Ndt Basics

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Engineers India Limited

SPECIALIST MATERIALS AND MAINTENANCE SERVICES

DEPT.

6 March 2013



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Ultrasonic waves are the sound waves or the mechanical vibrations in a

medium. They travel with the speed of the sound in a medium. Ultrasonic testing of material is, besides radiography, one of the main

NDT methods used in the modern industry.

The main applications are

- Thickness measurement

- Discontinuity detection- Study of metallurgical structure

Types of ultrasonic waves

- Longitudinal waves

- Transverse waves- Surface waves or Rayleigh waves

- Lamb or plate waves

ULTRASONIC FLAW DETECTION (UFD)



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Ultrasonic methods are classified as

- through transmission method- pulse echo method

Pulse echo Ultrasonic Flaw Detection

- The ultrasonic test equipment principally comprises of an ultrasound

pulse generator, a receiver, it’s amplification and the display system.

- The pulse echo equipment can be sub-divided into three groups basedon display of information.

A scan

B scan

C scan- By UT flaw detection we can inspect the weld joints for the probable

presence porosity, slag inclusions, cracks, laminations and other defects.



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TIME OF FLIGHT DIFFRACTION (TOFD)

The TOFD is an effective fully computerized inspection method for

the detection and assessment of flaws . This technique usesdiffraction instead of reflection phenomena.

TOFD technique uses a transmitter and receiver placed on equaldistances of the weld focused at the same location in the weld. Thetransmitter sends compression waves into the material. Thecompression wave will propagate from the sender towards the

receiver. The first signal received in the receiver is the lateral wave(waves

near the surface). Because of the beamspread the wave will alsobounce back from the backwall; the difference in time period will givean accurate measurement of the thickness of the element.

When beam encounters an anomaly or flaw, the diffraction signalswill occur on the edges of indication. The diffracted signals will besent to the receiver, the differences in timepath will give a accuratemeasurement of the through wall height of the anomaly or defect.

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Applications

- TOFD can be applied on all low or non-alloyed materials from wallthicknesses 6mm and diameter from 80mm

- Application is possible even temperatures above 400 Deg C, under

water sub-sea pipelines and on wall thicknesses even over 400mm.

- Major application is on heavy wall thicknesses to verify the absence of

cracks and lack of fusion which can’t be detected by conventionalradiography techniques.

Advantages

- Very accurate and relatively fast method.

- The data is highly reproducible and can be recorded on permanent

data storage.

- The improved accuracy and reliability of the data makes it one of the

ideal tool for in-service inspections, RBI programs and Fitness for

service calculations.



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Ultrasonic phased arrays use a multiple element probe

whereby the output pulse from each element is timedelayed in such a way so as produce constructiveinterference at a specific angle and a specific depth.

These time delays can be incremented over a range of angles to sweep the beam over the desired angle range.

For example 40 to 75 degree beam sweep would beproduced by calculating the time delays to produceconstructive interference at 40, 41, 42…. 75 degrees.

The main advantages are

- ability to sweep a range of angles- ability to display the image in real time for the swept

angles

- ability to focus

PHASED ARRAY ULTRASONIC TESTING (PAUT)


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Phase array UT vs Manual UT

- Manual UT produces single A scan at a specific angle. But

PAUT displays images in real time showing the depth and

location of indication relative to the probe.

- Manual UT is limited to single refracted angle; PAUT

simultaneously takes data from a range of angles andreconstructs an image in real time.

- PAUT image is easy to comprehend as it gives a display of

the ultrasound superimposed on the test piece.

- Using an encoder with PAUT probe, all raw A scan data canbe stored and can be retrieved; but there is no data storage

facility in manual UT.



How it works:

-Torsional or longitudinal guided waves are inducedinto the pipe body and propagated along the pipe

segment being inspected.

- When these guided waves identify a defect or an

anomaly they mode convert into laminar waves and

reflect back to the tools original location.

- Using Laptop these signals are digitally captured and

processed. The time of flight for each signature is

calculated to determine the distance from the tool and

the significance of the anomaly. The octants determine

the position around the pipe.

Long Range Ultrasonic Testing (LRUT)



Capabilities

- Pipe diameters- 1.5 “ to 48” can be employed.

- 100 % coverage is possible.

- Test range:

typical ± 30 mtrs

under very ideal conditions: ± 150 mtrs of length

- Productivity

Normally 600m per day; Even 2-3 Km under ideal

conditions.- Service temperatures up to + 125 Deg C

- Removal of insulation, coating/painting is not required

for the execution of the test.



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Advantages

- Rapid screening for in-service degradation- Reduction in cost of gaining access

- No need of removal/reinstatement of insulation or

coating except at the location of transducer tool.

- Ability to inspect inaccessible areas.

Limitations

- Can be used only as initial screening before closely

examining the subject area by the conventionalmethods.

- The test range drastically reducing for painted/coated

pipes.



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THANK YOU

Ndt Basics

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