str654: inspection, maintenance and repair of steel …
TRANSCRIPT
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.
33
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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