Basics of Non Destructive testing

INTRODUCTION TO NON-DESTRUCTIVE TESTING TECHNIQUE

Workshop for Skyline Aeronautical Engineering on 20th to 22nd May 2013

Testing

Destructive testing (DT) Semi-destructive testing Non-destructive testing (NDT)

Tensile test Bend test

Charpy test Hardness test Fatigue test Creep test

Metallography Chemical analysis, etc.

Coring test Tension test

Carbonation test Mortar test

Ultrasonic test Eddy current test Radiography test

Visual test Magnetic particle test

Penetrant test IR thermography

Leak test Acoustic emission

Laser shearography

SURESH SENANAYAKE | AEA 2

NDT Definition Non-destructive testing (NDT) is the testing of material to detect internal and surface defects or discontinuities using methods that do not damage or destroy the material under test.


Engineering Materials Materials

Aluminum

Copper

Zinc

Titanium

Tungsten

Nickel

Steel

Stainless steel

Cast iron

Ferrous Non-Ferrous

Metals Ceramics Polymers Composites


Material Discontinuities Should identify the types of metal manufacturing and service discontinuities (i.e. to know what causes the defects)

Defects

Inherent Processing Defects

Primary Processing Defects

Secondary Processing Defects Service Defects

The refining stage where metals are extracted from ores - Inclusions (Slag) - Porosity (Blown

hole) - Pipe - Segregation

Metal ingots are worked into usable forms such as billets or blooms by wrought processing or casting - Seams - Laps - Lamination - Forging bursts - Inclusions - porosity

Final stages of parts manufacturing - Grinding cracks

(thermal cracks) - Machining cracks - Welding defects - Heat treat cracks

(quenching cracks)

Occurs during the use of the part - Fatigue cracks - Stress cracks - Corrosion


Where is NDT used?

• where we need to ensure the serviceability of a specimen

• where we cannot afford the cost of a failure of the specimen because failure would be financially unacceptable or cause harm to us

• exist to prevent injury or death to the human user of the tested item


Why we need NDT? Generally, NDT is employed in various industries for the following reasons; i. To prevent accidents and save human lives ii. To improve product reliability iii. To give profit to the user by;

a. Ensuring customer satisfaction b. Helping in better product design c. Controlling manufacturing processes d. Lowering manufacturing processes e. Maintaining a uniform quality level


When is NDT used?

• NDT is used both before, during and after construction

• Using NDT "before or during construction" prevents a substandard material or part from wasting time and increasing scrap production

• Using NDT after to monitor performance after being service.


NDT METHODS There are six major NDT methods ; • Visual testing (VT) • Radiography testing (RT) • Ultrasonic testing (UT) • Magnetic particles testing (MT) • Liquid penetrant testing (PT) • Eddy current testing (ET)

NDT

Eddy Current Testing

Visual Testing


Other NDT methods;

• Leak testing • Strain gauging • Acoustic emission • IR Thermography • Laser shearography


NDT…. The choice of test method to be carried out on a certain piece depends on several factors, the most important ones being;

• Types of discontinuity expected

• Inherent limitations of each method

• Working conditions • Material to be tested


Indication • Each non-destructive test is designed to

provide visual evidence of discontinuities in parts, which are not normally visible to the unaided eye.

• The visual evidence left by each method is called an indication.

NOTE: There is no single NDT method capable of detecting all type of discontinuities


VISUAL TESTING Eye of a qualified technician can be supplemented with various optical aids and mechanical gauges for visual inspection.

• It is a most commonly used NDT method for detecting and evaluating defects.

• Inspect using human eye. • Optical equipment use to enhance sensitivity. But decreases the

area of coverage. SURESH SENANAYAKE | AEA 13

Visual Testing…. • Employs the eyes to look directly at the component • Mostly used on detection of corrosion • Needs lot of skill and training

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Training- 01


Visual Testing…..

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Training- 02


Visual Testing……

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Training- 03


Most basic and common inspection method.

Tools include Lights, Mirrors, fiberscopes, borescopes, magnifying glasses and CCTV.

Robotic crawlers permit observation in hazardous or tight areas, such as air ducts, reactors, pipelines.

Portable video inspection unit with zoom allows

inspection of large tanks and vessels, railroad tank

cars, sewer lines.

Visual Inspection


RADIOGRAPHY TESTING (RT) • Principle of Operation

– Using electromagnetic radiation (X-, Gamma-, Neutron-, etc.) to penetrate through materials.

– Discontinuities is recorded on film


Radiography….. Testing by means of radiation is based on the following factors: • The capacity that radiation has

to pas through matter • Different radiation absorption

depending on the piece being tested

• The possibility of measuring this absorption differences

The testing involves the use of, A radiation source A radiation detector (film)


Radiographic Image A radiographic image is a document composed of a photographic film on to which the image of an object which the radiation has passed through is recorded.


Radiography….. The radiographic technique gives a true image of the piece section (a photographic image) allowing many different types of material to be examined, even very thick materials.

Main disadvantages


RADIOGRAPHY……

Limitations – High capital and running cost – Require source of electricity (in the case of X-ray) – Trained and skill operators are necessary – Pose potential radiation hazard – Not sensitive to planar defect

Applications – Applicable to almost all metals and non-metals – Capable of detecting (and subsequently recording on the film)

surface and internal discontinuities


ULTRASONIC TESTING (UT) Principle of Operation

– Ultrasonic testing is based on the reflection that on acoustic wave is subjected to when, while moving through a certain material, it finds its propagation impared.

– Signal due to discontinuities is presented on cathode ray tube screen (CRT)


Ultrasonic…… The test requires a system comprising the following elements:

– A probe (which emits the acoustic beam) – A detection unit (which records the reflected beam on a screen)


Ultrasonic…..

The ultrasonic beam emitted by the probe passes through the test piece and, after reaching the bottom wall, is reflected back to the probe which signals it on the screen.

If the beam finds a discontinuity along its path, it returns to the probe more quickly, in this way signaling the presence of a defect.


Ultrasonic….. • The ultrasonic technique allows for immediate examination of the

piece with extremely low working times. • It is easy to carry out and proves particularly suitable for creating

automatic or semi-automatic systems as the discontinuity is detected by the presence or lack of signal from the reflected waves.


ULTRASONIC….. Applications • Applicable to almost all metals and non-metals • Capable of detecting surface and internal discontinuities • Automatic inspection and new computerized image processing allows

signal to be permanently recorded on paper • Measure thickness • Material characterization (e.g. measure elastic modulus, etc.)

Crankshaft - Northern Power Plant – Ultrasonic Testing


ULTRASONIC….. Limitations

– Relatively high capital cost – Requires highly trained and experienced operator – Interpretation of results can be extremely difficult – Not sensitive to defects parallel to the beam direction


MAGNETIC PARTICLE TESTING (MT) Principle of Operation

– Using magnetic or current flow to produce magnetic field in the materials

– The pattern of field distribution provides indication of the existence discontinuities


Magnetic Particle…..

• Magnetic testing is based on the following factors:

• The possibility of magnetizing the piece to be tested;

• Magnetic field variations generated by the piece discontinuities;

• The possibility of detecting surface and sub-surface variations piece in the piece’s magnetic field.


MAGNETIC PARTICLE……

Applications – Applicable to ferromagnetic materials – Capable of detecting surface and sub-

surface discontinuities – Easily operated –portable equipment

makes it suitable for field inspection


Magnetic Particle…… The test requires a system comprising the following elements: • Equipment for piece magnetization; • Magnetic powders (or particles) to be spread over the piece to

reveal the magnetic field variations.

Limitations – Not applicable to non-ferromagnetic materials – Requires a sources of electricity – Magnetization in two perpendicular directions is necessary – Inspected objects have to be demagnetized

Magnetization Equipment (Yoke)

Magnetic powders


Magnetic Particle crack indications


PENETRANT TESTING (PT) • Principle of Operation

– Using liquid to penetrate materials – Image of discontinuities become visible after

development


Penetrant Testing….. Penetrant testing requires a use of:

Penetrant liquids Developer


Penetrant Testing…… Advantages • It can be applied to any material (ferromagnetic

and non-ferromagnetic); • It can also be carried out on parts that are not

easily accessible; • It is relatively simple in terms of procedure and

interpretation; • Compared to other examinations, the equipment is

much more economical.

Limitations • It can detect only discontinuities opened to the

surface; • Discontinuities filled with extraneous matter

(dirt, oxides, etc.) cannot be detected; • Surface conditioning must be more accurate

than in other types of examination.

Applications – Inspect non porous materials (metals, glass, ceramic, etc.) – Detect surface defects – Simple equipment SURESH SENANAYAKE | AEA 36

EDDY CURRENT TESTING (ET) Principal of Operation

– Use electrical current in coils to induce eddy current within specimen

– Indicator will be deflected when discontinuities disturb the path of eddy current

The test probe in eddy current inspection is basically a coil of wire through which AC is passed. When AC is passed through the coil, a magnetic field is generated in and around the coil. When the probe is brought in close proximity to a conductive material, the magnetic field generates current flow in the material.

The induced current flows in closed loops in planes perpendicular to the magnetic flux and are named eddy currents.


Eddy Current….

Conductive material

Coil

Coil’s magnetic field

Eddy Currents

Eddy Current’s magnetic field


Eddy Current….

5/20/2013 MATERIAL

FLAW

PROBE PROBE PROBE PROBE

CRACK DETECTION


Eddy Current……

Applications – Inspect conducting materials – Detect surface and sub-surface disc – Measure hardness and thickness of layer and thin sheet

• Sub surface flaw detection – Multi-layered aircraft

structures. - Fuselage - Wings - Around Fasteners

– Phase indicates flaw depth – Magnitude indicates flaw

severity at depth


Eddy Current…..

Limitations – Defect detection is limited to only few mm below surface – Does not indicate the shape of discontinuities

• Conductivity measurement – Raw materials sorting – Manufacturing process

verification – Heat damage – Reference measurement


Eddy Current…..

• A typical NDT job – AC failure or fault found – Define DEFECT – Choose NDT method – Specify technique and

equipment – Risk analysis of fleet(s) – Inspection plan – Review design of faulty part


• Who is involved in Aerospace in service NDT? – Airline operators – Airworthiness authorities – Component part manufacturer – NDT equipment supplier – Aircraft manufacturer – Qualified NDT inspector


What needs to be avoided?


A defect that went undetected in an engine disk was responsible for the crash of United Flight 232.



Reliability of NDT

Method

Discontinuity

Superficial Internal

Detection Length evaluation

Height evaluation Detection Length

evaluation Height

evaluation Depth

evaluation

VT Adequate Adequate Inapplicable Inapplicable Inapplicable Inapplicable Inapplicable

PT Adequate Adequate Inapplicable Inapplicable Inapplicable Inapplicable Inapplicable

MT Adequate Adequate Applicable

with limitations

Low efficiency

Low efficiency Inapplicable Inapplicable

UT Applicable

with limitations

Low efficiency

Low efficiency Adequate Good Low

efficiency Adequate

RT Adequate Adequate Inapplicable Adequate Good Low efficiency

Applicable with double

exposure and calculus

ET Good Good Adequate Low efficiency

Low efficiency

Low efficiency

Low efficiency


Advantages of NDT • Tested objects or parts can be re-used (unless proven

defective) • Tests can be conducted to all samples (100% inspection) or

representative samples • More than one inspection techniques can be applied to a

similar object • Inspection on a certain product may be repeated • Requires minimum (or no) specimen preparation • Equipment are normally portable and suitable for field

inspection • Inspection may be performed while the objects or parts are in

service

Limitations of NDT • Results are normally qualitative • Requires highly trained and experienced personnel


Advantages of DT • Results are normally quantitative • Does not require highly trained and experienced personnel

Limitations of DT • Tested objects or parts become scrap and considered loss • Tests can only be conducted to representative samples

(100% inspection is not possible) • Each object or part can only be tested once • Repetitive inspection cannot be performed on a similar

object • Objects or parts to be inspected have to be taken away

from the system and replaced • Require systematic specimen preparation • Equipment are normally stationary


Thank you!


Documents

Basics of Non Destructive testing