic
wavelength is in the order of magnitude of embedded aggregate and pores. This causes
strong scattering which attenuates the transmitted pulses. To reduce this problem, signals
of low-frequency content in the range of 20–100 kHz are employed in the transmission
technique to minimize interaction with the coarse concrete structure. But simultaneously
the sensitivity of detection of the target objects is also decreased.
2.2 Problems arising in testing concrete by ultrasonic techniques
Non-destructive testing employing ultrasonic pulses/Impulse waves have been used for a
long time for the detection of internal objects in metals and other materials. Specifically
ultrasonic pulse-echo testing, allowing for one-sided access of a component, is used for
inspection tasks on a regular basis. The application to concrete, however, faces problems
induced by the inhomogeneous concrete structure. Aggregate and pores have acoustical
properties much different from those of the cement matrix, which gives rise to scattering,
attenuation and mode-conversion of propagating ultrasonic waves. Among the
consequences are pulse attenuation and structural noise, which can mask reflections of
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objects to be detected. The presence of water molecules or air gaps inside the concrete can
change the speed and other characteristics of the P- waves passing through it. This causes
faulty reading of the receiver signals and thus incorrect results.
2.3 Impact Echo Technique- Basics
This is the most common NDT method to inspect the subsurface discontinuities and
components of the concrete material. In the impact-echo method, a transient stress pulse is
introduced into a structure by mechanical impact at a point on the surface. This pulse has
the approximate shape of a half pulse sine curve. This pulse travels into the plate as
longitudinal (P-) and transverse (S-) waves and along the surface as a Rayleigh (R-) wave. The
P and S waves propagate into the structure along spherical wave-fronts and are reflected by
internal cracks or voids or interfaces and by the external boundaries of the structure. An
array of displacement transducer located close to the impact point is used to monitor the
surface displacements caused by the arrival of these reflected waves. These waves are, in
turn, reflected at the free surface, and they propagate back into the test object to be
reflected again by internal interfaces or boundaries. Therefore, a transient resonance
condition is set up by multiple reflections of the waves between the free surface and
internal defects or external boundaries. P-waves are of primary importance in impact-echo
testing of plate structures, because the displacements caused by P-waves are much larger
than those caused by S-waves at points located close to the impact point.
Fig.2 .1 - Schematic measurement setup for one-dimensional aperture scans
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The stress waves are generated when pressure or deformation is applied suddenly. These
waves consist mainly of three types-
1. P- waves : Longitudinal waves propagating parallel to the propagation direction
2. S- waves : propagate perpendicular to the propagation direction
3. Rayleigh (R-) waves : surface waves propagation only near surface
The velocity of P- waves inside concrete material depends on the Young’s modulus, mass
density and Poisson’s ratio of the concrete sample.
Vp =
Where, for M20 concrete grade (taken for analysis) the values of different variables are,
E = Elastic / Young’s modulus of concrete = 2.236 e10 Pascal
Ρ = Mass density of concrete = 2400 kg/m3
ν = Poisson’s ratio of concrete = 0.2
This method is effective for locating large voids or delaminations in plate like structures, e.g.
pavements or bridge decks, where the defect is parallel to the test surface. A mechanical
impact produces stress waves of 1 to 60 kHz. The wavelengths of from 50 mm to 2000 mm
propagate as if in a homogeneous elastic medium.
2.4 Method of determination of distance of reflecting surface by Impact-Echo method
The mechanical impact on the surface generates compression, shear and surface waves.
Internal interfaces or external boundaries reflect the compression and shear waves. When
the waves return to the surface where the impact was generated, they create displacements
in a transducer and subsequently a display on a digital oscilloscope. The resulting voltage-
time signal is digitized and transformed, in a computer, to amplitude vs. frequency plot. The
dominant frequencies appear as peaks on the frequency spectrum. The dominant frequency
is not necessarily the thickness signal. Using each of the frequencies identified as peaks on
the frequency spectrum, the distances to the reflecting surfaces can be calculated from
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d =
Where,
d = distance of the reflecting surface,
f = dominant frequency,
V = velocity of compression waves in the test material.
If the receiver is placed close to the impact point the reflected signals may not be seen
because the transducer is still ringing due to the impact.
Fig. 2 .2 Schematics of Impulse echo testing of the concrete slab
The data which is gathered by the array of receiver points are in this report Z- displacement
vs. time characteristics and fast Fourier transform is not required. The received data is put
for analysis in the SAFT reconstruction algorithm.
2.5 Range and limitations of impact-echo testing method
Velocity of the P-wave which causes most of the displacement in the receivers is frequency
times wavelength of the signal. Thinking in terms of a multiple of the wavelength-
Velocity = frequency × wavelength
9
V= f *λ
Where,
λ is wavelength and f is the frequency of the stress pulse.
For impact test to work i.e. the technique can only investigate cracks and defects which are
in order greater then half of the wavelength of the impact/input pulse. Recent research has
shown that the ‘near field’ detection capability of impact-echo is,
Minimum depth of detectable target = λ/2
In order to determine λ, the velocity of P- wave through the concrete is to be known. The P-
wave velocity through the concrete is 3200 m/s (rounded value) for M20 grade concrete.
And, λ = 3200/frequency in Hertz
When using impact-echo equipment, one selects the excitation frequency in order that the
appropriate size of spherical hammer is chosen. For example, in this report, the impact time
of 10 µs is used to detect the defect i.e. a 100 KHz excitation frequency hammer (half sine
pulse) is chosen, the near field minimum depth resolution would be
λ/2 = [ 3200/(100 KHz × 2) ] = 0.016 meter
This means this method can only work for crack depths greater than 16 mm for the taken
sample. A check needs to be undertaken on actual impact frequency achieved as the surface
of the concrete may crumble. If the surface crumbles, even a little, on impact-
Contact time increases
Lower frequency of excitation is achieved
Longer wavelength signal is generated
Lower “near field” resolution is achieved.
2.6 Parts of the Impact echo System
The Impact echo system consists of three components-
1. Impact source
2. Receiving transducer (receiver)
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3. Portable computer with a data acquisition card
2.6.1 Impact source
The choice of the impact source is very important. These days, hardened steel spheres on
spring-steel rods are used as an impact source. The force time characteristics of the impact
force can be approximated to be a half pulse sine curve. The duration of the impact
determines the frequency content of the stress pulse that is generated. A shorter duration
of impact produces a broader range of frequencies in the waves; however, the amplitude of
each component frequency is lower. The impact duration determines the size of the defect
which can be detected by impact-echo testing because the frequency content of the pulse is
determined by that. As the duration decreases, the pulse contains higher frequency (shorter
wavelength) components, and smaller defects or interfaces can be detected. Shorter-
duration impacts are needed to locate shallower defects. However, waves produced by
shorter-duration impacts (low frequency and low energy) will have limited penetrating
ability in concrete.
2.6.2 Receiving Transducer
The receiver consists of a transducer which detects the reflected pulse i.e. detects the
movement of the surface and provides the displacement vs . time characteristics of the
receiver points. These movements are converted into electrical pulses and are transferred
to the computer system with data acquisition capabilities.
2.6.3 Portable computer with a data acquisition card
A portable computer-based data-acquisition system is used to capture the output of the
transducer, store the digitized waveforms, and perform signal processing and analysis.
11
Chapter 3
Ultrasonic SAFT (Synthetic Aperture Focusing Technique)
3.1 Basics of SAFT Technology
Synthetic aperture techniques were originally conceived for radar systems in the 1950s and
were initially implemented using digital computers in the late 1970s and more advanced
techniques were introduced in the late 1980s. Ultrasonic SAFT (Synthetic Aperture Focusing
Technique) reconstruction is an imaging method utilizing the information content of several
pulse-echo measurements. The measurements are recorded on the surface of the concrete
on a one or two-dimensional grid, also called aperture. Since transducers or transducer
arrays of the size of a whole aperture are presently not manageable, one or more
transducers need to be moved to scan the grid.
The set of measured signals obtained at the receivers by numerous reflections and
scattering of source pulse inside the material (acquired by Impact-echo measurement) is
then processed using SAFT reconstruction. The underlying algorithm coherently
superimposes the signals for each image element, thus synthesizing a transducer of the size
of the aperture with variable focusing to each image element. Linear apertures lead to two-
dimensional images (B-scan) and planar apertures to three dimensional images (C-scan).
Fig. 3 .1 Position of aperture and imaging results
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SAFT is a signal processing tool that aims at improving the accuracy of ultrasonic signals,
thus leading to better sizing capabilities. SAFT reconstructions provide detailed information
about the imaged concrete section and can therefore be used for detection and localization
tasks. The superposition process reduces structural noise, which can be a severe problem in
single pulse-echo measurements (A-scan). But since physical limitations and wave
propagation specialties such as mode conversion may cause interfering indications, the
resulting images need to be interpreted, utilizing additional information if necessary.
3.2 Data acquisition in SAFT
There are various methods by which data can be gathered for analysis in the SAFT
reconstruction technique-
1. Fixed source and moving receiver
2. Moving source and receiver (same source as receiver)
3. Moving source and moving receiver
4. Fixed source and array of fixed receivers.
The method used in the analysis here is fixed source and fixed array of receivers. Since the
array of receivers doesn’t cover the whole top surface of the concrete sample, only the part
of cross sectional area that is below the receiver aperture is taken for the analysis.
Synthetic apertures offer a more flexible way of focusing. A synthetic aperture imitates a
large transducer by sampling its area at many points. This can be done either by an array of
transducers measuring simultaneously, or by a single transducer approaching the aperture
points in succession. Moved arrays, a combination of both, are also possible. The apertures
considered here are linear or planar, representing a large line. The planar aperture consists
of a grid of N points in the X- direction and M points in the Y direction; the linear aperture is
obtained for M=1.
For focusing the pulse-echo measurements at the synthetic aperture, the received signals
are processed using the SAFT algorithm (synthetic aperture focusing technique). The SAFT
algorithm focuses the received signals to any point of the reconstructed image by coherent
superposition. In this way, a large virtual receiver with variable focus is synthesized.
13
Resulting higher resolution image is two dimensional (2D SAFT) for the case of the linear
aperture, and three dimensional (3D SAFT) for the planar aperture. Two-dimensional SAFT
images are called B-scan; 3D SAFT images are often called D-scan sections capturing data
through the three-dimensional data field.
3.3 SAFT Reconstruction Algorithm
Impact signal (force vs. time characteristics) which is given by the source moves in the
model and due to the presence of the defects it is transmitted back and reflected in
arbitrary direction. The receivers that are present at the top of the model surface then pick
up this reflected signal from the defect as well as back wall echo that is coming from the
reflection by the back-wall of the model. These signals now vary from the signals that would
normally arise in the case of no defect or discontinuity. These signals are in the form of
amplitude vs. time. The signals received by the array of line receivers are now used for the
backtracking of the defect position. During reconstruction one has to calculate back the
signals received at the surface at receiver point (x, y) into that region inside the material (α,
β, z) with z as the depth coordinate. Here the reconstruction algorithm comes into picture
(W. Muller, V. Schmitz and G. Schafer, Reconstruction by the synthetic aperture focusing
technique (SAFT), 1986). Suppose the source (also the receiver) is moving towards the
defect zone in succession. Here the depth and width plane is messed and a grid of node
points is created. The integral to be solved now is-
A2 (α, β, z) = ∫∫ [ *A1 [ ]
Where,
A2 (α, β, z) is the image of the reflecting surface inside the sample at co- ordinate position of
(α, β, z).
r =
If sound velocity (P- wave velocity that causes maximum displacement) inside the material is
considered to be c, then t can be replaced by the expression;
t = * [ ]
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The two expressions are put in the main integral and solved. The result is the 3-dimensional
amplitude distribution inside the specimen, a so called D-scan.
Reducing the problem to the 1-dimensional case, so scanning only in one line (x-direction),
one has input data A(x, t) and equation reduces to
A2 (α, z) = ∫∫ 1/r *A1 [ ]
In this case the result is the amplitude distribution in an area below the scanned line and
perpendicular to the scanned surface, performing a side view, a so called B-scan.
In both cases, 2-dimensional and 1-dimensional SAFT, the data are picked up at discrete
points at the nodal points of the mess. They are reconstructed into discrete pixel points, and
the amplitude versus time-values is stored digitally, i.e. for discrete and limited time
intervals. Therefore one can overcome the integrals in the equations by simply performing
summations. The final reconstruction algorithm for 1-dimensional SAFT (LSAFT) for one
receiver is
A2 = ]
In other words,
To determine the image location of the defect point (r’j) by L-SAFT algorithm, the image
point I(r’j ) at the point r’j is calculated by using the time taken for the ultrasound to traverse
the distance | (rsi – r’j) | and again a distance of |(rs –r’j)| ,denoted by tij i.e. the time
taken to traverse from source to the image point and again from image point to the
receivers. This is calculated for each received signal position rsi. The set of values of
received signals S(rsi, tij) are backtracked and summed to determine I(r’j). Therefore if the
point r’j 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. Expressed more precisely, the SAFT image field is
I(r ‘) where
I(r’j) = 1/n
15
Where,
tij = t0/2 + 2/c( | (rsi-r’j) |+|(rs –r’j)| )
Here, n is the number of receivers, c the velocity of ultrasound in the medium, time t0 is the
impact time of the source and rsi is the receiver location and rs is the source location.
Fig3.2 Parameters involved in the calculation of SAFT image field amplitude
3.4 Imaging Properties
The result of the reconstruction algorithm is a focusing of the received ultrasonic echoes
back to the positions of the reflectors respectively, and because the area inside the scanned
aperture where echoes of the reflectors are received increases with increasing depth of the
reflectors, one gets a simultaneous focusing into all depths with just the same lateral
resolution.
The lateral resolution received for pulse echoes is half the ultrasonic wavelength or half the
probe width respectively. The axial resolution depends on the pulse length transmitted, in
most cases about 5 periods of the ultrasonic frequency used.
A major shortcoming of the method is the inherent assumption that the pulse reflected
from a flaw has a spectral content that is independent of the flaw's location relative to the
transducer.
16
Chapter 4
Modeling in LS-DYNA and SAFT Reconstruction Results
4.1 Specimen Specification
The concrete used in the study is M20 grade concrete with the dimension of the concrete
block taken as,
Length = 300 mm
Breadth = 300 mm
Depth = 50 mm
The velocity of the P- wave calculated by the formula comes out to be 3217.5 m/s for the
M20 concrete.
4.2 Various steps of Modeling of concrete block (without defect) in LS DYNA
This model helps us understand the back wall echo characteristics of the concrete block
without defect. This is taken for comparison with the analysis involving defected concrete
plate. Various steps and attributes of the modeling process are given below:
1. The dimension of the rectangular concrete bock is defined in the messing option
(about 20 elements per wavelength for X and Y; for Z, 3 or 4 more elements
depending on the thickness) and the messing in three axes is defined. In the model, 1
mm messing is used in all the three (X, Y and Z) directions. In this step, cuboidal
concrete block is made with required messing. The element chosen here is 8-node
point solid cubes.
2. Boundary condition of the model is defined in this step. The degree of the freedom
of the boundary nodes is fixed here. In the model the concrete block is assumed to
be simply supported on all sides. The translational motion is restricted in local X-, Y-
and Z- axes. The rotational movement is restricted about Z- axis for the boundary
node points. So, the entire lower edge of the block is constrained for movement.
17
3. Next step is defining the material. Material is assumed to be following Hooke’s law
of elasticity. The mass density (2400 kg/m3), Young’s modulus of elasticity (2.236 e10
Pascal) and Poisson’s ratio (0.2) value for the M20 grade concrete is taken.
4. For the type of integration process, a fully integrated quadratic 8 node element
(solid element) with nodal rotation is taken.
5. The impulse load to be applied at the center is defined. The impulse signal is taken to
be half sine pulse with impact time of 25 µs for testing concrete block with no
defect. The time (second) and force (Newton) characteristics are as in the figure. The
scale factor for the abscissa and ordinate values is taken to be 1.
Fig.4 .1 Impact pulse (Half sine) with impact time of 25µs used for inspecting concrete slab (without
defect)
6. The position of the node where impact pulse is to be applied is defined. The co-
ordinate of the node where the source is located is specified in the model. The node
number of the impact source node is also noted.
7. Next, the impulse load previously defined (half sine pulse) is applied at the impact
source co-ordinate. The node number of the impact node is supplied. The direction
of the applied impulse is provided here only. For the model the direction of the input
impulse is negative z direction (as seen in above figure). This is done because the
impulse applied is compressive in nature.
18
8. The termination time of the calculation is provided next. Termination time for the
model is taken to be 300µs.
9. Now, the node point values of all the receiver points are noted down where we want
to get the Z-displacement (many other variables can also be obtained) and time
characteristics. All the receiver node coordinates are entered in the model.
10. The nodout option is selected and the time interval between the outputs taken as
2e-7 seconds is specified for the receiver results. Then the model is saved to be run
in LS-DYNA. The LS-DYNA solver gives output nodout file. Various plots of time vs Z-
displacement for different receiver points are drawn from here to be studied in SAFT
algorithm.
Fig4.2 T he time vs. Z- displacement characteristics of the receiver node located 4 mm (Series1) and 12 mm
(Series2) from the impact source point of concrete slab (no-defect)
4.3 Modeling of Concrete block with defect
The concrete block with the defect of 40 mm width placed at a depth of 35 mm from the
top surface of the concrete model is evaluated next. The impact duration of the input
signal is taken to be 10 µs. The calculation is done with time interval between the
outputs taken as 2e-7 seconds.
19
Fig.4 .3 Cross- section of the concrete block with defect situated at 35 mm from the top surface
In LS DYNA, the model is made with source at the top center of the block and receivers put
at 2 mm distances in negative x directions. Mess size of 1 mm is taken for X- and Z- axes. The
termination time is taken to be 200 µs. We calculate the response only in left side of the
source exploiting the symmetry of the section. Only the node points which lie directly below
the receiver points are taken into consideration. So, we assume the mess having 21 node
points in X- direction and 50 node points in the Z- direction. This is taken because the
aperture of the receiver array extends only up to the position of the last transducer. The
output at the receivers consists of Z- displacement vs. time characteristics of its node points.
This plot is now used to backtrack the position of the defect. The rest of the modeling is
done as before.
The output at the receivers has some initial displacements caused by Rayleigh waves and it
needs to be overlooked for the SAFT reconstruction. Various peaks in the output (excluding
Rayleigh wave part) are multiple reflections of the P- waves from the defect. The first crest
corresponds to double reflection of P- wave (2 P) inside the material under investigation and
so on.
20
Fig.4 .4 –concrete block model with defect size of 40 mm (situated at 35 mm from top surface) to be
simulated in LS-DYNA
The response of various receivers (Z- displacement vs. time characteristics) is obtained from
running simulation in LS-DYNA.
Fig4.5 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)
21
4.4 Reconstruction algorithm in MATLAB™
Small code is written in MATLAB program for reconstructing the different data points
obtained at various receivers for concrete slab (with defect). The code is written in the
APPENDIX.
The MATLAB code (reconstruction algorithm) makes an array of the size of the messing of
size 21*50 in the X-Z direction. Then it assigns the co-ordinate value to the receiver points.
The velocity of the P-wave and the impact time of the input impulse signal are supplied to
the code. Now, for every nodal point in the cross-sectional part below the receivers, the
distance of the node point with the source and again with each of the receivers is
calculated. The sum of the distances is divided by the velocity of the P- wave through
concrete to get the time the input pulse takes to get to node point and back to the receiver.
In this time half the impact time of input pulse is added according to SAFT algorithm. This
time is then looked up in the receiver output characteristics (Z-displacement vs. time) to get
the value of the amplitude of the Z- displacement. The same is done for all the receivers for
a single nodal point and added to get the resultant amplitude value of the Z- displacement.
The Z- displacement value thus obtained is divided by the number of receivers to get the
amplitude value of Z- displacement value for the particular node. Now, the same is done for
all the nodal points in the array nodal points. Thus we get the Nodal point vs. Z -
displacement characteristics. This is plotted to check for what value of nodal points the Z-
displacement value coherently superimposes constructively to a get large value (clearly
greater from the rest nodal points). This gives the co-ordinate of the nodal points at which
defect is present.
The node points in the grid (21*50) are given integer values ranging from 1 to 1050 (total
number of node points in the grid). 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.
This way whole grid is assigned node numbers from 1 to 1050. Now, the 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). The
average of the two values gives the average node number of the defect. So, 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
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which is at 35 mm from the top surface the result is satis factory. Keeping in mind the flaw in
the model that the receiver’s aperture width is equal to the defect’s width, the result is
acceptable.
Fig. 4 .6 Plot of the reconstructed Z- displacement vs. the node number
23
Chapter 5
Results and Scope
The concrete block model with defect at 35 mm from the top surface had a design flaw. The
extent of the receiver array (20 mm in negative x-direction) was kept same as the width of
the defect below the aperture. After this also the result calculated by algorithm (28.33 mm)
had only 19% error. If the model had been equipped with more receivers for collecting data
or at least the width of the aperture had been twice the width of the defect the error could
be even more minimized. Usually the SAFT algorithm is done on data collected by moving
receiver and applying ‘delay and sum’ technique to it. Unfortunately, the method of SAFT
analysis discussed in report is less in use.
The project needs to go further to get 3-D images of the reconstructed defect positions and
analysis of the same.
In future, the SAFT reconstruction technology is more to be used in health monitoring of all
concrete superstructures. Its imaging technique can further increase the probability by
which the cancer cysts and other anomalies of the human body are detected. Its technique
can also be used to detect underwater characteristics of large water bodies. Its fairly simple
and detailed imaging properties will make other ultrasonic techniques for concrete defect
detection obsolete.
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REFERENCES
[1] http://www.ndt.net/article/concrete/concrete.htm Comparison of Pulse echo method for
testing concrete
[2] http://www.ndt.net/article/civil497/mschic/mschic.htm Towards SAFT-Imaging in
ultrasonic inspection of concrete
[3] http://www.ndt.net/article/kroggel/kroggel.htm Detection of thickness, voids, honeycombs and tendon ducts utilizing Impact echo technique.
[4] http://www.ndt.net/article/ndtce03/papers/v051/v051.htm Ultrasonic imaging of concrete
elements: State of the art using 2D synthetic aperture
[5] Mary Sansalone. Impact Echo: The complete story. ACI Structural journal: Title no.94-
S71, November-December 1997.
[6] M. Schickert, Progress in ultrasonic imaging of concrete, Materials and Structures 38
(November 2005), pp. 807-815, 6 April 2005
[7] 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
[8] Martin Schickert, Wolfgang Hillger, Automated ultrasonic scanning and imaging system
for application at concrete structures.
[9] Jorgen Arendt Jensen, Svetoslav Ivanov Nikolov, Kim Lokke Gammelmark,
Morten Hogholm Pedersen, Synthetic aperture ultrasound imaging, Ultrasonics 44 (2006)
e5–e15
[10] J. Opretzka, M. Vogt and H. Ermert, A Model-Based Synthetic Aperture Image
Reconstruction Technique for High-Frequency Ultrasound, 10.1109/ULTSYM.2009.0094
[11] S.F. Burch and J.T. Burton, Ultrasonic synthetic aperture focusing using planar-pulse-
echo transducers, Ultrasonics, November 1984 [12] 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
[13] 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
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[14] 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 [15] www.lstc.com
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APPENDIX
MATLAB code for the detection of effect by SAFT reconstruction technique
% loading the z- displacement vs. time characteristics of the entire receiver points below the
receiver aperture
load lt
for i = 130:2:148 % applying coordinate value to the receiver points
r1(i/2-64) = i;
r2(i/2-64) = 0;
end
% creating array for node points below the receiver aperture
d = zeros(50,21);
% Velocity of P- wave through concrete in mm/sec
v = 3200000;
% Impact time for input impulse in seconds
to = 10e-6;
for i = 2:51
for j = 130:150
temp = sqrt((150-j)^2+(i)^2);
for k = 1:10
temp2 = sqrt((r1(k)-j)^2+(r2(k)-i)^2) + temp;
time = (temp2/v) + to/2;
disp = getvalue(time,k,lt);
d(i-1,j-129) = d(i-1,j-129) + disp;
end
end
end
for i = 1:50
for j = 1:21
27
amp(((i-1)*21)+j) = d(i,j)/10;
end
end
for i = 1:1050
num_node(i) = i;
end
plot(num_node,amp);
getvalue function is defined here-
function [temp] = getvalue(temp2,k,lt)
for i = 1:596
if i ~=596
if lt(i,1) <= temp2 && lt(i+1,1) >= temp2
temp = lt(i,1+(k));
break
end
elseif i == 596
temp = lt(596,1+(k));
break
end
end
end
END OF THE CODE