Non Destructive Testing

Nondestructive testing (NDT) system without impairing has been defined as comprising those test methods used to examine an object, material or its future usefulness.

The term is generally applied to nonmedical investigations of material integrity .

Characteristics of NDT

• Applied directly to the product • Tested parts are not damaged • Various tests can be performed on the same product • Specimen preparation not required • Can be performed on parts that are in service • Low time consumption • Low labour cost

Objectives of NDT

(1) to ensure product integrity, and in turn, reliability; • • • • To detect internal or surface flaws To measure the dimensions of materials To determine the materials’ structure To evaluate the physical and mechanical properties of materials (2) to avoid failures, prevent accidents and save human life; (3) to make a profit for the user; (4) to ensure customer satisfaction and maintain the manufacturer's reputation; (5) to aid in better product design; (6) to control manufacturing processes; (7) to lower manufacturing costs; (8) to maintain uniform quality level; (9) to ensure operational readiness.

Aspects / Factors in NDT Method

• • • • • Energy source or medium used to probe the test object (such as X-rays, ultrasonic waves or thermal radiation); Nature of the signals, image or signature resulting from interaction with the test object (attenuation of X-rays or reflection of ultrasound, for example); Means of detecting or sensing resulting signals emulsion, piezoelectric crystal or inductance coil); (photo Method of indicating or recording signals deflection, oscilloscope trace or radiograph); and (meter Basis for interpreting the results (direct or indirect indication, qualitative or quantitative, and pertinent dependencies).

NDT Methods

1. Visual Inspection 2. Liquid penetrant method 3. Ultrasonic Inspection 4. Radiography methods • •

X-ray radiography & fluoroscopy γ- ray radiography

5. Eddy current testing 6. Magnetic particle testing 7. Thermography

LIQUID PENETRANT METHOD

Principle

A liquid penetrant is applied at the surface of the specimen. The penetrant is drawn by the surface flaws due to capillary action and this is subsequently revealed by a developer, in addition with visual inspection.

Procedure

i.

Cleaning the surface ii.

iii.

Application of the penetrant Removal of excess penetrant iv. Developing v.

Inspection

LIQUID PENETRANT METHOD

Characteristics of a penetrant & a developer

    

Penetrant

 Chemical stability & uniform physical consitency   High degree of wettability Quick & complete penetrability   Low viscosity Sufficient brightness & permanence of colour Chemical inertness Low toxicity Slow drying Ease of removal Low cost

Developer

 Highly absorptive  Fine grain size & particle shape for easy dispersion  Provision of contrast background   Easy application Formation of thin uniform coating over surface   Easily wettable Low toxicity

Applications

• Turbine rotor discs & blades • Aircraft wheels, castings, forged components, welded assemblies • Automotive parts – pistons, cylinders, etc.

• Bogie frames of railway locomotives & rolling stock • Electrical ceramic parts – spark plug insulators, glass-to-metal seals, etc.

• Moulded plastic parts

Advantages & Limitations of Liquid Penetrant Method

ADVANTAGES

• Simple & inexpensive • Versatile & portable • Applicable to ferrous, non-ferrous, non magnetic & complex shaped materials which are non-porous & of any dimension • Detects cracks, seams, lack of bonding, etc.

LIMITATIONS

• Detect surface flaws • Non-porous surface for material • Surface cleaning before & after inspection • Deformed surfaces & surface coatings prevent detection

ULTRASONIC FLAW DETECTION

Block Diagram for an Ultrasonic Flaw Detector

Master Timer Signal Pulse Generator Time Base Amplifier X Echo Signal Amplifier Work piece Probe (Transducer) Y CRT

Principle

Whenever there is a change in the medium, the ultrasonic waves are reflected.

Thus, from the intensity of the reflected echoes, the flaws are detected without destroying material.

the

Principle & Block Diagram

Applications

• Quality control & material inspection • Detection of failure of rail rolling stock axes, pressure columns, earthmoving equipments, mill rolls, mixing equipments, etc.

• Measurement of metal section thickness • Thickness measurements – refinery & chemical processing equipments, submarine hulls, aircraft sections, pressure vessels, etc.

• Inspect pipe & plate welds • Inspect pins, bolts & shafts for cracks • Detect internal corrosion

Advantages & Limitations

    

Advantages

 Sensitive to surface & subsurface discontinuities   Superior depth of penetration for flaw detection High accuracy – position, size & shape of defect Minimal part preparation Instantaneous result Automated detailed images Non hazardous Portable

Limitations

• Surface accessibility for ultrasonic transmission • Highly skilled & trained manpower • Irregular, rough, coarse grained or non homogenous parts, linear defects oriented parallel to the beam cannot be inspected – low transmission & high noise • Coupling medium required • Reference standards – equipment calibration & flaw characterization

ULTRASONIC SCANNINING METHODS

A-SCAN

 Amplitude Mode display  1-D information  Single fixed transducer  Spike – Strength of echo  Position – Penetration depth  Detects position & size of flaws

B-SCAN

 Brightness mode display  2-D information  Single movable transducer  Brightness & size of dot – Intensity & strength of echo  Position – Penetration depth  Exact information of internal structure of flaw

T. M. SCAN

•Time-motion mode display •Moving object information •Single fixed transducer •X-axis – dots – position of defect depending on depth •Y – axis – movement of object

Applications

• Measurement of velocity of fluids through pipes • Three dimensional image of specimen obtained • Detect corrosion in pipes and pressure vessels

Radiography

The formation of an image of the test piece either on a photographic film or on a fluorescent screen due to x-rays or γ-rays passing through the test piece.

Law of Absorption of X-rays

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I o e

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x

X-RAY RADIOGRAPHY

Principle

X-rays are passed through the specimen under inspection and it is differentially absorbed specimen.

by the The transmitted x-rays are received by the photographic film and the film is developed.

The dark and light shadows reveal the defects present in the specimen and hence the defects defected.

are

X-RAY RADIOGRAPHY – Displacement Method Principle

X-rays are exposed over the specimen by keeping the x-ray position source at ‘A’ and then at ‘B’ by displacing the source through a certain distance.

The images are recorded at positions ‘A’ and ‘B’.

From the displacements of the x-ray tube and the images, the exact position of the defect can be determined.

Merits & Demerits of X-ray Displacement Method

MERITS

• Material suitability • Used on castings and weldings • Determination of thickness • Used on uneven surfaces • Time consumption is less • Permanent record

DEMERITS

• Expensive • Development time consumption large • Skilled & trained personnel required • Tissue damage due to radiations

X-RAY FLUOROSCOPY

Principle

X-rays passed the specimen and is made to fall on are through a fluorescent screen.

respect defects specimen, to in With the the there will be a variation in intensity.

MERITS & DEMERITS

Merits

• No need of washing and developing films • Low cost • Image viewed immediately on screen • Time consumption is less • Movement of defects detected (real time images) • Permanent record can be made

Demerits

• Poor resolution • Low image contrast • Electronic image intensifier required for increasing the contrast

Differences

Radiography

• Image developed on photographic film • High resolution & contrast • Immediate image cannot be obtained.

• X-ray energy is converted into chemical energy.

• Expensive • Time consumption is high.

Fluoroscopy

• Image is developed on fluorescent screen.

• Fair resolution and low contrast.

• Immediate image can be viewed through the monitor.

• X-ray energy is converted into visible light.

• Inexpensive.

• Time consumption is low