Overview of NDT

7/29/2019 Overview of NDT

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NON-DESTRUCTIVE TESTING

Examination of materials and components insuch a way that allows material to be examinated

without changing or destroying their usefulness


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Non-Destructive Testing

The four basic methods

Magnetic particle inspection (MT)

Dye penetrant inspection (PT)

Radiographic inspection (RT)

Ultrasonic inspection (UT)


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Dye penetrant inspection (PT)


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Dye Penetrant Inspection

Surface breaking defects only detected

Penetrant applied to the component and drawn intothe defects by capillary action

Applicable to all non- porous and non absorbing

materials.

Penetrants are available in many different types

Water washable contrast

Solvent removable contrast Water washable fluorescent

Solvent removable fluorescent

Post-emulsifiable fluorescent


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Step 1. Pre-Cleaning

Cleaning preparation is very important on this method.

Usually solvent removal is been used


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Step 2. Apply penetrant

After the application of the penetrant the penetrant is normally

left on the components surface for approximately 15 minutes

(dwell time). The penetrant enters any defects that may be

present by capillary action


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Step 3. Clean off penetrant

After sufficient penetration time (dwell time) has been

given,excess removal penetrant stage take place. A damped

lint free tissue with solvent is used to clean the excess

penetrant.


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Step 3. Apply developer

After the excess penetrant is been removed, a thin layer

of developer is applied.A penetrant drawn out by reversed

capillary action.


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Step 4. Inspection / development time

Inspection should take place immediately after the developerhas been applied .Any defects present will show as a bleed

out during development time.


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Step 5. Post-Cleaning

After the inspection has been performed post cleaning isrequired to prevent corrosion.


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Colour contrast Penetrant

Fluorescent PenetrantBleed out viewed

under a UV-A lightsource

Bleed out viewed

under white light


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Advantages

Simple to useInexpensive

Quick results

Can be used on anymaterial provided it

is not porous

More sensitive thanvisual inspection

Low operator skill

required

DisadvantagesSurface breaking defectonly

little indication of depths

Penetrant and developer

may contaminate

component

Surface preparation

criticalPost cleaning required

Not applicable on porous

material


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Magnetic particle inspection (MT)


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Magnetic Particle Inspection


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Surface and slight sub-surface detection

Relies on magnetization of component being tested

Ferro-magnetic materials only can be tested

A magnetic field is introduced into a specimen being tested

Methods of applying a magnetic field, yoke, permanent

magnet, prods and flexible cables. Fine particles of iron powder are applied to the test area

Any defect which interrupts the magnetic field, will create aleakage field, which attracts the particles

Any defect will show up as either a dark indication or in thecase of fluorescent particles under UV-A light a green/yellowindication


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Collection of ink

particles due to

leakage field


Prods

DC or

AC

Electro-

magnet

(yoke) DC or

AC

Crack

like

indicati

on

Cracklike

indicati

on


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A crack likeindication


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Alternatively to contrastinks, fluorescent inks

may be used for greater

sensitivity. These inks

require a UV-A light

source and a darkenedviewing area to inspect

the component


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Magnetic Particle InspectionTypical sequence of operations to inspect

a weldClean area to be tested

Apply contrast paint

Apply magnetisism to the component

Apply ferro-magnetic ink to the component

during magnetisingInterpret the test area

Post clean and de-magnatise if required


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Advantages

Simple to use

Inexpensive

Rapid results

Little surface

preparation required

More sensitive thanvisual inspection

Disadvantages

Surface or slight sub-surface detection only

Magnetic materials

only

No indication of

defects depths

Detection is required

in two directions


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Radiographic inspection (RT)


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Radiographic InspectionThe principles of radiography

X or Gamma radiation is imposed upon a testobject

Radiation is transmitted to varying degrees

dependant upon the density of the material

through which it is travelling

Thinner areas and materials of a less density

show as darker areas on the radiograph

Thicker areas and materials of a greater

density show as lighter areas on a radiograph

Applicable to metals,non-metals and

composites


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Industrial Radiography

X - Rays

Electrically generated

Gamma Rays

Generated by the

decay of unstable

atoms


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X - Rays

Electrically generated


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Gamma Rays

Generated by the decay ofunstable atoms

R di hi I ti


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Source

Radiation beam Image quality indicator

Radiographic Inspection

Test specimenRadiographic film

R di hi I ti


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Source

Radiation beam Image quality indicator

Radiographic film with latent image after exposure


Test specimen

R di hi S iti it


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7FE12

Step / Hole type IQI Wire type IQI

Radiographic Sensitivity


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Wire Type IQI

Step/Hole Type IQI

Image Quality Indicators


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Radiographic Techniques

Single Wall Single Image (SWSI)- film inside, source outside

Single Wall Single Image (SWSI) panoramic

- film outside, source inside (internal exposure) Double Wall Single Image (DWSI)

- film outside, source outside (external exposure)

Double Wall Double Image (DWDI)- film outside, source outside (elliptical exposure)


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Single wall single image SWSI

IQIs should be placed source side

Film

Film

Single all single image SWSI


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Single wall single image SWSIpanoramic

IQIs are placed on the film side

Source inside film outside (single exposure)

Film

Double wall single image DWSI


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Film


IQIs are placed on the film side

Source outside film outside (multiple exposure)

This technique is intended for pipe diameters

over 100mm


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Radiograph

Identification

ID MR11

Unique identificationEN W10

IQI placing

A B Pitch marks

indicating readablefilm length

Double wall double image DWDI


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Film

Double wall double image DWDIelliptical exposure

IQIs are placed on the source or film side

Source outside film outside (multiple exposure)

A minimum of two exposures

This technique is intended for pipe diameters

less than 100mm


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Double wall double image DWDI

Shot A Radiograph

Identification

ID MR12

Unique identification EN W10

IQI placing

1 2 Pitch marks

indicating readablefilm length

4 3



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AdvantagesPermanent record

Little surface preparation

Defect identificationNo material type

limitation

Disadvantages

Expensive equipment

Bulky equipment ( x-ray )

Harmful radiation

Detection on defectdepending on orientation

Slow results

Required license to operate


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Ultrasonic inspection (UT)

Ultrasonic Inspection


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Ultrasonic Inspection Sub-surface detection

This detection method uses high frequency sound

waves, typically above 2MHz to pass through a material

A probe is used which contains a piezo electric crystal

to transmit and receive ultrasonic pulses and display the

signals on a cathode ray tube or digital display

The actual display relates to the time taken for the

ultrasonic pulses to travel the distance to the interface

and back

An interface could be the back of a plate material or adefect

For ultrasound to enter a material a couplant must be

introduced between the probe and specimen

Ult i I ti


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Ultrasonic testing is a good technique for the

detection of plate laminations and thickness surveys

Laminations detected using compression probes

Ult i I ti


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defect

0 10 20 30 40 50

defect

echoBack wall

echo

CRT DisplayCompression Probe

Material Thk

initial pulse

Ult i I ti


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Ultrasonic InspectionUT Set, DigitalPulse echo

signals

A scan Display

Compression probe Thickness checking the material



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Ultrasonic testing requires high operator for defect

identification

Most weld defects detected using angle probes



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Angle Probe

UT SetA Scan

Display

Ult i I ti


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0 10 20 30 40 50

initial pulse defect echo

CRT Display

sound path

Angle Probe

defect

Surface distance



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Advantages

Rapid resultsSub-surface detection

Safe

Can detect planar defectCapable of measuring the

depth of defects

May be battery poweredPortable

Disadvantages

Trained and skilledoperator required

Requires high operator

skill

Good surface finish

required

Difficulty on detecting

volumetric defectCouplant may

contaminate

Overview of NDT

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