k kapoor dgm(qa), nfc, hyderabad - iim | hyderabad … inspection the principles of radiography x or...

IIM Hyderabad 15th July 2016

Overview of NDT Methods

K Kapoor

DGM(QA), NFC, Hyderabad

Discontinuity

Flaw

Defect

Interruption in Normal Physical Structure

- Keyways, Grooves, Holes present by design

Discontinuity with undesirable

connotation

- Slag, Porosity, Lamination

Flaw which makes component unfit for service

- Cracks, Lack of Fusion in Weld

Types of Flaws:

• Planar Flaws (2D)

– Lack of Fusion , Cracks

• Volumetric Flaws (3D)

– Porosity , Inclusions

Important Flaw Characteristics in materials

Location Surface Flaw

Sub-surface Flaw

• Surface flaw more severe as compared to sub-surface flaw because of

higher stress intensity associated with it

• Code provides guidelines on classification of flaw in to surface or sub-

surface, if it lies just below the surface of the component

Shape

• Flaws with sharp tip like crack more severe than flaws with smooth

surface like porosity

• Small root radius leads to higher stress intensity


Proximity

• If two flaws are very close, they influence the stress intensity

associated with each other

• Two flaws shall be separated by the length of the longest flaws, or

else they shall be considered together as a singe flaw including

the sound region in between


Nature

• Weld Flaw

- Operator: Porosity, lack of fusion, slag inclusion, undercut

- Metallurgical origin: Cold crack, Hot crack, laminar tearing

• Planar or Volumetric Flaw

• Planar flaw more severe than volumetric flaw


Classification of NDT Methods

Non-Destructive Testing

Surface

Examination

Volumetric

Examination

Performance

Test

Visual

Liquid Penetrant

Magnetic

Particle

Eddy Current

Radiography

Ultrasonic

Testing

Acoustic

Emission

Leak Test

NDT Techniques for Welds

• Conventional

X- ray and Gama-ray Radiography

Ultrasonic testing

Eddy Current testing

Magnetic Particle

Dye Penatrant

• Advanced

Infra-red Thermography

Acoustic Emission

Micro-focal radiography

Real time radiography

Time of Flight Diffraction (UT)

Phased array ultrasonic testing

X-ray residual stress measurement

Dye Penetrant Inspection

Surface breaking defects only detected

Penetrant applied to the component and drawn into

the defects by capillary action

Applicable to all non- porous and non absorbing

materials.

Penetrants are available in many different types

Water washable contrast

Solvent removable contrast

Water washable fluorescent

Solvent removable fluorescent

Post-emulsifiable fluorescent


Step 1. Pre-Cleaning

Cleaning preparation is very important on this method.

Usually solvent removal is been used


Step 2. Apply penetrant

After the application of the penetrant the penetrant is normally

left on the components surface for approximately 15 minutes

(dwell time). The penetrant enters any defects that may be

present by capillary action


Step 3. Clean off penetrant

After sufficient penetration time (dwell time) has been

given,excess removal penetrant stage take place. A damped

lint free tissue with solvent is used to clean the excess

penetrant.


Step 3. Apply developer

After the excess penetrant is been removed, a thin layer

of developer is applied.A penetrant drawn out by reversed

capillary action.


Step 4. Inspection / development time

Inspection should take place immediately after the developer

has been applied .Any defects present will show as a bleed

out during development time.


Step 5. Post-Cleaning

After the inspection has been performed post cleaning is

required to prevent corrosion.


Colour contrast Penetrant

Fluorescent Penetrant Bleed out viewed

under a UV-A light

source

Bleed out viewed

under white light

Magnetic Particle Inspection


Surface and slight sub-surface detection

Relies on magnetization of component being tested

Ferro-magnetic materials only can be tested

A magnetic field is introduced into a specimen being tested

Methods of applying a magnetic field, yolk, permanent magnet, prods and flexible cables.

Fine particles of iron powder are applied to the test area

Any defect which interrupts the magnetic field, will create a leakage field, which attracts the particles

Any defect will show up as either a dark indication or in the case of fluorescent particles under UV-A light a green/yellow indication

Collection of ink

particles due to

leakage field


Prods

DC or

AC

Electro-

magnet

(yolk) DC or

AC

Crack

like

indicati

on

Crack

like

indicati

on


A crack like

indication


Alternatively to contrast

inks, fluorescent inks

may be used for greater

sensitivity. These inks

require a UV-A light

source and a darkened

viewing area to inspect

the component

Magnetic Particle Inspection Typical sequence of operations to inspect

a weld

Clean area to be tested

Apply contrast paint

Apply magnetisism to the component

Apply ferro-magnetic ink to the component

during magnetising

Interpret the test area

Post clean and de-magnatise if required


Advantages

Simple to use

Inexpensive

Rapid results

Little surface

preparation required

More sensitive than

visual inspection

Disadvantages

Surface or slight sub-

surface detection only

Magnetic materials

only

No indication of

defects depths

Detection is required

in two directions

Radiographic Inspection The principles of radiography

X or Gamma radiation is imposed upon a test

object

Radiation is transmitted to varying degrees

dependant upon the density of the material through

which it is travelling

Thinner areas and materials of a less density show

as darker areas on the radiograph

Thicker areas and materials of a greater density

show as lighter areas on a radiograph

Applicable to metals,non-metals and composites

Industrial Radiography

• X - Rays

Electrically generated

• Gamma Rays

Generated by the decay

of unstable atoms


• X - Rays

Electrically generated


• Gamma Rays

Generated by the decay of

unstable atoms

Source

Radiation beam Image quality indicator

Radiographic Inspection

Test specimen Radiographic film

Source

Radiation beam Image quality indicator

Radiographic film with latent image after exposure


Test specimen

7FE12

Step / Hole type IQI Wire type IQI

Radiographic Sensitivity

Wire Type IQI

Step/Hole Type IQI

Image Quality Indicators

Radiographic Techniques

• Single Wall Single Image (SWSI) - film inside, source outside

• Single Wall Single Image (SWSI) panoramic

- film outside, source inside (internal exposure)

• Double Wall Single Image (DWSI)

- film outside, source outside (external exposure)

• Double Wall Double Image (DWDI)

- film outside, source outside (elliptical exposure)

Single wall single image SWSI

IQI’s should be placed source side

Film

Film

Single wall single image SWSI panoramic

• IQI’s are placed on the film side

• Source inside film outside (single exposure)

Film

Film

Double wall single image DWSI

• IQI’s are placed on the film side

• Source outside film outside (multiple exposure)

• This technique is intended for pipe diameters over 100mm

Double wall single image DWSI

Radiograph

Identification

ID MR11

• Unique identification EN W10

• IQI placing A B • Pitch marks

indicating readable film length

Film

Double wall double image DWDI elliptical exposure

• IQI’s are placed on the source or film side

• Source outside film outside (multiple exposure)

• A minimum of two exposures

• This technique is intended for pipe diameters less than 100mm

Double wall double image DWDI

Shot A Radiograph

Identification

ID MR12

• Unique identification EN W10

• IQI placing

1 2 • Pitch marks indicating readable film length

4 3


Advantages

Permanent record

Little surface preparation

Defect identification

No material type

limitation

Disadvantages

Expensive equipment

Bulky equipment ( x-ray )

Harmful radiation

Detection on defect

depending on orientation

Slow results

Required license to operate

Basic Principles of Ultrasonic

Testing

• To understand and

appreciate the

capability and

limitation of UT

Ultrasonic Inspection Sub-surface detection

This detection method uses high frequency sound

waves, typically above 2MHz to pass through a material

A probe is used which contains a piezo electric crystal

to transmit and receive ultrasonic pulses and display the

signals on a cathode ray tube or digital display

The actual display relates to the time taken for the

ultrasonic pulses to travel the distance to the interface

and back

An interface could be the back of a plate material or a

defect

For ultrasound to enter a material a couplant must be

introduced between the probe and specimen

Ultrasonic Inspection

Ultrasonic testing is a good technique for the

detection of plate laminations and thickness surveys

Laminations detected using normal beam probes


defect

0 10 20 30 40 50

defect

echo Back wall

echo

CRT Display Compression Probe

Material Thk

initial pulse

Ultrasonic Inspection UT Set, Digital Pulse echo

signals

A scan Display

Compression probe Thickness checking the material


Ultrasonic testing requires high operator for defect

identification

Most weld defects detected using angle probes


Angle Probe

UT Set A Scan

Display


0 10 20 30 40 50

initial pulse defect echo

CRT Display

sound path

Angle Probe

defect

Surface distance


Advantages

Rapid results

Sub-surface detection

Safe

Can detect planar defect

Capable of measuring the

depth of defects

May be battery powered

Portable

Disadvantages

Trained and skilled

operator required

Requires high operator

skill

Good surface finish

required

Difficulty on detecting

volumetric defect

Couplant may

contaminate

No permanent record

Principles and Application of Eddy Current Testing

Basic Principle

Parameters influencing ECT

Equipment for ECT Probe (types, construction and design)

Supply, display and measuring unit

Reference Standards

General Applications Tube testing (Welded and Seamless)

Thickness measurement (bare surface and coating)

Surface Crack Detection

Conductivity measurement (sorting, heat-treatment etc)

Illustration

material

Basic Applications of ECT

• Crack detection

• Material thickness measurements

• Coating thickness measurements

• Conductivity measurements for:

o Material identification

o Heat damage detection

o Case depth determination

o Heat treatment monitoring

Advantages

• Sensitive to small cracks and other defects

• Detects surface and near surface defects

• Inspection gives immediate results

• Equipment is very portable

• Method can be used for much more than flaw detection

• Minimum part preparation is required

• Test probe does not need to contact the part

• Inspects complex shapes and sizes of conductive materials

Limitations

• Only conductive materials can be inspected

• Surface must be accessible to the probe

• Skill and training required is more extensive than other techniques

• Surface finish and and roughness may interfere

• Reference standards needed for setup

• Depth of penetration is limited

• Flaws such as delaminations that lie parallel to the probe coil winding and probe scan direction are undetectable

Parameters Influencing ECT

• Material Parameters

Conductivity

Permeability

• Magnetic Coupling (sensor and test object)

• Depth of penetration (frequency)

Conductivity ()

• Variables affecting :

Chemical composition

Impurity content

Heat-treatment

Lattice distortion and lattice defects (Cold work)

Hardness

Temperature

Discontinuities

Permeability

Coil’s Mag. Force (H)

Material’s Flux Density (B)

Non-magnetic Material

Coil’s Mag. Force (H)

Material’s Flux Density (B)

Magnetic Material

Example Cu, SS 304 Steels, Ni-base alloys

Low DOP for eddy currents High DOP for eddy currents

Depth of penetration

Depth of penetration

Material Conductivity

(% IACS)

Resistivity

(-cm)

Permeability DOP (in inch) at frequencies

1KC 4KC 16KC 64KC

Cu 100 1.7 1.0 0.082 0.041 0.021 0.010

Al (6061-T6) 42 4.1 1.0 0.126 0.063 0.32 0.016

Mg 37 4.6 1.0 0.134 0.067 0.34 0.017

Pb 7.8 22 1.0 0.292 0.146 0.073 0.037

Zr 3.4 50 1.0 0.446 0.223 0.112 0.056

304 SS 2.5 70 1.02 0.516 0.258 0.129 0.065

Alloy Steel 2.9 60 750 0.019 0.01 0.0048 0.0024

Cast Iron 10.7 16 175 0.018 0.009 0.0044 0.0022

Phase Lag (with depth)

• Phase lag is a parameter of the eddy current signal that

makes it possible to obtain information about the depth of

a defect within a material

In radians

In degrees

Where:

q=Phase Lag (Rad or Degrees)

x=Distance Below Surface (in or mm)

d=Standard Depth of Penetration (in or mm)

Probes Mode of

operation

Configuration Applications

Absolute Bobbin(ID),

Encircling(OD), Surface

Flaw detection, conductivity measurements and

thickness measurements.

Differential Bobbin(ID),

Encircling(OD), Surface

Flaw detection

Reflection

Probes

Bobbin(ID) (remote

field: driver and sensor)

Flaw detection and thickness measurement

Surface

Bobbin (ID) Encircling (OD)

Reference Standards Common eddy current reference standards include:

• Conductivity standards.

• Flat plate discontinuity standards.

• Flat plate metal thinning standards (step or tapered wedges).

• Tube discontinuity standards.

• Tube metal thinning standards.

• Hole (with and without fastener) discontinuity standards

Basic Applications

• Crack detection (tubes, components)

• Material thickness measurements

• Coating thickness measurements

• Conductivity measurements for:

o Material identification

o Heat damage detection

o Case depth determination

o Heat treatment monitoring

Application 1: Tube Testing: Analysis of Signals

Tube Testing: Analysis of Signals

Tube Testing: Examples

Typical flaws which show

up, include:

Pin-holes

Cross cracks lamination

Seam cracks

Porosity

Hook cracks

Lack of fusion Edge

damage

Open seams

Application 2:Thickness measurement-principle

1

4

3

2

Thickness measurement-example

Thickness of thin metal sheet and foil, and of

metallic coatings on metallic and nonmetallic

substrate

Cross-sectional dimensions of cylindrical tubes

and rods

Thickness of nonmetallic coatings on metallic

substrates

Application 3: Coating Thickness

Measurement of Thin Conductive Layers

• It is also possible to measure the thickness of a thin layer of metal on a metallic substrate, provided the two metals have widely differing electrical conductivities.

• A frequency must be selected such that there is complete eddy current penetration of the layer, but not of the substrate itself.

• The method has been used successfully for measuring thickness of very thin protective coatings of ferromagnetic metals (i.e. chromium and nickel) on non-ferromagnetic metal bases.

Thickness Measurements of Non-conducting Coatings on

Conductive Materials

o Principle: The thickness of nonmetallic coatings on

metal substrates can be determined using effect of

liftoff on impedance. The coating serves as a spacer

between the probe and the conductive surface.

o Applications: thickness measurement of paint and

plastic coatings.

o Accuracy: Thickness between 0.5 and 25 µm can be

measured to an accuracy between -10%/+ 4%.

Application 4: Surface Crack Detection

• Excellent method for detecting surface and near surface defects when the probable defect location and orientation is well known

• Defects such as cracks are detected when they disrupt the path of eddy currents and weaken their strength

Important Considerations

A knowledge of probable defect type, position, and orientation.

The probe should fit the geometry of the part and the coil must produce eddy currents that will be disrupted by the flaw.

Selection of a reasonable probe drive frequency. For surface flaws, use high freq. for maximum resolution and high sensitivity. For subsurface flaws, use lower frequencies to get the required depth of penetration and this results in less sensitivity. For ferromagnetic or highly conductive materials use of an even lower frequency to arrive at some level of penetration.

Reference specimens of similar material as component and with features that represent the defect or condition being inspected

Calibration for crack detection

k kapoor dgm(qa), nfc, hyderabad - iim | hyderabad … inspection the principles of radiography x or...

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