Crack detectection in pipes

1. 1
A
SEMINAR
ON
CRACK DETECTION IN PIPES
Submitted
By
Miss AKANSHA JHA
T.E. Mechanical
Examination Seat No: T8100801 Roll No: 01(A)
Prof. A.V.DEOKAR Prof.A.K.MISHRA
Guide Prof.& Head Of
Mechanical Engg.
Deptt.
Department Of Mechanical Engineering, Amrutvahini College
of Engineering,
Sangamner – 422608
2011-2012

2. 2
Amrutvahini College Of Engineering,
Sangamner
Department of Mechanical Engineering
2011-2012
CERTIFICATE
This is to certify that the Seminar entitled
“
CRACK DETECTION IN PIPES”
Has been Submitted By
Miss AKANSHA JHA
T.E. Mechanical
Examination Seat No: T8100801 Roll No: 01(A)
As a partial fulfilment for the Bachelor’s Degree in
Mechanical Engineering of
UNIVERSITY OF PUNE
Prof. A.V. DEOKAR Prof.A.K.Mishra
Guide Prof.&Head Mechanical Engg. Dept

3. 3
UNIVERSITY OF PUNE
Amrutvahini College of Engineering, Sangamner
Department of Mechanical Engineering
2011-2012
CERTIFICATE
This is to certify that
Miss AKANSHA JHA
Examination Seat No: T8100801 Roll No: 01(A)
Student of T.E. Mechanical has presented a Seminar Entitled
“CRACK DETECTION IN PIPES”
ON: 19/10/2011
At
Amrutvahini College of Engineering,
Sangamner-422608
GUIDE/INTERNAL EXAMINER EXTERNAL EXAMINER
(Prof. A.V. DEOKAR) (Prof. S.B.JADHAV)

4. 4
TABLE OF CONTENTS
Chapter Title Page No.
1 Introduction 1
1.1 Damage Assessment 1
1.2 Definition Of Crack 2
1.3 Classification & Types 2
1.3.1 Transverse Cracks 2
1.3.2 Longitudinal Cracks 3
1.3.3 Slant Cracks 3
1.3.4 Gaping Cracks 4
1.3.5 Surface Cracks 4
1.3.6 Subsurface Cracks 4
1.4 Causes 4
1.4.1 Stress Corrosion Cracking (SCC) 5
1.4.2 Hydrogen Induced Cracking (HIC) 6
1.4.3 Stress-Oriented Hydrogen Induced
Cracking (SOHIC)
6
1.4.4 Laps 6
1.4.5 Hook Cracks 7
1.4.6 Girth Weld Cracks 7
1.4.7 Fatigue Cracks 7
1.4.8 Narrow Axial External Corrosion
(NAEC)
7
1.5 Effects 8

5. 5
2 Concept Of Crack Detection 8
2.1 Leak Before Break 8
3 Methods Of Detection 10
3.1 Conventional Methods 10
3.1.1 Pipeline Injection Gauze (PIG) 10
3.1.2 Sewer Scanner & Evaluation Technology 11
3.1.3Automated Pipe Crack Detection 12
3.1.4 Ultrasound Acoustics Based Assessment
Technique
12
3.1.5 Piezoelectric Effect 12
3.2 Vibration Based Methods 13
3.3 Advantage Of Vibration Method 14
3.4 Limitations 15
5 Case Study 15
4.1 Modelling 16
4.2 Inverse Problem 17
4.3 Experimental Work 17
4.4 Results 18
6 Conclusions 24
References 25

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ABSTRACT
The detection of Crack location and size in structures has motivated several
researchers in recent years. A technique based on measurement of change of natural
frequencies has been employed to detect the cracks. A combined computational -
experimental method for determination of crack location in pipes is developed. The
graphs representing the first three natural frequencies are obtained using the FE
models of a pipe with variations in crack location and depth. In the inverse problem,
the interpolated graphs of FEA and the experimental data are used to locate the crack.
The method is implemented here of small and large diameter and the results are
discussed.
1 INTRODUCTION[1]
Mechanical accidents, fatigue, erosion, corrosion, as well as environmental
attacks, are issues that can lead to a crack in a mechanical structure. Cracks are
indications of an impending mechanical failure. In view of the fact that the presence
of a crack in a structure could lead to devastating results, investigating the structural
integrity of pipes was an extremely active area of research in the last two decades.
Mechanical structures in real service life are subjected to combined or separate effects
of the dynamic load, temperature and corrosive medium, due to the consequent
growth of fatigue cracks, cracks due to corrosion and other type damages. Although
the theory and technology of non-destructive testing is highly enhanced, inspecting
the integrity of a structure is a labour-intensive and protracted process that should
only be carried out when truly needed. One approach for reducing inspection related
shutdown time and cost is to provide a mechanism with an early warning failure
device. Such a device monitors, online, crack-related irregularity in the behaviour of a
system. If the device gives a sound signal that a crack is present, a message is given
out to the operator to shutdown the machine and have it checked. For the development
of such early warning devices, knowledge of the dynamics of cracked structures is
important.

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1.1 DAMAGE ASSESSMENT[1]
A crack in a structure may be realized from the local divergence in structure stiffness
affecting the global dynamic behaviour of the structure. Also, a crack may manifest
its presence in a beam-like structure through the change in natural frequency and
mode shape of the system. These indicators may also be used to measure the extent of
the damage and to determine its location. A crack in a pipeline can be thought of as a
local flexibility and as such depends on crack depth. The existence of a crack further
reduces the natural frequencies of the structure so consequently by measuring
changes in natural frequencies the location of crack can be identified. Definition of
damage is given as any deviation introduced to a structure, either deliberately or
unintentionally, which adversely affect the performance of the system.
One damage identification system commonly classifies four levels of damage
assessment:
• Level 1: Determining the presence of damage:
• Level 2: Locating the damage geometrically:
• Level 3: Quantifying the damage severity,
• Level 4: Prediction of the remaining serviceability or service life of the structure
1.2 DEFINITION OF CRACK
Any unwanted deformation leading to the fracture of any component without
complete separation of its parts is termed as a Crack. Partial breaking of the material
which is induced due to various stresses or forces acting on it, is known as crack.
1.3 CLASSIFICATION OF CRACKS[3]
In all of the method to estimate the location and the depth of the crack from the changes in the
natural frequencies, the model of the damage is important. Based on their geometries, cracks
can be broadly classified as follows:
1.3.1 Transverse Cracks
Cracks perpendicular to the pipe axis are known as “transverse cracks”. These are the
most common and most serious as they reduce the cross-section and thereby weaken
the pipe. Fig 1.1

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Fig 1.1 Transverse Crack
1.3.2 Longitudinal Cracks
Cracks parallel to the pipe axis are known as “longitudinal cracks”. They are
not that common but they pose danger when the tensile load is applied is at right
angles to the crack direction i.e. Perpendicular to pipe axis or the perpendicular to
crack. Fig 1.2
Fig 1.2 Longitudinal Crack
1.3.3 Slant Cracks
“Slant cracks” (cracks at an angle to the pipe axis) are also encountered, but
are not very common. Their effect on Lateral vibrations is less than that of transverse
cracks of comparable severity. Fig 1.3
Fig 1.3 Slant Crack

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1.3.4 Gaping Cracks
Cracks that always remain open are known as “gaping cracks”. They are more
correctly called “notches”. Gaping cracks are easy to mimic in a laboratory
environment and hence most experimental work is focused on this particular crack
type. Fig 1.4
Fig 1.4 Gaping Crack
1.3.5 Surface Cracks
Cracks that open on the surface are called “surface cracks”. They can
normally be detected by techniques such as dye-penetrates or visual inspection.
1.3.6 Subsurface Cracks
Cracks that do not show on the surface are called “subsurface cracks”. Special
techniques such as ultrasonic, magnetic particle, radiography are needed to detect
them. Surface cracks have a greater effect than subsurface cracks on the vibration
behaviour of shafts.
1.4 CAUSES OF CRACK[4]
Pipelines are pressure tested in addition to non-destructive testing prior to being put into
service. Normally, pipelines are hydrostatically stressed to levels above their working
pressure and near their specified minimum yield strength. This pressure is held for several
hours to ensure that the pipeline does not have defects that may cause failure in use. This
proof test of pipelines provides an additional level of confidence that is not found in many
other structures. Generally damage in a pipeline element may occur due to normal operations,
accidents, deterioration or severe natural events such as earth quake or storms. Some of the
causes of pipeline failures are listed below:

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a) Mechanical damage
b) Fatigue cracks
c) Material defects
d) Weld cracks
e) Incomplete fusion
f) Improper repair welds
g) Incomplete penetration
h) External or internal corrosion
i) Hydrogen blistering
Mechanical damage normally consists of gouges and dents. They generally are
created by excavation or handling equipment during construction. Cracks are also
initiated during the manufacturing processes. Small cracks are known to propagate
due to fluctuating stress conditions. During fabrication, cracks may originate from
casting defects or plate rolling, and a family of cracks and crack-like defects may
arise during the seam and girth welding operations.
During pipeline operation, existing defects may grow due to fatigue. Other in-service
crack growth mechanisms include sour service cracking (HIC and SSCC) and external
stress corrosion cracking (SCC).[5]
1.4.1 Stress Corrosion Cracking (SCC)
External stress corrosion cracking on high-pressure pipelines is recognized in two
forms: high ph and near-neutral ph. SCC cracks can initiate and grow in a range of
conditions, including predominantly inter-granular cracking in alkaline conditions and
trans-granular cracking in neutral ph environments. SCC can occur in a wider range of
restricted aqueous environments at the pipe surface, and in extreme cases SCC has
been confirmed on above-ground pipelines.
The corrosion creates crack-like features aligned at right angles to the principal
stress. In most cases, the product pressure in the pipeline creates the principal stress,
so the cracks are aligned parallel to the axis of the pipeline. External stresses such as
ground movement can give rise to cracks at almost any angle through to fully
circumferential.

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SCC risk can be minimized on new pipelines by careful coating selection and
preservation of coating condition through the construction process. To reduce SCC
risk, priority should be placed on the long-term adhesion performance of the coating
and its resistance to adhesion loss from water uptake, cathodic disbanding.
1.4.2 Hydrogen Induced Cracking (HIC)
Sour service pipelines are vulnerable to HIC in the presence of water. It can occur in
pipeline steels of any strength and is generally associated with non-metallic
inclusions, particularly elongated manganese sulphides.
Features within the pipe wall appear as cracks, but features near the surface
appear as blisters or bumps. Acid corrosion takes place on water-wetted areas inside
the pipeline. Hydrogen is produced by this corrosion reaction, but in the presence of
sulphide, scales on the steel surface rather than being liberated as a gas. The atomic
hydrogen diffuses into the steel, forming blisters in the microscopic voids around non-
metallic inclusions. The gas pressure in these blisters generates very high localized
stress, which initiates cracking along lines of weakness in the steel.
HIC develops as flat cracks in the rolling plane of the pipe material. Crack
colonies develop, and failure often occurs as colonies link together in a stepwise
fashion. For this reason, HIC is sometimes called stepwise cracking.
1.4.3 Stress-Oriented Hydrogen Induced Cracking (SOHIC)
A special form of HIC may occur when local stress concentration is very high
in a sour service pipeline. High stress fields allow the hydrogen to accumulate without
the need for inclusions or other interfaces. For example, some types of spiral-welded
pipe exhibit highly stressed regions close to the seam weld, caused during the edge
forming process. Stacked arrays of HIC can form in these regions, leading to rapid
stepwise cracking failures.
1.4.4 Laps
These crack-like surface defects originate during the rolling process used to
produce the plate or strip from which pipe is fabricated. Surface cracks in the hot slab
become oxidized, which prevents them from welding to the adjoining metal during

12. 12
subsequent rolling. The cracks remain on the outer layer of the steel and are rolled
over to become surface-breaking defects at a very shallow angle. They can occur in
any position around the pipe.
1.4.5 Hook Cracks
These defects in the longitudinal weld occur during manufacture of the pipe,
when inclusions at the plate edge are turned out of the plane of the steel during the
welding process. They may pass the manufacturer’s initial hydro test, but fail later
due to metal fatigue. It is the turning out of the metal at the weld that gives the
characteristic “hook” or “J” shape to the crack.
1.4.6 Girth Weld Cracks
Although girth weld cracks can occur in any position around the weld, they are
most often found at the 6 o’clock mark inside the pipe, which is the position of
maximum stress during movement of the internal clamp, when only the root bead has
been made. The cracks are formed almost exclusively during construction because of
inadequate fit-up and excessive stress.
1.4.7 Fatigue Cracks
Metal fatigue is caused by repeated or fluctuating stresses whose maximum
value is less than the tensile strength of the material. They start as minute cracks
which grow steadily in reaction to pressure cycling, physical deformation of the
pipeline and other mechanical stresses.
1.4.8 Narrow Axial External Corrosion (NAEC)
Although this is not strictly a crack, it is one of a number of defects associated
with the seam weld, which are difficult to detect with standard metal loss tools
because of their axial orientation. It is caused when the pipe wrap “tents” over the
seam weld bead, allowing moisture to enter and encouraging corrosion. The resulting
loss of metal parallel to the seam can result in rupture.

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1.5 EFFECTS OF CRACK
1) It produces high costs of production and maintenance.
2) Cracks present in vibrating components could lead to catastrophic failure.
3) Presence of cracks in structures or in machine members leads to operational
problem as well as premature failure.
4) Crack/damage affects the industrial economic growth.
2 CONCEPT OF CRACK DETECTION[6]
There are two types of problems related to crack:
a) Direct problem: To determine of the effect of damages on the dynamic
characteristics of pipes.
b) Inverse problem: To detect, locate and quantify the extent of the damages.
Assuming there is a surface crack on pipe (or container) wall, whose initial
length is ai and depth ci , the crack will simultaneously enlarge along length direction
and depth direction respectively under the effect of external loads. If critical length
that the structure creates overall unstable invalidation is acrit under the effect of a
certain load, corresponding length which the crack penetrates wall thickness is aleak ,
this structure will only cause penetrating leakage when aleak ＜ acrit , not cause instable
overall breakage. But under the effect of external load, the penetrating crack length is
aleak will continually enlarge until the structure causes overall breakage when the
crack length reaches acrit . This invalidation process is called LBB. People can
discover leakage via various methods and measures in this process, and immediately
take measures to prevent occurrence of catastrophic overall breakage accidents.
The LBB invalidation process can be divided into four phases: subcritical
enlargement of surface crack, local instability (crack penetration), subcritical
enlargement of penetrating crack and overall instability. The LBB analysis requires
that the crack produces enough liquid leakage before overall instable breakage in
order to guarantee it detected as soon as possible, and there is enough time to take
measures (such as pressure discharge and maintenances) before leakage detected to
crack instability. The LBB criterions are used to differentiate whether a certain initial

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crack on pressure pipes (or containers) causes LBB (LEAK BEFORE BREAK)
under the effect of operating load and environmental factors. The LBB criterions
usually include following requirements:
(1) Load Requirements: The load should include static strength and static bending
moment under normal operating conditions, strength and bending moment pertinent to
earthquake and shock. Assuming that the crack is positioned at the point where the
combination of stress and material quality is worst, where the stress is highest while
the ductility and intensity of the material are lowest.
(2) Crack Dimensions And Positions: Assuming that the dimensions of the crack are
large enough in order to guarantee the leakage measured as soon as possible. The
crack leakage rate under normal operating loads is commonly required to be at 10
times of minimum leakage quantity detected by leakage detection system.
(3) Crack Stability Conditions: While determining crack stability, add the normal
operation loads to earthquake and shock loads, and then multiply a coefficient of
safety (according to different load calculation methods, commonly take 2 or 1.0), and
the crack is required to be stable under this load.
(4) Crack Dimension Margin: Satisfy the minimum crack length prescribed in
condition (2) i.e. 2aleak , satisfy the crack length of critical instability enlargement
prescribed in condition (3) i.e. 2acrit . The LBB criterions require:
Acrit ≥ Sa aleak ----- (1)
Of which, the safe margin factor of crack length Sa commonly takes 2.
(5) Other Requirements: enough time is required to implement leakage detection
and safe protective measures, that is:
Tlbb > St T ----- (2)
Of which, tlbb is the time of penetrating crack from the detectable leakage beginning to
unstable enlargement, T is the responding time of leakage detection system, including

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the time required to detect leakage under normal operating conditions and to take
necessary measures, St is safe margin factor of crack enlargement time.
3 METHODS OF CRACK DETECTION
There are various methods of crack detection primarily comprising of:
 Conventional Method
 Non Conventional Method
3.1 CONVENTIONAL METHODS
(NON-DESTRUCTIVE TECHNIQUES,NDT)
Non-destructive evaluation is widely used in industry to evaluate the structural
integrity of civil and mechanical structures. ).[7] The difference between NDE and
Pipeline Injection Gauze (PIG) is that PIG is an online system that is intended to be
performed while the structure is in service. NDT methods include ultrasonic,
magnetic particle, liquid penetrant, radiographic, eddy current testing, low coherence
inferometry.[8]
3.1.1 Pipeline Injection Gauze (PIG) [9]
Ultrasonic non-destructive testing Pipeline inspection gauze are the ‘smart’
and intelligent inline inspection tools which are sent through the pipe via the
circulation of fluid (i.e. Water or gas) contained in the pipe shown in Fig. 3.1. It
travels and conduct inspection over very large distances pigs use several non
destructive tests to conduct the inspection. Pigs with ultrasound have got several
transducers that produces a high frequency sound pulse perpendicular to the pipe wall
and receives echo signals from the inner and external surface of the pipe. It can detect
as small a s 1cm by 1 cm crack.
They work in a pulse-echo mode with a rather high repetition frequency.
Straight incidence of the ultrasonic pulses is used to measure the wall thickness and
45º incidence is used for the detection of cracks. Straight incidence of the ultrasonic
pulses is used to measure the wall thickness and 45º incidence is used for the
detection of cracks. Several hundred sensors have to be controlled, their echoes have

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to be recorded, on-line data processing has to be able to reduce the amount of data
recorded and ensure that all relevant data are stored.
Fig 3.1 Pipeline Injection Gauze (PIG)
The inspection speed of the tool depends on the medium and may vary within
a certain range. The inspection process has to be fully automatic and cannot be
supervised during a run. The data are stored on solid state memories that are the safest
and most reliable means of storing data in such a hostile environment. The distance is
measured using several odometer wheels.[10]
3.1.2 Sewer Scanner & Evaluation Technology[11]
An innovative technology for obtaining images of the interior of pipe (Isley
1999). SSET is a system that offers a new inspection method minimizing some of the
shortcomings of the traditional inspection equipments that rely on CCTV inspection.
This is accomplished by utilizing scanning and gyroscopic technology. The
mechanics of inspecting the pipes by SSET camera are similar to the CCTV
inspection. The SSET is designed to operate from a tractor platform to propel the tool
through the pipe. Since the SSET utilizes state-of-the-art scanner technology, it can
travel through the pipe at a uniform speed. The major benefit of the SSET system over
the current CCTV technology is that the engineer has higher quality image data that
enables him to make critical rehabilitation decisions as shown in Fig. 3.2
Fig. 3.2 Sewer Scanner & Evaluation Technology

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At its current stage of development, SSET provides the basis for future sewer
management tools that will become much more powerful as automated defect
recognition software is developed (CERF 2001).
3.1.3 Automated Pipe Crack Detection[11]
This algorithm is successfully able to detect cracks in varying pipe
backgrounds, colour, and crack patterns. Figure 2 shows an original image, contrast
enhanced image and a finally segmented image with cracks accurately detected by the
above discussed algorithm. Detailed descriptions on the algorithm and
implementation aspects can be found in (Iyer & Sinha 2004). In light of our proposed
method, better 2-D features from segmented crack images are available which can be
supplemented with additional third dimension depth data acquired from ultrasound
acoustics based non-destructive inspection techniques. Fig. 3.3. Original, contrast
enhanced and automatically segmented pipe crack image.
Fig. 3.3 Original, contrast enhanced and automatically segmented pipe crack
image
3.1.4 Ultrasound Acoustics Based Assessment Technique
Superposing an ultrasonic image or signal (from a region with surface cracks)
on the optical image information in the pipe creates a visual context in three
dimensions in which interpretation and analysis about the extent of crack propagation
is easily achieved. Ultrasonic’s is the name given to the study and application of
ultrasound, which is sound of a pitch too high to be detected by the human ear, i.e. of
frequencies greater than about 18 kHz.

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3.1.5 Piezoelectric Effect
The sensors and actuators used here are piezoelectric materials. Piezoelectrics
are used due to their ability to be used as sensors and actuators as well as their ability
to respond at high frequencies. Smart materials are classified as any material that
exhibits direct coupling between two physical domains. ). Piezoelectric materials
exhibit coupling between mechanical strain and electric charge. This effect is known
as electromechanical coupling. . This effect is attributed to an asymmetry of charge in
the crystal structure of a piezoelectric molecule. When the material is stressed, the
charged particles in the material move. This motion is called “electric displacement”
and will cause a net voltage which can be measured at the electrodes. The converse
effect is when an electric field is applied to the material causing electric displacement
and therefore producing mechanical strain). The piezoelectric effect is highly useful in
a variety of applications. Piezoelectric can be used as sensors and actuators for NDE
and SHM systems.
3.2 VIBRATION BASED METHODS[1]
Dimarogonas and Chondros modelled the crack as a local flexibility they
obtained the local flexibility by experiments.[6] They also developed a spectral
method to identify cracks relating the crack depth to the change in the first three
natural frequencies of the structure for known crack position.Vibration-based damage
assessmentwhichisthe mostlyusedglobaldamage identification method is usually carried
out in three steps:
1. Data collection,
2. Extraction of condition index,
3. Assessment of structure condition through the analysis of indices.
Vibration based methods, depending on the assumptions, the type of analysis, the
overall pipe characteristics and the kind of loading or excitation, a huge number of
publications containing a variety of different approaches. It can be classified into two
categories:
 Linear approaches
 Non linear approaches

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Linear approaches detect the presence of cracks in a target object by monitoring
changes. In the resonant frequencies in the mode shapes or in the damping factors.
Vibration methods study the structural or modal parameters of a structure. Structural
parameters include mass, stiffness, and flexibility. Modal parameters are functions of
the structural parameters and are more often used. The modal parameters include
natural frequencies, modal damping, and mode shapes.
3.3 ADVANTAGE OF VIBRATION BASED METHOD OVER
CONVENTIONAL METHODS[1]
1) The use of vibration-based methods of damage/crack diagnostics is promising:
The immediate visual detection of damage is difficult or
impossible in most of the cases and use of local non-destructive testing (NDT)
methods of damage detection requires time and financial expense and is
frequently inefficient. These methods are based on the vibration characteristics
of structures such as natural frequencies.
2) The vibration-based methods can help to determine the location, size &
severity of crack from the Vibration signatures of the structures:-
Conventional methods for NDE crack detection include visual
inspection, radiography, and acoustic emission according to Aydin (2008).
These methods are rather limited in the information they provide and can be
inaccurate. They can only provide level one (existence of damage) and limited
level two information (location of damage). Generally, conventional methods
can only tell that the structure is damaged.
3) By using a vibration model it is possible to minimize the number of necessary
sensors and maximize the information provided through data analysis.:
In order to determine the location of the damage, many sensors are
necessary. With vibration based methods it may be possible to acquire global
damage assessment using a single sensor (Cawley (1979).Using fewer sensors
is less costly in terms of initial cost and maintenance costs.
4) The comparison with the crack sites identified by measuring both axial and
flexural vibrations showed better results for the flexural vibration case. Visual,
acoustic, magnetic field and eddy current techniques are some examples.

20. 20
5) A direct procedure is difficult for crack identification and unsuitable in some
particular cases, since they require minutely detailed periodic inspections,
which are very costly. In order to avoid these costs, researchers are working
on more efficient procedure in crack detection through vibration analysis.
6) Access to pipe through ultrasonic based technique is difficult owing to the
layer of soil above the pipe periphery.
3.4 LIMITATIONS OF VIBRATION BASED METHODS:
Since natural frequencies can be sensitive to atmospheric conditions and temperature
it may be difficult to determine whether a structure is damaged or if other conditions
have affected the experimental results. A compensation technique would be required
to account for atmospheric conditions.
Much research has been conducted in crack modeling for tracking natural
frequency shifts. There are many ways of measuring natural frequency shifts. A low
power method with no direct human interaction would be ideal. According to
Doebling et al. (1998) natural frequencies may be poor indicators of damage. Either
very large cracks or very precise measurements are necessary to detect damage with
vibration based methods. Compared to other methods, much power is necessary to
excite multiple vibration modes. This could mean that self-excited structures, such as
airplane wings, would be more conducive to vibration based techniques.
4 CASE STUDY: CRACK LOCATION IN PIPES USING MODAL
FREQ. AND FEM [12]
Due to presence of cracks the dynamic characteristics of structure changes.
These are the natural frequencies, the amplitude responses due to vibration and the
mode shapes.Fatigue cracks are a potential source of catastrophic structural failure.
vibration-based methods can offer an effective and convenient way to detect fatigue
cracks.
Most of the crack identification techniques found in literature rely on concurrent
monitoring of the changes in the first few natural frequencies of the system which
need a high precision algorithm to estimate the required system frequencies. Also,
these techniques are applied to a uniform cross section pipe like structure, without any

21. 21
attached masses to it. However, the techniques which take the attached masses into
consideration used complicated and lengthy modelling procedures. The modal
methods exploit the global changes in the dynamic characteristics of a structure to
detect cracks, the modification resulting from attached masses will interfere with that
resulting from cracks. The present work proposes a simple technique based on
mathematical model to identify crack in pipe. In this technique monitoring to only one
system natural frequency is needed, which requires a simple algorithm to estimate the
considered system frequency. This technique utilizes the variation in the difference
between a cracked and intact surface algorithm is used to estimate the first natural
frequency of the system.
4.1 MODELLING
The FE model of cracked pipe is developed using an APDL (ANSYS
Parametric Design Language). Tetrahedral and hexahedral elements are used to mesh
the pipe. Using hexahedral elements all over the entire pipe will also reduce the
accuracy of the results. Therefore, tetrahedral elements are used in the vicinity of the
crack while hexahedral elements are employed elsewhere throughout the pipe as
shown in Fig 4.1.
.
Fig 4.1 Modelling
A rectangular cut (crack) through the pipe perpendicular to the axis is modelled here.
The crack is specified by its dimensions: depth and width. Different types of cracks

22. 22
were modelled by FEM. The results show that the width does not have much effect on
the frequency changes. The width of the crack is therefore immaterial here, however
to be consistent with the tests a small 1mm cut is assumed in the simulation.
In general. Zero setting method was used to reduce the errors in the FE model of the
pipe, fixing the imprecise values of module of elasticity and density. This way, the
module of elasticity was changed for each frequency to match the uncracked
frequencies of the FE model with the experimental values. The module of elasticity
was used for other cracked pipe FE models with the same location of crack.
4.2 INVERSE PROBLEM
By running the APDL for different depths and locations of crack, the first
three natural frequencies are plotted against depth and location forming three surfaces.
The x-coordinate of the surfaces is the normalized distance of the crack (x/l); x
indicating the crack location and l the length of the pipe. The y-coordinate represents
the crack dimensions (d/w). By intersecting three planes corresponding to the first
three natural frequencies (taken from experimental data of the cracked pipe) with the
above mentioned three surfaces, three curves (contours) are obtained. These three
curves have a point of intersection in the x-y plane which shows the anticipated
location and the depth of the crack.
4.3 EXPERIMENTAL WORK
Modal testing was conducted using two pipes: one with 900 mm length, 16mm
and 20 mm inner and outer diameters, and the other width 650 mm length, 104.5 mm
inner and 112.5mm outer diameter. The density and nominal elastic modulus for the
pipe material were 7800 kg/m3 and 200 GPa respectively. Both pipes were suspended
from the ends by two elastic chords. A DJB accelelorometer A/120/VT with 100mv/g
sensitivity was mounted on the pipes 1/3 length from one ned. In order to excite the
system, an impact hammer (model: Rion Ph-51 instrumented with charge amplifier
with an integrated sensitivity of -4Pc/g) was used. The data acquisition system was
B&K Pulse 3560C, including hardware and the associated software. A photograph of
the test setup is shown in fig 4.2

23. 23
During the test, the pipes were lightly tapped by the hammer in the transverse
direction. The frequency response function (FRF) obtained from the FFT analyser is
processed further to determine the first three natural frequencies. The resonance
frequencies corresponding to the peaks in the FRF’s are taken as the natural
frequencies due to the fact that the structure is lightly damped. A typical record of the
FRF of the first pipe (case without crack) is illustrated in Fig 4.3
Fig. 4.2 Test Setup for the 1st pipe
Fig 4.3 Frequency Response function of the cracked pipe
4.4 RESULTS
The analytical, experimental and raw FE data (before zero setting) for the first
pipe is shown in table1. The analytical results are calculated from Euler –Bernoulli
beam equations. Using zero setting, the FE results are fitted to the experimental data.
Table 4.1

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A high resolution precise FE model was used here for investigation; however
it is not usually necessary to find the exact location of a crack in industrial
applications. If, for instance, a crack exists within 5cm intervals of the pipe fem, then
the FE model can predict the location of the crack with a ±2.5 cm error in the inverse
problem.
Table 4.1 Analytical, experimental and raw FE data of the uncracked first pipe (l=900
mm, di=16 mm, do=21 mm)
Analytical Experimental FE model
ω1 (Hz) 146.9 147.50 146.54
ω2 (Hz) 404.98 399.63 401.69
ω3 (Hz) 793.95 786.75 787.17
The depth of the crack is also a main parameter in the investigations. The
point in pipe crack detection problems is that the wall thickness is small and therefore
the crack size can be estimated with less computational effort.
The crack depths assumed here are: 1, 1.5,2 and 2.5mm for the first pipe, and
1, 2, 3 and 4 mm for the second pipe.The three surfaces of the first three natural
frequencies from FE model for the first pipe after zero setting are shown in Fig 4.4,
Fig 4.5 and Fig 4.6.
The first pipe is tested for a specific known crack. The crack was made by
means of a thin cutter perpendicular to the pipe axis, 300 mm from one end of the
pipe. The test was conducted for two crack depths: 1mm and 2mm. The measured
natural frequencies are shown in table 4.2
Fig 4.4 First natural frequency from FE model of the pipe1

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Fig 4.5 Second natural frequency from FE model of pipe1
Fig. 4.6 Third natural frequency from FE model of the pipe1
Table 4.2 Experimental data of the cracked pipe#1(300 mm from one end for d=1mm
and d=2mm)
Depth=1mm Depth=2mm
ω1 (Hz) 147.25 147.0
ω2 (Hz) 398.63 397.75
ω3 (Hz) 786.63 786.38
The intersection of the three constant frequency planes and the three surfaces
obtained from fe model is shown in Fig 4.7 and Fig 4.8. As it is demonstated in these
figures, the depth and location of the crack is predicted to be 1.8mm and 300mm; and
2.6 mm and 300mm which are very close to the real data.

26. 26
Fig. 4.7 Intersection of experimental natural frequency planes (1mm crack) and
surfaces of FE model of the first pipe
Fig 4.8 Intersection of experimental natural frequency planes (2mm crack) and
surfaces of FE model of 1st pipe
The FE results for the second pipe were also taken from the APDL runs. The
experimental and FE results for the second pipe are shown in the table 4. 3.the
analytical results based on euler-bernoulli beam formulas are not valid in this case
since the pipe is not slender.

27. 27
Table 4.3 Experimental and raw FE data of the uncracked first pipe (l=650 mm, do
=104.5 mm, di=112.5 mm)
FE results Experimental
ω1 (Hz) 1084.2 1083.5
ω2 (Hz) 1413.0 1411.1
ω3 (Hz) 1659.4 1658.1
Table 4.4 Experimental data of the 2nd pipe, cracked at 325mm from one end for
d=1.5mm
depth=1.5mm
ω1 (Hz) 1077.6
ω2 (Hz) 1412.0
ω3 (Hz) 1652.3
Fig. 4.9 Intersection of experimental natural frequency planes (1.5mm crack in
325mm from end) and surfaces of FE model of 2nd pipe
The test was run for a 1.5 mm crack depth cut at the middle of the second
pipe. The test results are tabulated in Table 4.4. The intersection curves of the ωn
planes with the three surfaces of the second pipe that were obtained from FE model is
shown in Fig. 4.9 and Fig. 4.10.

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Fig. 4.10 The intersection of FE natural frequencies plane (assumed 3 mm crack in
250mm from the end in a FE model) and surfaces of FE model of the 2nd pipe
5 CONCLUSIONS
Significant changes in natural frequencies of the vibrating pipes are observed
at the vicinity of crack location. When the crack location is constant but the crack
depth increases, the natural frequency of the pipe decreases.
Damage detection was carried out using natural frequencies obtained from FE
modal analyses of a pipe with variable crack location and size combined with
experimental natural frequencies. The method presented has limitations that should be
considered. If the pipe is relatively long which conforms to slender beam theory, the
FE simulation is sufficiently good to detect the crack using the first few modes. The
reason is that the bending modes are dominant and the modal stiffness is highly
affected by the presence of cracks. In Contrast, once the pipe diameter becomes large
compared to its length, the vibration pattern is mostly dominated by shell type modes.
The developed method is no longer simple and effective. This is due to two main
problems: first the interference of too many shell type mode which disturb the
frequency pattern. Secondly, the test facility and results does not usually have enough
resolution to detect the frequency changes due to the induced cracks, especially for
shell-modes. The low variation of modal stiffness of such modes with crack
parameters hardly appears in the test results. Further investigation has to be conducted
to extend similar techniques for large tubes and vessels.

29. 29
REFERENCES
[1] “Fault Detection Of Cracked Cantilever Beam Using Smart Technique”; Sutar
M.K, M.Tech Thesis; National Institute Of Technology, Rourkela; 2009.
[2] “Damage Detection In Beams By Wavelet Analysis”; Hüseyin Yanilmaz, M.S
Thesis; Middle East Technical University; 2007.
[3] ”Vibration Analysis Of Cracked Beam”; Prabhakar M.S, Thesis Of M. Tech;
National Institute Of Technology, Rourkela; 2009.
[4]http://site.ge-energy.com/prod_serv/serv/pipeline/en/insp_srvcs/crack_detection/
types_cracks.htm
[5] www.materialsengineer.com/CA-pipeline-failure.htm
[6] “The Applied Researches Of The Leak Before Break (Lbb) On The Marine
Pressure Pipes”; Zhaojun Li, Xiangbo Lv, Jinhua Bai, and Xinkai Liu; Institute Of
Nuclear Engineer, Beijing, China,2007.
[7] “Crack Detection In Aluminum Structures”; Brad A. Butrym, M.S. Thesis;
Virginia Polytechnic Institute And State University; 2010.
[8] “Material Science And Metallurgy”; V.D.Kodgire and S.V Kodgire; Everest
Publishing House; 25th Edition.
[9] “ Ultrasonic In-Line Inspection: High Resolution Crack Detection For Pipelines
Using A New Generation Of Tools”; A. Barbian, M. Beller, K. Reber, N. Uzelac, and
H. Willems; Ndt Systems & Services Ag, Stutensee, Germany, Ndt Systems &
Services Ag, Toronto, Canada.
[10] "How Do Defect Assessment Methods Influence The Design Of New In-Line
Inspection Tools?"; K. Reber and M. Beller, Proceedings Of The 5
th
International
Conference And Exhibition On Pipeline Rehabilitation & Maintenance, Pennwell,
Bahrain, 2002.
[11] “Improving Condition Assessment Of Buried Pipes Using Non-Contact
Ultrasound Based 3-D Crack Map Generation”; Shivprakash Iyer, and Sunil K.
Sinha; North American Society For Trenchless Technology (Nastt), Orlando, Florida;
2005.

30. 30
[12] “Crack Location In Pipes Using Modal Frequencies And Fem”; M.J. Mahboob,
A. Marzban and A.Shahsavri; The 11th International Conference On Vibration
Engineering, Timissoara, Romania, 2005.

31. 31
ACKNOWLEDGMENT
I Avail this opportunity to thank all those people who helped me in making
this seminar report a success.
I would especially like to extend my grateful thanks to Prof. A.V. DEOKAR
(seminar guide), who helped and enlightened me in every possible way. I am
indebted to him for bringing order to this report out of the chaos that was many times
presented to him.
I would like to express my respect, deep gratitude and regards to Prof. S.B.
JADHAV (Seminar Co-ordinator),and Prof. A.K.MISHRA (H.O.D. Mechanical
Department) for their moral support.
AKANSHA JHA
(T.E.MECHANICAL)