failure bridge

8/10/2019 failure bridge

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CHAPTER 1: INTRODUCTION

Malaysia is moving towards achieving the status of a developed nation by the year 2020.

The government has invested substantially, particularly in public infrastructure to develop the

economy. The Public Works Department (PWD) Malaysia has to ensure that the defects and

failures of infrastructure are avoided. Otherwise, the investment by the government will be

jeopardized and huge cost needed to reconstruct structural damages. Thus precautions steps

should be taken by monitoring the crack on the structure.

Detection of crack in concrete structure is critical to the safe maintenance of the structure.

Cracks may result from the material defects that occur during the build process due to excessive

crack propagation and material failure. Thus it is important to have crack detector, it is a one

method to verify the robustness of any structure. Besides that, this method is a vital for the

maintenance process of the structure. Crack inspection is a valuable nondestructive test method

and it is design to check the propagation of crack in the structure.

In order to avoid the failure occurs, it is important to check the structure of the

infrastructure from time to time. Therefore, in this paper we proposed our inspection system to

solve all these kind of failure. This inspection system used the theory of the eddy current flow todetect the crack in the structure. This paper will discussed more on the design the system, how

the system work and the expected results that will be responds.



CHAPTER 2 LITERATURE REVIEW

2.1 Background

Internal crack on the building usually occurs when there have elastic deformation. The

elastic deformation may occur when the walls are unevenly loaded due to variation in stress in

different parts of wall. So, the crack may form in the walls. Other than that, shrinkage also may

occur in the walls that may lead to the internal crack. Shrinkage may occurs when the building

material absorb moisture from atmosphere and shrink whey they are dry. Cement made materials

shrink due to drying up of the moisture used in the construction. While the internal crack of the

bridge and building can escalate causing a catastrophic failure. Catastrophic failure is the

situation where there is a sudden and total failure of some system from which recovery is

impossible. Catastrophic failure often leads to cascading system failure. While cascading failure

is the situation where the failure in a system of interconnected parts in which the failure of a part

can trigger the failure of successive parts. Cascading failure usually begin when one part of the

system fails. When happened, the other components or nearby nodes also get failed due to the

failed components. Some example of the catastrophic failure of engineered structure includes:

Figure 1: Tay Bridge before collapse.

The Tay Bridge disaster on 1879 where the centre half mile of the bridge was completely

destroyed. The bridge was badly designed and its replacement was built as a separate structure

upstream of the old.

While in Malaysia, the most internal crack was due to excessive crack propagation and

material failures. The crack propagation is the situation where after the fracture initiation, any



widening, lengthening or increase in the number of crack. This is normally happen to the brittle

material, but a stable crack growth can lead to significant energy absorption and therefore a

relatively tough behavior. Mean while material failure is the loss of load carrying material unit. In

fact, the material failure can be examine in different scales. From microscopic to macroscopic the

definition per se were introduced. The material failure can be distinguished in two broader

categories depending on the scale in which the material is examined. Problem in existing bridges

are often variously identified by term. Some of the term was “defects”. “Damage”,

“deterioration”, and even “bridge failure”.

2.2 BRIDGE COMPONENTS AND THEIR PROBLEM

A bridge is fundamentally a structural system comprising many components with their own

role that have different structural and functional roles. Deck slab, girders, abutments and piers are

the primary members that transmitting and bringing the load from the bridge deck down through

the foundation. The secondary member also important that included of expansion joints and

bearings, provide the articulation needed by the system to accommodate movement due to load

and temperature changes. Parapets serve to support the bridge with the load of the vehicle within

the carriageway while gullies and downpipes bring the water from the deck down to the ground

and away from the bridge components. Primary or secondary components, the role to the bridge

are still important. Once one of the components fails to function as designed, the problem would be arise and would make a bigger problem. The Road Engineering Association Malaysia

(REAM) bridge inspection guide has identified this entire problem as “damage”. The “damage”

describes each of them as a basis for condition rating. Some of the problem that usually occurs to

the bridge in Malaysia is:

1. Problems in concrete members

2. Problem in steel members

3. Bearing problems

4. Joint problems

5. Hydraulic problems

6. Excessive vibrations

7. Impact of vehicles

8. Vegetation growth



The example of the damage of bridge components that occurs in Malaysia:

Figure 2: Vegetation growth at bridge abutment

Figure 3: Damage at underside due to vehicular impact

2.3 PREVIOUS RESEARCH ON CRACK DETECTION USING EDDYTHERM.

In many industries, crack inspection is a valuable non-destructing test method. According

to the valuable term, there is a growing need for powerful crack detection methods. For that

purpose, thermography has increasingly gained in significant these past several years. The

developments of the thermography were focused in the combination with eddy current inspection

has resulted in EddyTherm inspection method. In the process, high-frequency generators

generate, through induction coils, continuous alternating magnetic fields that cause the current to

flow and the part under inspection to heat up rapidly. The EddyTherm method uses a high

frequency current to heat the part to moderate, defined overall temperature. In the process, cracks

disturb the flow of the induced currents and affect the temperature distribution on the part surface

so that cracks can be visualized using a suitable thermographic camera.



Figure 4: The example of the current induced [2].

In the figure 4 shows that the induced current that flow in the inspection part. The flow of

the induced current was stable if the inspection part in the good condition (without crack). While

the cracked part make the flow of the Eddy Current not in linear and make the temperature

separation worst. The induced current would make the temperature rise. The induced current floe

in the uncraked part is smooth compared to the cracked part. As the result, the temperature

separation would be different. The different of the temperature can be seen by using suitable

thermographic camera. The schematic diagram shows that, the changed current flow through the

crack and the different in temperature at the crack tips and the colder regions at the crack flanks.

Figure 5: Computed temperature profile in the cracked metallic specimen [2].

In the figure 5 show that the different between the colder region and the hotter region due

to the difference in the induced current flow to the specimen. While the cracked specimen would

make the difference in the induced current flow that results the difference in temperature rising of

the specimen.



CHAPTER 3 PRINCIPAL OF MEASUREMENT AND INSPECTION

The paper proposed non-destructive test by using eddy current. The principle of the eddy

current technique is based on the interaction between a magnetic field source and the test

material. Eddy currents are closed loops of induced current circular motion in planes

perpendicular to the magnetic flux. The fields generated can cause repulsive, attractive,

propulsion, drag and heating effects. The bigger the magnetic field produced, the faster the field

changes thus the higher the eddy current generated. When alternating current is applied to the

conductor, such as copper wire, a magnetic field develops in and around the conductor. This

magnetic field expands as the alternating current rises to maximum and collapses as the current is

reduced to zero. The current will be induced in this second conductor, if another electrical

conductor is brought into the close to this changing magnetic field. Eddy currents are induced

electrical currents that flow in a circular path. Eddy Current gets their name from “eddies” that

are formed when a liquid or gas flows in a circular path around obstacles when conditions are

right. Eddy currents travel parallel to the coil‟s winding and the flow is limited to the area of the

inducing magnetic field.

Figure 6: How Eddy Current works

Figure 6 shows how the Eddy Current works where at step „a‟ the alternating current

flowing through the coil at chosen frequency generates magnetic fields around the coil. At step

„b‟ shows when the coil is placed closed to electrically conductive material, eddy c urrent will

induced in the material. In step „c‟ shows that if the flaw in the conductive material disturbs the

eddy current circulation, the magnetic coupling with the probe changed and a defect signal can be

read by measuring the coil impedance variation.



Eddy current focus near to the perpendicular surface to an excitation coil and their

strength decreases with distance from the coil. This phenomenon is known as the skin effect as

illustrate in Figure 6. Skin effect produce when the eddy currents flowing in the test object at any

depth produce magnetic field which oppose the primary field, thus reducing the net magnetic flux

and causing a decrease in current flow as the depth increases. The penetration depth can be

calculated using formula in equation 1 below:

√ Equation 1

Where:

= the penetration depth (m)

= the frequency (Hz)

= the magnetic permeability of material (H/m)

= the electrical conductivity of the material (S/m).

Besides that, eddy current suitable in the inspection of ferromagnetic and non-

ferromagnetic material. From the formula, non-ferromagnetic materials have the value of

magnetic permeability, value of 1 while ferromagnetic materials have the value of magnetic

permeability, value of larger than 1 [5].

The depth penetration of non ferromagnetic material is better than ferromagnetic material.

This means that, eddy current suitable to inspect the surface of ferromagnetic material such as

carbon steel surface material and it also suitable to inspect internal crack such as concrete

structural.

Figure 7: Illustrate diagram eddy current field depth of penetration and density [5].



Every coil is characterized by the impedance parameter Z 0, which is a complex number

shows in the equation 2 and which represents the voltage-current ratio (V 0/ I 0) for a single

frequency sinusoidal excitation f . Impedance Z 0 has a magnitude | Z | and a phase :

Equation 2

A time-varying magnetic field creates when an alternating current energizes a coil. The

magnetic lines of flux tend to focus at the center of the coil. Eddy current inspection is involved

with Faraday‟s electromagnetic induction law. Equation 3 shows the Faraday electromagnetic

induction law. Faraday had found that a time-varying magnetic induction flux density induces

currents in an electrical conductor. The electromotive force ε is directly proportional to the time-

rate change of the magnetic induction flux density Φ B:

Equation 3

When an alternating energized coil of impedance Z 0 approaches an electrically conductive

non-ferromagnetic material, the primary alternating magnetic field penetrates the material and

generates continuous and circular eddy currents. The induced currents flowing within the test will

generate a secondary magnetic field that tends to oppose the primary magnetic field, as shown in

figure 8.

Figure 8: Primary and secondary magnetic field



The primary magnetic field will has weakening effect because this opposing magnetic

field, coming from the conductive material. So, the new imaginary part of the coil impedance

decreases proportionally when the eddy current intensity in the test piece increases. Eddy currents

also contribute to the increasing of the power dissipation of energy that changes the real part of

coil impedance.

In order to generate currents for an inspection, a probe is used. A length of electrical

conductor which is formed into a coil is called the probe. The figure 5 shows the probe.

Figure 9: Example probes

The cracks can be detected by the change of the Eddy Current flow. The figure 6 shows

the comparison between cracks and non cracks that affect the flow of Eddy Current

Figure 10: comparison between cracks and non cracks that affect the flow of Eddy Current.



CHAPTER 4 METHOD OF INSPECT

4.1 Eddy current scanning Method using differential probe

Eddy current scanning method is using probe as device to collect data from the specimen.

The test is implemented to gain reflection signal from the test piece and the signal being

processed and display based on the instrument settings. The instrument voltage range of peak

from 0 to 9v and the frequency range must within 5 to 10 MHz to penetrate the specimen. Both

impedance plane and voltage plane are used for testing. The differential probe of bi-directional

sensitivity is used to scan the specimens. The probe is can detect crack in plain and weld areas

especially steel components. It consists of two circular coils with axes parallel to the surface and

perpendicular to each other. The coil is used to compares the area that influenced by its own

magnetic field to the area influenced by the other coil. In the event, both coils are simultaneously

affected by the same material conditions; there will no signal, meaning that the bridge is

balanced. Any electromagnetic condition that is not common areas of the specimen being tested

will produce an imbalance in the system and will be detected in process, since the coils are

located in close proximity to each other, gradual changes caused by conductivity or permeability

are limited. Lift off effects are minimized, but not suppressed, as both coils are simultaneously

affected when the distance to probe surface changes. The maximum probe response is observed

when the crack is perpendicular to the direction of either winding coil. Minimal response occurwhen the crack is at a 45 degree angle from the direction of the winding coil having

approximately the same impedance, both coil are electrically connected to oppose each other.

Figure 11: Shows the scanning angle and direction of the Eddy Current method [1].

Figure 11 show a block diagram of eddy current device and its configuration. The device

used a single tone generator which is functioning to energize the test coil sensor. The frequency,

phase and amplitude are adjusted to optimum parameter for the work pieces. From the block

diagram the amplifier is used to control the frequency of eddy current in order it to penetrate

work pieces deeply and detect crack not only on the surface of test piece but fulfill the

requirement to test thick beam of metal since bridges are commonly made up from the greatthickness beam to support heavy load. When crack detected the coil impedance adaptation



experience changes then the demodulator will extract the data signal that caused by the

impedance variation and visualize and display it on the screen of oscillator as XY plot graph.

Figure 12: Show block diagram and configuration of eddy current scanning device.

4.2 Standard Calibration Blocks and Crack Specimens

By using the surface of 2 reference standard steel specimens made from ASTM A588 (A709

Grade 50) and A36 (A709 Grade 36), 3 electro discharge machining (EDM) notches were

produced with different depths of 0.2mm, 0.5mm, and 1.0mm (±0.1mm). These specimens were

measured on the basic condition, where annealing and demagnetize was not apply to the

specimen. This allowed the study in effect of conductive coating on crack indications by the

surface of reference standard A588 steel specimen with inorganic zinc-based coating and lead-

based coating.

Power

source

Amplifier Impedance

adaptation Probe

Pre-filterDemodulator

Test piece

Oscillator



Figure 13: EDM Standard Calibration Drawing.

In order to study and analyze the crack-depth profile and to compare a standard EDM

calibration curve with a fatigue crack-depth curve, a second set of 5 fatigue crack specimens was

used with specification of A36 (A709 Grade 36) as material, with depths of 0.8mm, 1.0mm,

1.5mm, 2.0mm, and 3.0mm (±0.25mm) and lengths of 20mm.

Figure 14: Fatigue Crack Specimen Drawing.



Lastly, a third set of 3 steel specimens A588 is being tested with surface-breaking cracks.

2 of the specimens are welded with a tee joints representing either web/stiffener or web/flange

connections, then the third specimen is welded with a butt joint typically used for flange or web

splices. Both of the joints are a complete fillet weld and penetration groove welds respectively.

These specimens are subjected to the corrosive environment condition and represented as cracks.



CHAPTER 5 EXPECTED RESULT

Figure 15: Eddy current distribution with depth in a thick plate and resultant phase lag

(IAEA-TECDOC, 2008) [8].

From the figure shown, the density of eddy current is exponential with depth. It also been

related to the standard depth of penetration because the density is decreasing to 1/e or in other

word 36.8% of the surface. The standard penetration showed that the plane wave electromagnetic

field excitation within the test sample. Based on the standard penetration, a formula has beenderived [7]:

Or

Where



The eddy current density is influenced the sensitivity of a defect location. They are

decreasing rapidly in depth although eddy current ability can penetrates deeper than one standard

depth of penetration.

Figure 16: The Standard Depth of Penetration for Carbon Steel, Stainless Steel,

Aluminum and Copper [9].

From the figure 16 above, it shown that, the frequency affects the standard depth of

penetration. Based on the formula given it showed that the higher the frequency the lower the

depth of penetration. The effective depth of penetration usually defined as three time of the

standard depth where the eddy current density is decreasing to 5% of its initial value.

Figure 17: Typical eddy current instrument and display [9].



From the figure 17 above, it shown that typical reading of eddy current machine based on

cracking of varying depths. In the graph shown, the cracking section can be detected through the

amplitude and the phase of response.

Figure 18: Impedance diagram showing the conductivity curve, and the thickness curve [7].

From the figure 18 above, the test frequency is set to 90 to be tested for the thickness of

the material. The operating point is absolutely at the bottom of the thickness curve, and the lift-

off curve from this point is approximately at 90 to the thickness curve. The eddy currentinstrument display is normally set up with the lift-off trace horizontal to the left. Material

significantly thicker use because the sensitivity to changes in thickness reduces rapidly below at

90 operating point. It also can be concluded that the thickness curve crosses the conductivity

curve as the thickness approaches the effective depth of penetration. This means that, when

operating in this region of the curve, the same lift-off trace would be obtained from two different

thicknesses of material.



CONCLUSION

As the conclusion, this paper is about solving the structural problem in building and

bridges which is made ferrous and non ferrous material by recommends the use of crack

detections system. The system involves eddy current working principle as the basic knowledge to

design the crack detector for non ferrous and ferrous material. From the research, eddy current

principle is valid to check the crack propagation in neither building nor bridges structure. The

depth of eddy current for inspection can be defined by using the equation 1. This paper proposed

the crack detection system as shown in figure 12. The expected result from this system predicted

to be in graph. From the graph the types of crack can be determined based on the output signal

shown.



REFERENCES

1. Highway Administration, November 2000. “Detection on Sizing of Cracks in

Structural Steel Using Eddy Current”. U.S department Transportation.

2. G. ZENZINGER , J. BAMBERG, M. DUMM, P. NUTZ. “Crack Detection Using Eddy

Therm”. MTU AERO E NGINES, D-80995 MUNICH, GERMANY.

3. Koichi Morita and Kazuya Noguchi. “Crack Detection Methods Using Radio

Frequency Identification and Electrically Conductive Materials”. National Institute

for Land and Infrastructure Management, Tsukuba-shi, Ibaraki-ken, 305-0802,

Japan.

4. Libor TOPOLAR, Lubos PAZDERA, Jaroslav SMUTNY, Vlastimil BILEK,

Marta KORENSKA, Veronika RODRIGUEZOVA (2012). “Crack Detection of

Concrete Slab made from Different Concrete Mixtures by Acoustic Emission

Method”. 30th European Conference on Acoustic Emission Testing & 7th

International Conference on Acoustic Emission University of Granada, 12-15

September 2012.

5. ND. Eddy Current Theory. Retrieve from:

www.innospection.com/pdfs/Eddy%20Current%20Theory.pdf. (Surf on 20

November 2011).

6. Hyeong-Gyeong Moon and Jung-Hoon Kim. “Intelligent Crack Detecting

Algorithm On The Concrete Crack Image Using Neural Network. Department of

Civil and Environmental Engineering, Yonsei University, Seoul, Korea.

7. Smith C.H., Schneider R.W., Dogaru T., and Smith S.T. (2003). EDDY-

CURRENT TESTING WITH GMR MAGNETIC SENSOR ARRAYS. Review of

Progress in Quantitative Non destructive Evaluation. Vol. 2323

8. Eddy Current Testing at Level 2: Manual for the Syllabi Contained in

IAEATECDOC-628/Rev. 2, „Training Guidelines for Non-Destructive Testing

Techniques‟

9. Hansen J. (2004). The Eddy Current Inspection Method, Part 1. History and

electrical theory. Insight. Vol. 46 No 5

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