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Introduction to NDT and Penetrant Testing (PT)

G.Jothinathan

Scientific Officer Gr.I

Department of Metallurgical andMaterials Engineering

I I T M, Chennai 600 036

Emailgjndt@yahoo.com

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Introduction To NDT

Non Destructive Testing (NDT) is anoninvasive method and the testing is carried

out without impairing further usefulness of

the material, component and structure

The component can be put into use after the

Testing. The signals that are employed do not

alter the properties permanently .

Method Signal Remarks

UT US waves - elastic waves Elastic deformation

RT X-rays and gamma rays No damage duringthe course of testing

Radiation damage!!!

MT Magnetic field Remnent field-

Demagnetisation

- interesting and highly paying field- interdisciplinary of metallurgy, physics and

chemistry

NDT accounts for 1/3 of the cost of an aircraft

(without raw material and fabrication cost !!)

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Definition of terms

NDT

NDI Used interchangeably- detection and

reporting

NDE

The scope is broaderserviceability isascertained

Also includes material characterisation

Material anomalies-interruption,break,disruption

Discontinuity- can meet the service requirements

Defect/flaw - renders unsuitable for service

Anamoly whether Discontinuity or defect/flaw

1.Nature of the imperfectionlinear-volumetric2.Materialstructural or high strength3.Criticality of usestorage vessel or boiler

boiler in thermal power plant

boiler in nuclear power plant

Linear -cracklength > 3 times widthVolumetric - porosity and inclusion

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Applications of NDT

The main application of NDT is

flaw detection and evaluation.

It is also used for

geometric dimension measurement.

material characterization,

bond integrity testing ,condition monitoringetc

Material characterisation:1. E and determination

2. Grain size evaluation

3. Proportion of microstructural phases4. Extent of deformation

5. Nodularity of nodular cast iron etc

6. Fracture toughness determination

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Purpose of NDT

Fracture mechanics approachdamagetolerance approach (propagates the idea of

live with discontinuities)A material property, fracture toughness (KIc)

is defined and this characterizes the material

behaviour in presence of discontinuities

which the earlier strength of materials designapproach has not taken into account.

KIc can be experimentally determined.

The stress intensity factor (KI) the stress

distribution around a discontinuityis given by KI = G a where G isgeometric constant, is the working stressand a is flaw size.Failure occurs when K

Iis equal to or greater

than KIcTo determine KI,, flaw size information

needed and is provided by NDT personnel.

Life extension and life predication methods

also require information on the flaw size.

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APPLIED

STRESS

YIELD TENSILE

STRENGTH

(a) The Strength of materials approach

APPLIED

STRESS

FLAW

SIZE

FRACTURE

TOUGHNESS

(b) The Fracture Mechanics approach

Comparison of classical design philosophy with fracture mechanics approach

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NDT methods

1.Visual testing (VT)

2.Liquid Penetrant Testing (PT)

3.Magnetic particle Testing (MT)

4.Radiographic Testing (RT)

5.Ultrasonic Testing (UT)

6.Eddy Current Testing (ET) - highly sensitive7.Acoustic Emission Testing (AET) monitors activity

8.Thermal Infrared imaging (TIFR)

9.Leak testing (LT)

Surface NDT

Surface &internal

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Visual Testing (VT)

This ancient and original method of

examination by human eyes is still widely

employed to find gross discontinuities,

surface irregularities, roughness and

corrosion products on the surface.

Many gadgets like lenses, cameras are used.

For surfaces inaccessible to the human eyes

as in the case of inside surfaces of pipes and

boilers, boroscopes and flexible fibroscopes

are employed. These are called endoscopes

(internal vision). They are nothing but lens

arrangement to transfer the image or the

optical fibers arrangement for transferring the

image.

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VT of welds(contd)

Mainly for noncritical welds

Before, during and after

welding Before -Surface roughness and

cleanliness (oil)

During - Electrode size andwelders performance

After -Dimensional accuracy of

weldments Conformity ofwelds to size and contour

requirements

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VT of welds (contd)

Acceptability of weld appearancewith regard to:

Presence of surface discontinuities

such as: surface roughness weld spatter

cleanliness underfill pores

Undercuts overlaps Cracks spatter

Establishing definite procedure toensure uniformity and accuracy

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Liquid Penetrant Testing (PT)

-is applicable to discontinuities that are

open to the surface or surface connected.

-is extension of visual testing

-an indication is obtained whose widthis very much larger than the actual

width of the crack so as to be seen by

the unaided eye.

Crack indication

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Sensitivityand applications

-Sensitivityequal or better than MTbetter than RT for surface

discontinuities

1 m x 10m x 50 mcan be detected

Applicationson all materialsmetals(ferrous and nonferrous)

nonmetals(rubber,plastic etc)

all type of defects (open)leak testing

Normally not applicable to porous materials

(unfired ceramics and powder metallurgical

parts)Presently Filtered particle penetrant for porous

materials

Rough surfaces pose problem- > 125 m-

background poses problem

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Principle of Penetrant Testing PT

Highly coloured (visible or fluorescent)

organic dye liquid which is also surface

active in nature (called penetrants) is appliedon to theclean surfaceof the component and

allowed sufficient time for penetration into

discontinuities. The excess surface penetrant

on the component is removed. This leaves a

clean surface of the component with pentrant

residing in the discontinuities. At this point

Of time, developer, which is highlyabsorptive

in nature, is applied. The developer brings

back or bleeds out the penetrant thereby

providing an indication in a contrasting

background of white colour of developer.

1.Preparation of part2. Application of penetrant

3. Removal of excess surface penetrant

4. Development

5. Inspection and Evaluation

6. Post Cleaning

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Inspection and InterpretationApplication of developer

Penetrant application

Fig.1. Principle of Penetrant Testing

Clean surface of Component

Removal of excess surface penetrant

After removal of excess surface penetrant and

before application developer

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Principle Of PT

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Properties of penetrant

The entire penetrant testing is based on theability of the liquid to penetrate into

discontinuities and later ability to come out.

The required properties are 1. Wettability

2. Capillarity

The ability of the liquid to wet the solid surfaceor spread over the solid surface is determined by

the surface energies of the liquid-gas interface,

the solid-liquid interface and the solid-gas

interface.Mathematically expressed

SSL = SG - (Lg + SL)

Where: SSL is the wetting ability of liquid on a clean solid.

Lg is the surface energy of the liquid-gas interface.

SL is the surface energy of the solid-liquid interface.

SG is the surface energy of the solid-gas interface.

The liquid to spread over the solid surface, should replace the previously

existing solidgas interface. This can happen when the energy differenceSG -SL) is positive or ifSG> >SL). Or in other words the surface energy

of the solid gas interface should exceed, the surface energy of the solid

liquid interface. The difference in these energies is responsible for the

liquid to spread over the solid surface.

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The ability of the liquid to spread or wet the soli

surface is related to the contact angle , whic

quantifies the resultant adhesive and cohesive forces

The contact angle is defined as the angle between th

solid surface and the tangent drawn to the liquid a

he point of contact. It can be seen that spreading

ability and contact angle are inversely related. Th

Figs. Show the contact angle and wetting ability.

a. low contact angle contact angle 90 deg. High contact angle

Normally, the penetrants need to have a very low

contact angle and the commercial penetrants

have contact angles between 0 5.. Contact angledepends on the solid surface to be wetted. Water-

glass has a contact angle of 0 deg. compared to

water-silver which is 90 deg

Once the liquid wets the surface, the ability of theiquid to rise in the capillary or enter into the

openings is determined by surface tension (T)

Hence the main properties of penetrant are T and

However, the speed of penetration is determined byviscosity

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Other penetrant required properties

1.Visibility2.Nontoxicity (noncorroding etc)

Visibility is next most important property of the

penetrant

Colour contrast ratio of visible dye is ~1:10(The light reflected by the white background to bright

red of the dye)

Colour contrast ratio of the fluorescent dye is ~1:100

(light emitted by the indication to the light emitted by

the dark background)

Because of this colour contrast ratio, the indication is

better seen in the case of fluorescent indication . The

human eye brings an effect called halation effect, the

ability to magnify the indication

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Classification penetrants

There are mainly three types of penetrants namely

1. Visible dye or colour contrast penetrant (Type II )

2. Fluorescent or brightness contrast penetrant (Type I )

3. Dual mode (visible and fluorescent) (Type III)

Other unclassified type is . filtered particle penetrant

(Type I, II, or III is based based on the type of dyethat is incorporated -visible or fluorescent or both )

Each of these are further classified as methods

1. Water washable WW ( Method A)

2. Post emulsifiable PE lippophilic (Method B)3. Solvent removable SR (Method C )

4. Post emulsifiable Hydrophilic ( Method D)

-classification is based on the method by which

the excess penetrant is removed in the excess

penetrant removal step.

Simple water washing water washableSolvent wiping solvent removableEmulsifying &removing post emulsifiable

lippo & hydro-philic

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Composition of penetrants

1. Oil base

2. Dye material (visible or fluorescent)

3. Solvents and stabilising agents.

The composition of solvent removable and postemulsifiable penetrant are essentially same.

Solvent removableby solvent action

Post emulsifiableby dispersing the penetrant

into fine particles by the

applied emulsifier,

making it water removable

Water washable - Simple water washing

Simple water washing cannot remove the oil basepenetrant. Water washable penetrant has one more

constituent namely built in emulsifier The momen

water is applied, penetrant is dispersed by the in-

built emulsifier making it amenable for water

washing

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Sensitivity of penetrants

All fluorescent methods are more sensitive

than visible dye penetrant

Penetrant Type

Properties

Visisble dye Fluorescent

dye

Indication Bright redcolour

Yellowishgreen(on

excitation)

Background White colour

of the

developer

Dark (max. 2

ft.candles)

Colour contrast ratio ~1:10 ~1:100Visibility(seeability) Medium Very high

Halation

effect(ability of the

eye to magnify the

indication)

Normal High

Sensitivity Medium HighIn weld inspection, lower sensitivity methods,

namely water Washable and solvent removable are

employed due to surface roughness of the weld.

Hence, use of high sensitivity penetrant namely

fluorescent dye penetrant may be preferred.

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Preparation of parts

3.1.4 Interference by contaminants

1.physical blocking of the discontinuity (eg

Rust or scale paint or conversion coatings)

2.disturbing the balanced composition of the

penetrant (eg. Oil, water etc)

3. Entering and occupying the discontinuities. Oil,water etc)

3.1.2 The possible contaminants on the component

are

1.Oil and grease

2. Rust or scales (Oxidation products)

3. Paints and conversion coatings

4. Carbon, Varnish etc5. water

3.1.3 Sources

1. Fabrication processes and subsequent treatment

2. Surface protection against corrosion3. Surface treatment for improvement of properties

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Simple wiping

Vapour degreasing

Ultrasonic cleaning

Oil and grease1. Solvent cleaning

Acid cleaning

Mechanical cleaning

Rust, scale

Chemical cleaning Detergent cleaning oxide etc

Alkaline cleaning

Machining, grinding etc

Wire brushing, sand

blasting etc

Rust, scale.

Oxide etc

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Application of penetrant

Dwell time : The total time the penetrant is

contact with the test surfaceincluding the time required for application and

for drain .

Dwell time = application time + drain time

Normally 5-30 mts. Depends on the size and

nature of discontinuity and the material and

surface condition of the material etc.

Penetrant can be applied by

immersion, dipping ,spraying, swabbing and pouring

The only requirement is that a thin layer of

penetrant should be present for the specifiedtime (dwell time) on the surface to be inspected.

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Removal of excess penetrant

t is obvious that in PT to achieve a high S/N ratiohe excess penetrant on the surface should b

emoved as completely as possible and the penetran

s the discontinuity should not be lost by overwashing

The excess surface penetrant on the surface is

carefully removed without affecting the penetranthat is residing in the discontinuity.

This is an important step as the unremoved excess

surface penetrant will affect subsequently by

affecting the contrast of the indication (excessive

background) and if any penetrant in thediscontinuity is disturbed the volume of the

penetrant indication will get reduced (The volume of

penetrant is already very small)

n any NDT method the Signal to Noise ratio i

mportant and this should be as high as possible

S- Volume of penetrant that has entered into the discontinuity

N- Unremoved excess surface penetrant in the discontinuity

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Dissolve and remove (SR)

. Disperse (emulsify) and remove by water PE

. Simply water wash if penetrant contains built in

Emulsifier (WW)

n each case, care need be exercised so as to

ompletely remove the excess penetrant on the

urface completely and at the same time theenetrant in the discontinuity is not affected.

Washdont over wash .n the case, welds, when need be inspected

without flushing the crown, the surface

oughness poses problems giving backgroundolouration thereby affecting the visibility of the

ndication. Hence Solvent removable and water

washable variations are normally employed.

Here overwashing tendency of these methods, is

aken to advantage, meaning, the surface is

much free from the background colouration.

Of course, the sensitivity suffers. Sensitivity

depends on the amount of retained penetrant in

he discontinuity

Penetrant removal methods

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Developing an indication

3.6.1 Developer functions

The developer functions are

1.To assist the natural seepage of the penetrant in

the discontinuity and extract or blot out the

penetrant so as to form an visual indication(mechanism of visual indication)

2. To provide a contrasting base which enhances

the detection an indication. Blue-black to

yellowish green: white to red

3. To spread the penetrant so as to increase theapparent size of the indication

4. To mask some confusing indications

3.5.0 DEVELOPING

After the removal of the excess surface penetrant ,in developing step, the penetrant from the

discontinuity is brought to the surface so as to

form an visible indication.

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Mechanism of development

3.6.3 Mechanism of developing action :

Natural seepage is assisted and the absoptivedevelopers blot out more penetrant from the

discontinuity. The thickness of the penetrant layer

is increased to the levels above the threshold

visibility .( Some fluorescent penetrant indications

can be seen without developing with high intensity

black light -3000 microwatt/sq.cm)

No

Developer

Developer Absorbed

Penetrant

Width

is

more

Particles

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Type of developers and the developer

used in weld inspection

Developer has two variations: Dry and Wet. In

the case of wet, aqueous and non aqueous.

As the sensitivity of wet developers are better

than, dry developers and since in weld

inspection, less sensitive methods namely waterwashable and solvent removable are employed ,

it is better to use wet developers (mostly

nonaqueous) in weld inspection in spite surface

roughness. Normally for high surface roughness,

dry powder is better suited .

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Developing action at the end of developer time

reveals an indication. In this step the indications

are interpreted as to the origin of the indicationnamely whether True or false indications and if

true indication whether relevant or nonrelevant

and if relevant as to the nature of

discontinuities linear or volumetric.

Inspection environement depends on the type of

dye - Visible or fluorescent dye

white light for visible and blacklight

for fluoresecent

Inspection, interpretation and evaluation

True indicationRelevant

Non relevant

False indication Improper processing

tandard test specimen in PT

. Aluminum comparator blockspenetrant quality

. Chrome plated steel specimensensitivity

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.8.1 Inspection environment

sible dye penetrant indications are inspected under white lighnd fluorescent dye penetrant indications are viewed undeach light (ultraviolet light?)

lectromagnetic radiation spectrumisible and ultraviolet spectrum

sible light spectrum - 380 nm to 770 nm or ( V to R ) o800 Ao to 7000 Ao

ltraviolet spectrum - 10 nm to 380 nm or 100 -3800 Aoack light spectrum - 320 nm to 380 nm or (with peak a

65 nm)

g arc vapour lamp produces ultraviolet light in addition tsible light. The filters are used to remove the harmful portiof UV (below 320 nm) and also visible light which affects th

uorescence of the fluorescent dye. The filters that arommonly used give black light with maximum peak intensity a

65 nm..

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Colour and fluorescent colours

Colourpart of the white light that is reflected by the surface thatis flooded with the white light

Fluorescent colourEmission of visible light (any colour) on theimpingement of high energy radiation on the surface.

In PT & MT, the impingement of black light (UV) leads toemission of yellowish green light

Fluorescent dye

Black light

Yellowish

green light

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Weld defects that can be detected by PT

The preponderance of weld joints arefusion welded. The following

discontinuities can be detected.

1. Porosity

2. Lack of root penetration (on single side)

3. Lack of fusion (at toe of welds)

4. Cracks in heat affected zone

5. Crater and other cracks

Weldments at the site is inspected with

visible dye penetrant using solvent removable

method

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Hints for weld inspection(contd)

5.Water washable fluorescent penetrantscan be used on large pressure vessels or other

large structures by washing with a hose and air

drying.

Developent with solvent suspended developercan provide good sensitivity

Water washable penetrants can be washed more

readily from most weld

6. It is important to remove all the of the slag from

welds before penetrant testing.

Grinding off the ripples on rough welds is

desirable. Grinding should not smear the metal

7. Brazed joints can discontinuities similar to weld

beads that can be detected by PT.A braze that does not wet the surface is

indication of a poor joint.

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Method Application Advantages Disadvantage

Visual Surface

discontinuities

Economical test Limited to visual

acuity of the

inspector

Liquidpenetrant

Surface cracksand porosity

Relativelyinexpensive &

Reasonably fast

Cleaning is neededbefore and after

inspection .surface

films hide

discontinuities

Magnetic

particlei

Surface and

subsurfacediscontinuities

Relatively

economical andexpedient

Applicable only

ferromagneticmaterials

Radiographic Volume

discontinuities

Surface &

subsurface

Provides

permanent record

Plane

discontinuities

must be favourably

aligned with

radiation beanUltrasonic Most

discontinuities

Sensitive to

planar

discontinuities.

High penetration

capability

Small thickness

plate may be

difficult to inspect.

Requires skilled

operator

Eddy currenttesting

Surface andsubsurface can

be inspected

Painted or coatedsurface signal

High speed

Many variablesaffect the test