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General Introduction to NDT

TWI

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NDT Methods

Penetrant Inspection

Magnetic Particle Inspection

Eddy Current Inspection Ultrasonic Inspection

Radiographic Inspection

Magnetic Flux Leakage

Acoustic Emission

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Liquid Penetrant Inspection

Surface inspection method

Applicable to all non-porous,non-absorbing materials

A.K.A. Dye Penetrant Inspection (DPI)

Penetrant Flaw Detection (PFD)

Penetrant testing (PT)

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Penetrant Inspection

Penetrating fluid applied to component anddrawn into defect by capillary action

Penetrating fluid removed from componentsurface (but not from defect)

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Principle : Capillary Action

Interaction of adhesive and cohesiveforces

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Penetrant Inspection

Penetrating fluid applied to component anddrawn into defect by capillary action

Developer applied to surface

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Penetrant Inspection

Penetrating fluid applied to component anddrawn into defect by capillary action

Developer applied to surface

Penetrant drawn back out of the defect byreverse capillary action

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Penetrant Inspection

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System classification

PENETRANT

Colour

contrast Fluorescent

Dual

REMOVAL

Solvent

Waterwashable

Postemulsifiable

DEVELOPERS

Dry powder

Aqueous Non-Aqueous

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Advantages of DPI

Applicable to non-ferromagneticmaterials

Able to test large parts with a portable

kit Batch testing

Applicable to small parts with complex

geometry Simple,cheap easy to interpret

Sensitivity

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Disadvantages of DPI

Will only detect defects open to thesurface

Careful surface preparation required

Not applicable to porous materials

Temperature dependant

Cannot retest indefinitely

Compatibility of chemicals

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Magnetic Particle Inspection

Test method for the detection of surfaceand sub-surface indications in

ferromagnetic materials Magnetic field induced in component

Defects disrupt the magnetic flux

Defects revealed by applyingferromagnetic particles

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Principle of MPI : Flux Leakage

N S SN

No Defect Defect

Lines of flux follow the path of least resistance

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Magnetic Particle Inspection

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Advantages of MPI

Will detect some sub-surface defects

Rapid and simple to understand

Pre-cleaning not as critical as with DPI

Will work through thin coatings

Cheap rugged equipment Direct test method

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Disadvantages of MPI

Ferromagnetic materials only

Requirement to test in 2 directions

Demagnetisation may be required

Odd shaped parts difficult to test

Not suited to batch testing Can damage the component under test

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Magnetic Flux Leakage

A saturation of flux is introduced

N S

M i Fl L k

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Magnetic Flux Leakage A saturation of flux is introduced

Flux forced into air by reductions in thickness

N S

M ti Fl L k

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Magnetic Flux Leakage A saturation of flux is introduced

Flux forced into air by reductions in thickness

Magnetic flux detected by sensors

N S

Sensor

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Affected by Climatic conditions

Cleanliness

Surface condition

Magnetic Flux Leakage

Limitations

Coverage Qualitative not Quantitative

Top side / Bottom side differentiation

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Eddy Current Inspection

An alternating current ispassed through a coil

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Eddy Current Inspection

An alternating current ispassed through a coil

A.C. generates an

alternating field

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Eddy Current Inspection An alternating current is

passed through a coil

A.C. generates analternating field

Alternating fieldgenerates eddycurrents in conductors

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Eddy Current Inspection An alternating current is

passed through a coil

A.C. generates analternating field

Alternating fieldgenerates eddycurrents in conductors

Eddy currentsgenerate opposingfield which modifies

current in coil

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Eddy Current Inspection

Electrical currents induced in metals byalternating magnetic fields

The size of the current is affected by

Electrical conductivity

Stand off distance

Flaws Permeability

Specimen dimensions

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Advantages of ET

Sensitive to surface defects

Can detect through several layers

Can detect through surface coatings

Accurate conductivity measurements

Can be automated

Little pre-cleaning required

Portability

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Disadvantages of ET

Very susceptible to permeability changes

Only on conductive materials

Will not detect defects parallel to surface Not suitable for large areas and/or

complex geometry's

Signal interpretation required

No permanent record (unless automated)

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Ultrasonic Inspection

High frequency sound sound waves areintroduced into a material

Interfaces between materials of differingacoustic properties reflect or transmit

sound

Reflected sound is displayed on a CRT

Principle

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Defect Orientation

0 degree Probes

DepthMetal

Depth

Screen

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Defect OrientationAngle Probes

Range

Range

Depth = Range x Cos (Probe angle)

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Defect Orientation

1 2 3

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Advantages

Sensitive to cracks at variousorientations

Portability

Safety

Able to penetrate thick sections

Measures depth and through wall extent

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Disadvantages

No permanent record (unless automated)

Not easily applied to complex geometries

and rough surfaces.

Unsuited to course grained materials

Requires highly skilled and experienced

technicians

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Acoustic Emission

Transient stress waves from microstructural changes detected by sensors

Stress waves

Stress

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Radiographic Inspection

Electromagnetic radiation is imposedupon a test object

Radiation is transmitted to varyingdegrees dependant upon the density ofthe material through which it is travelling

Variations in transmission detected by

photographic film or fluorescent screens

Applicable to metals,non-metals andcomposites

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Radiographic Image

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Advantages of Radiography

Permanent record

Internal flaws

Can be used on most materials

Direct image of flaws

Real - time imaging

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Disadvantages of Radiography

Health hazard

Sensitive to defect orientation

Limited ability to detect fine cracks

Access to both sides required

Limited by material thickness

Skilled interpretation required

Relatively slow

High capital outlay and running costs

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The End

Any Questions Please ?

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Vacuum created within a perspex box Soapy liquid applied to surface

Vacuum Box Testing

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Vacuum Box Testing

Bubbles indicate through thicknessdefect

Vacuum created within a perspex box Soapy liquid applied to surface