19. Short Non Destructive Testing

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Copyright 2004, TWI Ltd World Centre for Materials Joining Technology

TWICSWIP 3.1

WIS 5WELDING INSPECTION

NDT


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What is NDT?

A method of determining:

the integrity of a material,

component or structure the characteristic of an object

Without doing harm!


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NDT methods

Visual Inspection

Penetrant Inspection

Magnetic Particle Inspection

Eddy Current Inspection

Ultrasonic Inspection

Radiographic Inspection


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Uses of NDT methods?

Flaw Detection and Evaluation Dimensional Measurements

Structure and Microstructure

Characterization Estimation of Mechanical and Physical

Properties

Material Sorting and Chemical CompositionDetermination


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Certification systems

BS EN 473 (ISO 9712) PCN

ASNT

CSWIP In House


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Welding gauge

Visual inspection of welds using themulti-purpose welding gauge


7/46Copyright 2004, TWI Ltd World Centre for Materials Joining Technology

Using the welding gauge

Angle of preparation(from 0 to 60 in 5increments)

Linear Misalignment



Using the welding gauge

Undercut / mechanicaldamage (grinding etc)

Excess weld metal/rootpenetration



Using welding gauge

Fillet weld actualthroat thickness (upto 20 mm or 3/4 inch)

Fillet weld leg length(up to 25 mm)



Other gauges for weld inspection

Tapered gauge

Hi-Lo gauge



Liquid penetrant inspection

Surface inspection method

Applicable to all non-porous, non-absorbing

materials

Dye Penetrant Inspection (DPI)

Penetrant Flaw Detection (PFD)

Penetrant testing (PT)




Penetrating fluid applied tocomponent and drawn into defect bycapillary action

Penetrating fluid removed fromcomponent surface (but not fromdefect)



Principle of capillary action

Interaction of adhesive and cohesiveforces





Developer applied to surface





Developer applied to surface Penetrant drawn back out of thedefect by reverse capillary action



System classification

PENETRANT

REMOVAL DEVELOPERS

Colourcontrast

Fluorescent

Dual

Solvent

Waterwashable

Postemulsifiable

Dry powder

Aqueous Non-aqueous



Advantages of LPI

Applicable to non-ferromagneticmaterials

Able to test large parts with a portablekit

Batch testing

Applicable to small parts with complexgeometry

Simple,cheap easy to interpret Sensitivity


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Disadvantages of LPI

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 inferromagnetic 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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MPI equipment


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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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An alternating current ispassed through a coil


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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 inconductive materials


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Eddy Current Inspection An alternating current

is passed through a coil

A.C. generates analternating field

Alternating fieldgenerates eddycurrents inconductive materials Eddy currents

generate opposingfield which modifies

current in coil


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The size of the electrical currents inducedin metals by alternating magnetic fieldsis affected by:

Electrical conductivity

Stand off distance

Flaws

Permeability

Specimen dimensions


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Advantages of ECI

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 ECI

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 are

introduced into a material

Interfaces between materials ofdiffering acoustic properties reflect or

transmit sound Reflected sound is displayed on a CRT

Principle


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Ultrasonic inspection equipment


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Ultrasonic inspection equipment

0 degree probe Angle probe


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Defect orientation

0 degree Probes

D

epth

Metal

Depth

Screen


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Defect orientation

Angle probes

Range

Range

Depth = Range x Cos (Probe angle)

Screen


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Advantages of UI

Sensitive to cracks at variousorientations

Portability

Safety

Able to penetrate thick sections

Measures depth and through wallextent


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Disadvantages of UI

No permanent record (unless automated)

Not easily applied to complex geometries

and rough surfaces. Unsuited to course grained materials

Requires highly skilled and experiencedtechnicians


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Radiographic inspection

Electromagnetic radiation is imposedupon a test object

Radiation is transmitted to varying

degrees dependant upon the density ofthe material through which it istravelling

Variations in transmission detected by

photographic film or fluorescent screens

Applicable to metals,non-metals andcomposites


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Radiographic Inspection

Lowerdensity

Higherdensity

Radiation source

Film

Specimen


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Radiographic image


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Radiographic equipment


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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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19. Short Non Destructive Testing

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