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.
27
Embed
Synthetic Aperture Focussing Technique for testing the defects in Concrete using LS Dyna
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
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.
2
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
4
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.
[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
25
[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
26
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