Introduction to NDT and penetrant testing

non destructive testing

Introduction to NDT and Penetrant Testing (PT)

G.JothinathanScientific Officer Gr.IDepartment of Metallurgical and

Materials Engineering

I I T M, Chennai 600 036Email – gjndt@yahoo.com

Introduction To NDT

Non Destructive Testing (NDT) is a

noninvasive method and the testing is carriedout 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 during the course of testing Radiation damage!!!

MT Magnetic field Remnent

- 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 !!)

Also includes material characterisation

Material anomalies-interruption ,break,disruption

Discontinuity- can meet the service requirementsDefect/flaw - renders unsuitable for service

Anamoly whether Discontinuity or defect/flaw

1.Nature of the imperfection –linear-volumetric2.Material –structural or high strength

3.Criticality of use – storage vessel or boiler

boiler in thermal power plant boiler in nuclear power plantLinear -crack – length > 3 times width

Volumetric - porosity and inclusion

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 ,

Material characterisation:

1 E and μ determination

2 Grain size evaluation

3 Proportion of microstructural phases

4 Extent of deformation

5 Nodularity of nodular cast iron etc

6 Fracture toughness determination

Purpose of NDT

Fracture mechanics approach – damage

tolerance 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 design approach has not taken into account

KIc can be experimentally determined

The stress intensity factor (KI) the stress

distribution around a discontinuity

is given by KI = G σ √πaa where G is

geometric constant, σ is the working stress and ’ a’ is flaw size

Failure occurs when KI is equal to or greater than KIc

To 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

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

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 sensitive

8.Thermal Infrared imaging (TIFR)

9.Leak testing (LT)

Surface NDT

Surface &internal

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

as in the case of inside surfaces of pipes andboilers, boroscopes and flexible fibroscopes

are employed These are called endoscopes

(internal vision) They are nothing but lensarrangement to transfer the image or the

optical fibers arrangement for transferring theimage

VT of welds(contd)

• Mainly for noncritical welds

• Before, during and after

requirements

• surface roughness weld spatter

• cleanliness underfill pores

• Undercuts overlaps

• Cracks spatter

• Establishing definite procedure to ensure uniformity and accuracy

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 width

is very much larger than the actual

width of the crack so as to be seen by the unaided eye.

Crack indication

Sensitivity and applications

-Sensitivity – equal or better than MT

– better than RT for surface discontinuities

1 μm x 10 μm x 50 μm

can be detected

Applications – on all materials

– metals(ferrous and nonferrous) nonmetals(rubber,plastic etc) – all type of defects (open)

Rough surfaces pose problem- > 125

μm-background poses problem

Principle of Penetrant Testing PT

Highly coloured (visible or fluorescent)

organic dye liquid which is also surface

active in nature (called penetrants) is applied

on to the clean surface of 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 highly absorptive

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

Inspection and Interpretation Application 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

Principle Of PT

Properties of penetrant

The entire penetrant testing is based on the ability 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 surface

or 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 solid –gas interface This can happen when the energy difference SG -SL) is positive or if SG> > 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

The ability of the liquid to spread or wet the solid surface is related to the contact angle , which quantifies the resultant adhesive and cohesive forces, The contact angle is defined as the angle between the solid surface and the tangent drawn to the liquid at the point of contact It can be seen that spreading ability and contact angle are inversely related The 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 angle depends 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 the liquid 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 by viscosity η

Other penetrant required properties

1.Visibility

2.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

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 dye that 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 washable

Solvent wiping – solvent removable

Emulsifying &removing – post emulsifiable

lippo & hydro-

philic

Composition of penetrants

1 Oil base

2 Dye material (visible or fluorescent)

3 Solvents and stabilising agents

The composition of solvent removable and post

emulsifiable penetrant are essentially same

Solvent removable – by solvent action

Post emulsifiable – by 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 base penetrant Water washable penetrant has one more constituent namely built in emulsifier The moment water is applied, penetrant is dispersed by the in-built emulsifier making it amenable for water

washing

Sensitivity of penetrants

All fluorescent methods are more sensitive

than visible dye penetrant

of the developer

Dark (max 2 ft.candles)

Colour contrast ratio ~1:10 ~1:100

Visibility(seeability) Medium Very high

Sensitivity Medium High

In 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

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 components are

1.Oil and grease

2 Rust or scales (Oxidation products)

3 Paints and conversion coatings

4 Carbon, Varnish etc

5 water

3.1.3 Sources

1 Fabrication processes and subsequent treatment

2 Surface protection against corrosion

3 Surface treatment for improvement of properties

Simple wiping

Vapour degreasing

Machining, grinding etc

Wire brushing, sand blasting etc

Rust, scale Oxide etc

Application of penetrant

Dwell time : The total time the penetrant is contact with the test surface including 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 specified

time (dwell time) on the surface to be inspected

Removal of excess penetrant

It is obvious that in PT to achieve a high S/N ratio, the excess penetrant on the surface should be removed as completely as possible and the penetrant

is the discontinuity should not be lost by overwashing

The excess surface penetrant on the surface is carefully removed without affecting the penetrant that 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 the discontinuity is disturbed the volume of the penetrant indication will get reduced (The volume

of penetrant is already very small)

In any NDT method the Signal to Noise ratio is important 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

Dissolve and remove (SR)

1 Disperse (emulsify) and remove by water PE

2 Simply water wash if penetrant contains built in

Emulsifier (WW)

In each case, care need be exercised so as to

completely remove the excess penetrant on the

surface completely and at the same time the

penetrant in the discontinuity is not affected

Wash – don’t over wash

In the case, welds, when need be inspected

without flushing the crown, the surface

roughness poses problems giving background

colouration thereby affecting the visibility of the Indication Hence Solvent removable and water washable variations are normally employed.

Here overwashing tendency of these methods, is taken 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 the discontinuity

Penetrant removal methods

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 the apparent 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

Mechanism of development

3.6.3 Mechanism of developing action :

Natural seepage is assisted and the absoptive developers 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)