Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 1

STR654:

INSPECTION, MAINTENANCE

and REPAIR of STEEL

STRUCTURES

Lecture 4a

Professor of Steel Structures and Bridges

Faculty of Engineering,

Cairo University1

Sherif A. Mourad

TOPICWeld Defects

Weld Inspection.

Radiography.

Magnetic particle crack detection.

Dye penetrant testing.

Ultrasonic flow detection.

Eddy current & electromagnetic.2

Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 2

TOPIC

Other NDT methods.

Electrical potential drop.

Sonics.

Infra-red.

Accoustic emmition.

Spectrography.

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WELD DEFFECTS

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Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 3

WELD DEFFECTS

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WELD DEFFECTS

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Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 4

VISUAL

INSPECTION

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VISUAL

INSPECTIONOptical and visual testing, is a Non destructive testing

where the test parts are visually checked. The

following defects can be found with this inspection:

• Impurities

• Faulty or incomplete assembly, connection

• Burr formation untercuts and cracks

• change of color (tarnishing with for example

austenitic steels)

• Surface roughness

• Lack of fusion

Visual testing is limited to only detect defects on the

surface of a test part. 8

Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 5

MAGNETIC

PARTICLE

Suitable for detection of surface

and near-surface discontinuities.

Suitable for magnetic materials

only – mainly ferric steel and iron.

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CONCEPT

Generate magnetic flux.

Flux lines running at right angle to suspected defect.

At discontinuity, flux lines stray.

Crack edge becomes magnetic and attracts iron particles.

Detects defects which may not be visible.

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Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 6

Magnetic Particle

Inspection

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APPLICATION

Particles are typically iron oxide, 20

to 30 microns.

Suspension in fluid provides

mobility.

May be applied as dry powder.

May be coated with fluorescent

color.12

Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 7

HOW TO

GENERATE FLUXPermanent magnet at the surface.

Direct generation of electric current: Pass current through the subject, and magnetic flux is normal to the current direction.

Indirectly:

Pass current through a coil.

Use the test piece as a yoke.13

HOW TO

GENERATE FLUX

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Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 8

Magnetic Particle

Inspection

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MAGNETIZATION

DIRECTIONIt is necessary to magnetize a test

specimen in at least two

perpendicular directions (slow).

Modern techniques (multi-

directional magnetization) requires

only one inspection.

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Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 9

ILLUSTRATION

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ADVANTAGES

Simple to apply and operate.

Quantitative.

May be automated.

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Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 10

DISADVANTAGES

Restricted to ferromagnetic materials.

Restricted to surface or near-surface

flaws.

Lack of indication may be either

because there are no defects or that

the process was not carried properly.

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DYE PENETRANT

For surface breaking flaws in non-

ferrous metals.

Based on the penetration of a low

viscosity fluid into the defects.

Penetrant is usually a very thin oil,

dyed bright red or ultraviolet

fluorescent.20

Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 11

PROCEDURE

Surface is chemically cleaned to remove any material, grease, dirt, ...

Penetrant is applied and allowed to remain in contact for 15 minutes.

Capillary action draws penetrant into cracks.

Surplus penetrant is removed.

A thin coat of powdered chalk is applied.

Chalk draws the dye out of the cracks, giving visual indication in contrast with background.

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DYE PENETRANT

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Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 12

DYE PENETRANT

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ADVANTAGES

Simplicity in operation.

Best method for surface breaking

cracks in non-ferrous metals.

Suitable for automation.

Quantitative.

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Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 13

DISADVANTAGES

Restricted to surface breaking

cracks only.

Decreased sensitivity.

Uses a considerable amount of

consumables.

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RADIOGRAPHY

X & Gamma rays. Suitable for damage detection in ferrous

and non-ferrous metals.

Penetrating radiations is differentially absorbed by the material through which it passes.

X-rays are generated electrically.

Gamma rays are emitted from radioactive isotopes.

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Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 14

RADIOGRAPHY

X & Gamma rays.

X-ray and Gamma rays also have the property (like light) to convert silver halide crystals in a photographic film to metallic silver, in proportion to the intensity of the radiation reaching the film, forming a latent image.

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Radiography

(X & Gamma rays)

A latent image is formed on the film,

developed and fixed like normal photos.

The greater the thickness, the greater the

absorption.

The denser the material, the greater the

absorption.

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Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 15

Radiography

(X & Gamma rays)

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X rays

Generated electrically.

Penetration power is determined by the

volt applied to the X-ray tube.

For steel, 1000 volts per inch thickness.

Exposure time is expressed in

milliampere minutes, and governed by

the amperage of the cathode.

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Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 16

Gamma rays

Emitted from radioactive isotopes.

Penetration power governed by isotope.

Iridium 192 for steel 12 to 25 mm thick.

Caesium 134 for steel 18 to 62 mm thick.

Intensity of radiation is set at the time of supply, and reduces over a period of time.

Time of exposure increases as the isotope decays.

Half life: Time to decay to half the amount.

Iridium: 74 days, caesium: 2.1 years.

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Procedure

Materials is placed between film and source.

Voids show as darkened areas, where more radiation has reached the film.

Film package may include intensifying screens.

As the isotope is continuously emitting radiation, it must be placed in a shielded box.

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Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 17

Advantages

Information is presented

pictorially.

Provides a permanent record.

May be used for thin sections.

Sensitivity is declared on each film.

Suitable for any material.

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Disadvantages

Ineffective for thick sections.

Possible health hazard.

Needs accurate direction.

Require film processing facility.

Not suitable for automation.

Not suitable for surface defects.34

Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 18

ULTRASONIC

Used to detect internal and surface defects in sound-conducting materials.

Similar to echo sounding. A short pulse is generated by applying

an electric charge to a piezo-electric crystal.

The crystal vibrates for a very short period at a frequency related to the thickness of the crystal.

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ULTRASONIC

In flaw detection the frequency is in

the range of 1 to 6 MHz.

Sound waves at such high frequency

can travel long distances with little

attenuation.

Travel velocity is related to Young’s

Modulus (in steel, 5900 m/s, in .36

Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 19

ULTRASONIC

Ultrasonic waves attenuate in air.

The waves shall reflect upon reaching an interface.

Oscillating crystal is incorporated into a hand-held probe.

A couplant is applied to ensure full contact between the probe and test piece.

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ULTRASONIC

The crystal oscillates in short pulses,

and is quiescent in between.

The crystal coverts electric pulses to

mechanical oscillations and vice

versa.

Returning pulses are detected and

converted to electricity.38

Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 20

ULTRASONIC

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ULTRASONIC

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Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 21

ADVANTAGES

Can test thickness & length up to 10 m.

Position, size & type of defect can be determined.

Instant test results.

Portable.

Extremely sensitive (if required).

Access to one side only.

No consumables.41

DISADVANTAGES

No permanent record.

Operator may decide whether the test

piece is defective or not during the test.

Indications require interpretation.

Requires skill for interpretation of

measurement.

Difficult to test very thin sections.42

Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 22

EDDY CURRENT

Used to detect surface or subsurface

flaws, conductivity measurements,

and coating thickness measurement.

Technique is sensitive to material

conductivity, permeability, and

dimensions.

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EDDY CURRENT

Eddy currents may be produced in any electrically conducting metal subject to alternating magnetic field.

The alternating magnetic field is generated by passing an alternating current through a coil.

The presence of cracks is noted by impedance change.

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Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 23

EDDY CURRENT

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COIL

CONFIGURATION

Coil with single winding.

Driven pair.

Transformer type: Primary coil, and

one or two secondary ones. The

primary is typically wound over the

secondary.46

Sherif A. Mourad - Faculty of

Engineering, Cairo University

November 2020

STR654 Lecture 4a 24

ADVANTAGES

Suitable for determining defects, composition, hardness, conductivity, permeability, thickness,…

Simple information may be provides, and more sophisticated (phase) data is available.

Extremely compact and portable. No consumables. Different probes for different applications. Suitable for total automation.

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DISADVANTAGES

The wide range of parameters that affect the eddy current response means that the signal from the desired material characteristic may be masked by an unwanted parameter.

Testing is typically restricted to surface breaking conditions and slightly subsurface flaws.

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