NON DESTRUCTIVE TESTING
Electrical potential drop, sonics, infra-red, acoustic emission and
spectrography,
Radiography
Magnetic Particle Crack Detection
Dye Penetrant Testing
Ultrasonic Flaw Detection
Eddy Current and Electro-magnetic Testing
Radiography - X And Gamma
This technique is suitable for the detection of internal defects in ferrous and nonferrous
metals and other materials
X-rays, generated electrically, and Gamma rays emitted from radio-active isotopes,
are penetrating radiation which is differentially absorbed by the material through
which it passes; the greater the thickness, the greater the absorption. Furthermore,
the denser the material the greater the absorbtion.
In X-radiography the intensity, and therefore the exposure time, is governed by the
amperage of the cathode in the tube. Exposure time is usually expressed in terms of
milliampere minutes. With Gamma rays the intensity of the radiation is set at the time
of supply of the isotope. The intensity of radiation from isotopes is measured in
Becquerel’s and reduces over a period of time.
Recent developments in radiography permit ‘real time’ diagnosis. Such techniques
as computerised tomography yield much important information, though these
methods maybe suitable for only investigative purposes and not generally employed
in production quality control.
3 Magnetic Particle Inspection
This method is suitable for the detection of surface and near surface discontinuities in
magnetic material, mainly ferritic steel and iron.
The principle is to generate magnetic flux in the article to be examined, with the flux
lines running along the surface at right angles to the suspected defect. Where the
flux lines approach a discontinuity they will stray out in to the air at the mouth of the
crack. The crack edge becomes magnetic attractive poles North and South. These
have the power to attract finely divided particles of magnetic material such as iron
fillings.
in some instances they can be applied in a dry powder form.
The particles can be red or black oxide, or they can be coated with a substance,
which fluoresces brilliantly under ultra-violet illumination (black light). The object is to
present as great a contrast as possible between the crack indication and the material
background.
then the indirect method must be used. This can be one of two forms:
1. Passing a high current through a coil that encircles the subject.
2. Making the test piece form part of a yoke, which is wound with a current
carrying coil. The effect is to pass magnetic flux along the part to reveal
transverse and circumferential defects
4 Dye Penetrant Testing
This method is frequently used for the detection of surface breaking flaws in nonferromagnetic
materials.
The subject to be examined is first of all chemically cleaned, usually by vapour
phase, to remove all traces of foreign material, grease, dirt, etc. from the surface
generally, and also from within the cracks.
Next the penetrant (which is a very fine thin oil usually dyed bright red or ultra-violet
fluorescent) is applied and allowed to remain in contact with the surface for
approximately fifteen minutes. Capillary action draws the penetrant into the crack
during this period.
The surplus penetrant on the surface is then removed completely and thin coating of
powdered chalk is applied.
After a further period (development time) the chalk draws the dye out of the crack,
rather like blotting paper, to form a visual, magnified in width, indication in good contrast to the background
5 Ultrasonic Flaw Detection
This technique is used for the detection of internal and surface (particularly distant
surface) defects in sound conducting materials.
The principle is in some respects similar to echo sounding. A short pulse of
ultrasound is generated by means of an electric charge applied to a piezo electric
crystal, which vibrates for a very short period at a frequency related to the thickness
of the crystal. In flaw detection this frequency is usually in the range of one million to
six million times per second (1 MHz to 6 MHz). Vibrations or sound waves at this
frequency have the ability to travel a considerable distance in homogeneous elastic
material, such as many metals with little attenuation.
Ultrasonic energy is considerably attenuated in air, and a beam propagated through a
solid will, on reaching an interface (e.g. a defect, or intended hole, or the backwall)
between that material and air reflect a considerable amount of energy in the direction
equal to the angle of incidence.
For contact testing the oscillating crystal is incorporated in a hand held probe, which
is applied to the surface of the material to be tested. To facilitate the transfer of
energy across the small air gap between the crystal and the test piece, a layer of
liquid (referred to as ‘couplant’), usually oil, water or grease, is applied to the surface.
the crystal does not oscillate continuously but in short
pulses, between each of which it is quiescent. Piezo electric materials not only
convert electrical pulses to mechanical oscillations, but will also transduce
mechanical oscillations into electrical pulses; thus we have not only a generator of
sound waves but also a detector of returned pulses. The crystal is in a state to detect
returned pulses when it is quiescent. The pulse takes a finite time to travel through
the material to the interface and to be reflected back to the probe.
The standard method of presenting information in ultrasonic testing is by means of a
cathode ray tube, in which horizontal movement of the spot from left to right
represents time elapsed. The principle is not greatly different in digitised instruments
that have a LCD flat screen. The rate at which the spot moves is such that it gives
the appearance of a horizontal line on the screen. The system is synchronised
electronically so that at the instant the probe receives its electrical pulse the spot
begins to traverse the screen. An upward deflection (peak) of the line on the left
hand side of the screen is an indication of this occurrence. This peak is usually
termed the initial pulse.
Since the velocity of sound in any material is characteristic of that material, it follows
that some materials can be identified by the determination of the velocity.When the velocity is constant, as it is in a wide range of steels, the time taken for the
pulse to travel through the material is proportional to its thickness. Therefore, with a
properly calibrated instrument, it is possible to measure thickness from one side with
an accuracy in thousandths of an inch. This technique is now in very common use.
A development of the standard flaw detector is the digital wall thickness gauge. This
operates on similar principles but gives an indication, in LED or LCD numerics, of
thickness in absolute terms of millimetres. These equipments are easy to use but
require prudence in their application.
6 Eddy Current and Electro-Magnetic Methods
The main applications of the eddy current technique are for the detection of surface
or subsurface flaws, conductivity measurement and coating thickness measurement.
The technique is sensitive to the material conductivity, permeability and dimensions
of a product.
Eddy currents can be produced in any electrically conducting material that is
subjected to an alternating magnetic field (typically 10Hz to 10MHz). The alternating
magnetic field is normally generated by passing an alternating current through a coil.
The coil can have many shapes and can between 10 and 500 turns of wire.
The magnitude of the eddy currents generated in the product is dependent on
conductivity, permeability and the set up geometry.
When a crack, occurs in the product surface the eddy currents must travel
farther around the crack and this is detected by the impedance change.
Most eddy current electronics have a phase display and this gives an operator the
ability to identify defect conditions. In many cases signals from cracks, lift off and
other parameters can be clearly identified. Units are also available which can inspect
a product simultaneously at two or more different test frequencies. These units allow
specific unwanted effects to be electronically cancelled in order to give improved
defect detection.
The eddy current test is purely electrical. The coil units do not need to contact the
product surface and thus the technique can be easily automated. Most automated
systems are for components of simple geometry where mechanical handling is
simplified.
Aerospace
Industry
Testing components including aero-engine, Landing gear and air frame
parts during production
Aircraft Overhaul
Testing components during overhaul including aero-engine and landing
gear components
Automotive
Industry
Testing Brakes-Steering and engine safety critical components for flaws
introduced during manufacture. Iron castings – material quality. Testing of
diesel engine pistons up to marine engine size.
Petrochemical &
Gas Industries
Pipe-Line and tank internal corrosion measurement from outside. Weld
testing on new work. Automotive LPG tank testing
Railway Industry
Testing locomotive and rolling stock axles for fatigue cracks. Testing rail
for heat induced cracking. Diesel locomotive engines and structures.
Mining Industry
Testing of pit head equipment and underground transport safety critical
components.
Agricultural
Engineering
Testing of all fabricated, forged and cast components in agricultural
equipment including those in tractor engines.
Power Generation
Boiler and pressure vessel testing for weld and plate defects both during
manufacturing and in subsequent service. Boiler pipe work thickness
measurement and turbine alternator component testing.
Iron Foundry
Testing ductile iron castings for metal strength on 100% quality control
basis.
Shipbuilding
Industry
Structural and welding testing. Hull and bulkhead thickness measurement.
Engine components testing.
Steel Industry
Testing of rolled and re-rolled products including billets, plate sheet and
structural sections.
Pipe & Tube
Manufacturing
Industry
Raw plate and strip testing. Automatic ERW tube testing. Oil line pipe
spiral weld testing.
Electrical potential drop, sonics, infra-red, acoustic emission and
spectrography,
Radiography
Magnetic Particle Crack Detection
Dye Penetrant Testing
Ultrasonic Flaw Detection
Eddy Current and Electro-magnetic Testing
Radiography - X And Gamma
This technique is suitable for the detection of internal defects in ferrous and nonferrous
metals and other materials
X-rays, generated electrically, and Gamma rays emitted from radio-active isotopes,
are penetrating radiation which is differentially absorbed by the material through
which it passes; the greater the thickness, the greater the absorption. Furthermore,
the denser the material the greater the absorbtion.
In X-radiography the intensity, and therefore the exposure time, is governed by the
amperage of the cathode in the tube. Exposure time is usually expressed in terms of
milliampere minutes. With Gamma rays the intensity of the radiation is set at the time
of supply of the isotope. The intensity of radiation from isotopes is measured in
Becquerel’s and reduces over a period of time.
Recent developments in radiography permit ‘real time’ diagnosis. Such techniques
as computerised tomography yield much important information, though these
methods maybe suitable for only investigative purposes and not generally employed
in production quality control.
3 Magnetic Particle Inspection
This method is suitable for the detection of surface and near surface discontinuities in
magnetic material, mainly ferritic steel and iron.
The principle is to generate magnetic flux in the article to be examined, with the flux
lines running along the surface at right angles to the suspected defect. Where the
flux lines approach a discontinuity they will stray out in to the air at the mouth of the
crack. The crack edge becomes magnetic attractive poles North and South. These
have the power to attract finely divided particles of magnetic material such as iron
fillings.
in some instances they can be applied in a dry powder form.
The particles can be red or black oxide, or they can be coated with a substance,
which fluoresces brilliantly under ultra-violet illumination (black light). The object is to
present as great a contrast as possible between the crack indication and the material
background.
then the indirect method must be used. This can be one of two forms:
1. Passing a high current through a coil that encircles the subject.
2. Making the test piece form part of a yoke, which is wound with a current
carrying coil. The effect is to pass magnetic flux along the part to reveal
transverse and circumferential defects
4 Dye Penetrant Testing
This method is frequently used for the detection of surface breaking flaws in nonferromagnetic
materials.
The subject to be examined is first of all chemically cleaned, usually by vapour
phase, to remove all traces of foreign material, grease, dirt, etc. from the surface
generally, and also from within the cracks.
Next the penetrant (which is a very fine thin oil usually dyed bright red or ultra-violet
fluorescent) is applied and allowed to remain in contact with the surface for
approximately fifteen minutes. Capillary action draws the penetrant into the crack
during this period.
The surplus penetrant on the surface is then removed completely and thin coating of
powdered chalk is applied.
After a further period (development time) the chalk draws the dye out of the crack,
rather like blotting paper, to form a visual, magnified in width, indication in good contrast to the background
5 Ultrasonic Flaw Detection
This technique is used for the detection of internal and surface (particularly distant
surface) defects in sound conducting materials.
The principle is in some respects similar to echo sounding. A short pulse of
ultrasound is generated by means of an electric charge applied to a piezo electric
crystal, which vibrates for a very short period at a frequency related to the thickness
of the crystal. In flaw detection this frequency is usually in the range of one million to
six million times per second (1 MHz to 6 MHz). Vibrations or sound waves at this
frequency have the ability to travel a considerable distance in homogeneous elastic
material, such as many metals with little attenuation.
Ultrasonic energy is considerably attenuated in air, and a beam propagated through a
solid will, on reaching an interface (e.g. a defect, or intended hole, or the backwall)
between that material and air reflect a considerable amount of energy in the direction
equal to the angle of incidence.
For contact testing the oscillating crystal is incorporated in a hand held probe, which
is applied to the surface of the material to be tested. To facilitate the transfer of
energy across the small air gap between the crystal and the test piece, a layer of
liquid (referred to as ‘couplant’), usually oil, water or grease, is applied to the surface.
the crystal does not oscillate continuously but in short
pulses, between each of which it is quiescent. Piezo electric materials not only
convert electrical pulses to mechanical oscillations, but will also transduce
mechanical oscillations into electrical pulses; thus we have not only a generator of
sound waves but also a detector of returned pulses. The crystal is in a state to detect
returned pulses when it is quiescent. The pulse takes a finite time to travel through
the material to the interface and to be reflected back to the probe.
The standard method of presenting information in ultrasonic testing is by means of a
cathode ray tube, in which horizontal movement of the spot from left to right
represents time elapsed. The principle is not greatly different in digitised instruments
that have a LCD flat screen. The rate at which the spot moves is such that it gives
the appearance of a horizontal line on the screen. The system is synchronised
electronically so that at the instant the probe receives its electrical pulse the spot
begins to traverse the screen. An upward deflection (peak) of the line on the left
hand side of the screen is an indication of this occurrence. This peak is usually
termed the initial pulse.
Since the velocity of sound in any material is characteristic of that material, it follows
that some materials can be identified by the determination of the velocity.When the velocity is constant, as it is in a wide range of steels, the time taken for the
pulse to travel through the material is proportional to its thickness. Therefore, with a
properly calibrated instrument, it is possible to measure thickness from one side with
an accuracy in thousandths of an inch. This technique is now in very common use.
A development of the standard flaw detector is the digital wall thickness gauge. This
operates on similar principles but gives an indication, in LED or LCD numerics, of
thickness in absolute terms of millimetres. These equipments are easy to use but
require prudence in their application.
6 Eddy Current and Electro-Magnetic Methods
The main applications of the eddy current technique are for the detection of surface
or subsurface flaws, conductivity measurement and coating thickness measurement.
The technique is sensitive to the material conductivity, permeability and dimensions
of a product.
Eddy currents can be produced in any electrically conducting material that is
subjected to an alternating magnetic field (typically 10Hz to 10MHz). The alternating
magnetic field is normally generated by passing an alternating current through a coil.
The coil can have many shapes and can between 10 and 500 turns of wire.
The magnitude of the eddy currents generated in the product is dependent on
conductivity, permeability and the set up geometry.
When a crack, occurs in the product surface the eddy currents must travel
farther around the crack and this is detected by the impedance change.
Most eddy current electronics have a phase display and this gives an operator the
ability to identify defect conditions. In many cases signals from cracks, lift off and
other parameters can be clearly identified. Units are also available which can inspect
a product simultaneously at two or more different test frequencies. These units allow
specific unwanted effects to be electronically cancelled in order to give improved
defect detection.
The eddy current test is purely electrical. The coil units do not need to contact the
product surface and thus the technique can be easily automated. Most automated
systems are for components of simple geometry where mechanical handling is
simplified.
Aerospace
Industry
Testing components including aero-engine, Landing gear and air frame
parts during production
Aircraft Overhaul
Testing components during overhaul including aero-engine and landing
gear components
Automotive
Industry
Testing Brakes-Steering and engine safety critical components for flaws
introduced during manufacture. Iron castings – material quality. Testing of
diesel engine pistons up to marine engine size.
Petrochemical &
Gas Industries
Pipe-Line and tank internal corrosion measurement from outside. Weld
testing on new work. Automotive LPG tank testing
Railway Industry
Testing locomotive and rolling stock axles for fatigue cracks. Testing rail
for heat induced cracking. Diesel locomotive engines and structures.
Mining Industry
Testing of pit head equipment and underground transport safety critical
components.
Agricultural
Engineering
Testing of all fabricated, forged and cast components in agricultural
equipment including those in tractor engines.
Power Generation
Boiler and pressure vessel testing for weld and plate defects both during
manufacturing and in subsequent service. Boiler pipe work thickness
measurement and turbine alternator component testing.
Iron Foundry
Testing ductile iron castings for metal strength on 100% quality control
basis.
Shipbuilding
Industry
Structural and welding testing. Hull and bulkhead thickness measurement.
Engine components testing.
Steel Industry
Testing of rolled and re-rolled products including billets, plate sheet and
structural sections.
Pipe & Tube
Manufacturing
Industry
Raw plate and strip testing. Automatic ERW tube testing. Oil line pipe
spiral weld testing.