NDT

Non Destructive Testing

(NDT)Techniques

Basic NotesBasic Notes

What is NDT? The abbreviation NDT refers to NON DESTRUCTIVE TESTING

AS FOUND IN ASME SECTION V (2007) Assess the condition of a component without causing any damage to it.

Applied physical parameters such as mechanical vibration, radiation,

localised heating, vibration, sound and electromagnetic interaction is used localised heating, vibration, sound and electromagnetic interaction is used

test material / components.

Indications are found based on the response from certain material or

damage related properties.

Evaluation and interpretation of these indications results in classifications

such as non-relevant, relevant and defective.

Related engineering actions are taken.

What are Some Uses of NDT Methods?

Flaw Detection and Evaluation

Leak Detection

Location Determination

Dimensional Measurements Dimensional Measurements

Structure and Microstructure Characterization

Estimation of Mechanical and Physical Properties

Stress (Strain) and Dynamic Response Measurements

Material Sorting and Chemical Composition

Determination

When are NDT Methods Used? There are NDE application at almost any stage

in the production or life cycle of a component.

To assist in product development.

To screen or sort incoming materials.

To monitor, improve or control manufacturing To monitor, improve or control manufacturing

processes.

To verify proper processing such as heat treating.

To verify proper assembly.

To inspect for in-service damage.

Six Most Common NDT (Basic)

Methods

Visual

Liquid Penetrant

Magnetic Magnetic

Ultrasonic

Eddy Current

Radiation

Visual Inspection

Most basic and common inspection method.

Tools include fiberscopes, borescopes,

magnifying glasses and mirrors.

Portable video inspection unit with zoom

allows inspection of large tanks and vessels,allows inspection of large tanks and vessels,

railroad tank cars, sewer lines.

Robotic crawlers permit observation in

hazardous or tight areas, such as air ducts,

reactors, pipelines.

Visual Inspection

What is ET? The abbreviation ET refers to Electromagnetic Testing

Thus includes all Non Destructive Testing methods that rely on some kind of electromagnetic interaction with test sample / component

Methods include (for example): Barkhausen noise analysis

Ground penetrating radar

Microwave inspection Microwave inspection

Wire Rope testing

Magnetic Resonance testing

Remote field testing

Flux Leakage testing

Potential drop testing

Magnetostrictive testing (EMAT) EDDY CURRENT TESTING (ECT)

What is EDDY CURRENT TESTING ? An alternating current is used to excite a primary coil which results in a varying primary

magnetic field. (Oersted)

Brought into close proximity with a conductive material, eddy currents are generated inthe material based on a concept referred to as electromagnetic induction. (Faradays law)

The generated eddy currents forms a secondary magnetic field that opposes the primarymagnetic field. (Lenzs Law).

The consequent change in the coil / probe impedance is used to detect the nett changebetween the primary and secondary magnetic field.

Variation in the nett magnetic field results from material property changes (conductivityand permeability), material dimensional changes, distance from test surface and mostimportantly, due to the presence of surface and subsurface indications.

What is EDDY CURRENT TESTING ? Eddy current testing is an indirect detection method, viz. the presence of

defect modifies a secondary parameter i.e. the generated eddy currents &

consequent secondary magnetic field.

The change in coil impedance caused by the resultant change between

secondary and primary magnetic fields are used to detect the defect.

THUS

SUITABLE CALIBRATION AND REFERENCE

STANDARDS USED TO QUANTIFY AND

CHARACTERIZE DEFECTS ARE CRUCIAL.

What is EDDY CURRENT TESTING ? Eddy Current Testing could be used in the place of PT and MT with the

following added advantages:

Depth of penetration (extent of subsurface defect detection) exceeds MT,

depending on frequency selection and material properties.

Very little surface preparation is required, viz. non conductive coating /

rubber linings of up to 10mm thick need not be removed.

No direct surface contact is required.

Remote inspections are possible limited access / safety risk.

Possible to determine defect depth (under quantifiable conditions).

What is EDDY CURRENT TESTING ?

NO SINGLE NDT METHOD IS SUITABLE FOR

DETECTING ALL TYPES OF DEFECTS.

THUS A COMBINATION OF METHODS AND

TECHNIQUES ARE REQUIRED FOR COMPLETEINSPECTION.

SURFACE, SUBSURFACE & VOLUMETRIC.

Eddy Current Testing

CoilCoil's

magnetic field

Conductivematerial

Eddy

currents

Eddy current's

magnetic field

Liquid Penetrant Testing (PT) Relatively simple but effective technique for locating

surface-breaking cracks.

Applicable to many materials.

Involves flooding surface of test object with low-viscosity fluid. Fluid soaks into crevices via capillaryaction.

Penetrant is cleaned and developer (powder) is applied Penetrant is cleaned and developer (powder) is appliedto draw out penetrant remaining between crack faces.

Developer provides contrasting background andblooming effect to enhance crack visibility.

Visual inspection is the final step in the process. Thepenetrant used is often loaded with a fluorescent dyeand the inspection is done under UV light to increasetest sensitivity.

Liquid Penetrant Testing (PT)

Liquid Penetrant Testing (PT) Penetrants can be fluorescent and viewed under

suitable light.

Portable and well suited for fieldwork.

Inexpensive and requires minimal skills. Sensitive

to small discontinuities. Can be applied to non-to small discontinuities. Can be applied to non-

porous materials. Flaw orientation does not

usually pose a problem.

Only surface anomalies. Must have access to

surface. Stresses at surface can cause problems.

Significant surface preparation required. Fairly

slow due to preparation, dwell, clean-up.

Radiography Testing (RT)

The radiation used in radiography testing is a higher energy (shorter wavelength) version of the electromagnetic waves that we see as visible light. The radiation can come from an X-ray generator or a radioactive source.


The part is placed between the radiation

source and a piece of film. The part will stop

some of the radiation. Thicker and more

dense area will stop more of the radiation.

X-ray film

The film darkness (density) will

vary with the amount of radiation

reaching the film through the test

object.

= more exposure

= less exposure


X-Rays pass through matter that is opaque to many otherwavelengths of electromagnetic radiation.

As X-Rays pass through matter, they become absorbed. The greater the thickness, the greater the level of absorption. Some materials absorb more than others (metal versus flesh). Some materials absorb more than others (metal versus flesh). Higher atomic number and density gives higher absorption. Though X-Rays are invisible, they cause photographic film to

expose. Photographic film can be used to measure the amount of

radiation passing through a body, and from the resulting imagewe can get information on the internal structure/compositionof the object.


Basic Principles:

Electrons are emitted from a tungsten filament

cathode.

A high voltage accelerates them towards the A high voltage accelerates them towards the

tungsten anode.

Electrons in atoms of anode are excited to higher

energy states.

As they return to original states, they give off

energy in form of X-Rays.


Defect Sizing

Finding length/area of defect is easy and straight forward.

Thickness can be estimated.

Volume can be estimated.

Defect Orientation Defect Orientation

Depending on defect geometry, its orientation can

be very important in determining its detectability.

Volume defects (e.g. porosity, cavity) equally easy

(or hard) to see from all directions.

Cracks can be very difficult to see in some directions.


As film is exposed, it goes from white (transparent) to black opaque (when later developed).

Black areas correspond to places where more X-Rays get through.

Light areas/shadows are regions of increased thickness/density, for example

Light areas/shadows are regions of increased thickness/density, for example Inclusion of some denser material

Variation in cross-section

Dark areas are regions of decreased density/thickness, for example Voids

Cracks

Radiography Testing (RT) Gamma Rays

X-Ray machines are generally large and heavy.

For field-tests (e.g. chemical plant pipework) a gammaray source is more convenient.

Source is a Radioisotope (radioactive material).

Source is portable, requires no power.

Radioisotope cannot be turned off: always Radioisotope cannot be turned off: alwaysdangerous.

X-Ray machine only dangerous when activated.

Safety For gamma rays and X-Rays, exposure of living tissue

to the radiation must be avoided and minimized.

Chronic exposure can cause cancer and death.Extreme acute exposure can kill more quickly.

Ultrasonic Testing (UT)

Basic Concepts

Similar to Sonar or Radar.

Sound waves are propagated into the test-

piece.piece.

Waves interact with defects and boundaries:

Reflect or Attenuate (i.e. not propagate).

From observation of interaction, information

can be gained on internal defects and flaws.


Reflective type Transmittive type


Defect Orientation

As for radiography, depending on defect geometry,

its orientation can be very important in

determining its detectability.

Cracks can be very difficult to see unless they are Cracks can be very difficult to see unless they are

perpendicular to the propagating waves.

Angle beam probes can help detect

inconveniently aligned defects.

Applying refraction principle by the use of a

Perspex wedge between the probe and the object.

Can use multiple reflections too.


Defect Orientation

Refraction Method

Ultrasonic Testing (UT) Couplant

Getting sound waves from transducer intospecimen, and back from specimen intotransducer is problematic.

Specimen needs to be smooth surfaced.

Couplant (e.g. vaseline) is used which fills in the Couplant (e.g. vaseline) is used which fills in thetiny imperfections which are still present.

Immersion tests can be used for less smoothparts.

Specimen and transducer(s) are placed in a waterbath. Water is the couplant between transducersand specimen.

Magnetic Particle Inspection (MT) Ferromagnetic test piece is magnetised. Magnetic flux

leakage fields are detected through the addition ofsmall particles which migrate to the flux leakage.

Can detect surface or near-surface flaws.

Complex shapes readily tested. Fairly portable andrelatively inexpensive. Sensitive to small surface (ornear-surface) defects. Requires moderate skill levels.near-surface) defects. Requires moderate skill levels.Relatively quick results.

Limited to ferromagnetic materials. Surfaces must beaccessible. Large electric currents can be required. Mayneed to demagnetise object. Limited to surface andnear-surface defects. Surface finishes may need to beremoved. Flaw detection sensitive to magnetic fieldorientation.

Magnetic Particle Inspection (MT)


The part is magnetized. Finely milled iron

particles coated with a dye pigment are then

applied to the specimen. These particles are

attracted to magnetic flux leakage fields and

will cluster to form an indication directly over

the discontinuity. This indication can bethe discontinuity. This indication can be

visually detected under proper lighting

conditions.

NDT

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