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*Unit –IV*ULTRASONIC TESTING

*By:Zoha NasirAssistant professorJIT, Barabanki

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*Ultrasonic Inspection

Ultrasonic detection equipment makes it possible to locate defects in all types of materials.

Minute cracks, checks, and voids too small to be seen by x-ray can be located by ultrasonic inspection.

An ultrasonic test instrument requires access to only one surface of the material to be inspected and can be used with either straight line or angle beam testing techniques.

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* 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 the material.

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*Types Of Ultrasonic Waves

Longitudinal waveTransverse waveSurface or Rayleigh wavesLamb or plate waves

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* Wave Type - Particle Vibration

Longitudinal (Compression)-Parallel to wave directionTransverse (Shear)-Perpendicular to wave directionSurface – Rayleigh- Elliptical orbit - symmetrical modePlate Wave – Lamb-Component perpendicular to surface

05/01/2023 Jahangirabad institute of technology 6*Methods For Testing

Two basic methods are used for ultrasonic inspection. The first of these methods is immersion testing. In this method of inspection, the

part under examination and the search unit are totally immersed in a liquid couplant, which may be water or any other suitable fluid.

The second method is called contact testing, which is readily adapted to field use and is the method discussed in this chapter. In this method, the part under examination and the search unit are coupled with a viscous material, liquid or a paste, which wets both the face of the search unit and the material under examination.

There are three basic ultrasonic inspection methods: (1) Pulse Echo. (2) Through Transmission. (3) Resonance.

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*Equipment And Transducers

Piezoelectric Transducers:

The conversion of electrical pulses to mechanical vibrations and the conversion of returned mechanical vibrations back into electrical energy is the basis for ultrasonic testing.

This conversion is done by the transducer using a piece of piezoelectric material (a polarized material having some parts of the molecule positively charged, while other parts of the molecule are negatively charged) with electrodes attached to two of its opposite faces.

When an electric field is applied across the material, the polarized molecules will align themselves with the electric field causing the material to change dimensions.

In addition, a permanently-polarized material such as quartz (SiO2) or barium titanate (BaTiO3) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. This phenomenon is known as the piezoelectric effect.

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*Transducer Types

Transducers are classified into two major groups according to the application

Contact TransducersImmersion Transducers

05/01/2023 Jahangirabad institute of technology 9*Couplants

A couplant is a material (usually liquid) that facilitates the transmission of ultrasonic energy from the transducer into the test specimen.

Couplant is generally necessary because the acoustic impedance mismatch between air and solids is large. Therefore, nearly all of the energy is reflected and very little is transmitted into the test material.

The couplant displaces the air and makes it possible to get more sound energy into the test specimen so that a usable ultrasonic signal can be obtained.

In contact ultrasonic testing a thin film of oil, glycerin or water is typically used between the transducer and the test surface.

When shear waves are to be transmitted, the fluid is generally selected to have a significant viscosity.

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Some couplants are:

Water OilGlycerinPetroleum greaseSilicon greaseWallpaper paste

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* Pulse Echo Technique

Ultrasonic pulse-receivers are well suited to general purpose ultrasonic testing.

Along with appropriate transducers and an oscilloscope, they can be used for flaw detection and thickness gauging in a wide variety of metals, plastics, ceramics, and composites.

Ultrasonic pulse-receivers provide a unique, low-cost ultrasonic measurement capability.

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* Data Presentation

Ultrasonic data can be collected and displayed in a number of different formats. The three most common formats are known in the NDT world as

A-scan, B-scan C-scan presentations.

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*A Scan

The A scan presentation displays the amount of received ultrasonic energy as a function of time.

The relative amount of received energy is plotted along the vertical axis and the elapsed time (which may be related to the traveled distance within the material) is displayed along the horizontal axis.

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*B Scan

It is possible for automated linear scanning systems where it shows a profile (cross-sectional) view of the test specimen.

In the B-scan, the time-of-flight (travel time) of the sound waves is displayed along the vertical axis and the linear position of the transducer is displayed along the horizontal axis.

From the B-scan, the depth of the reflector and its approximate linear dimensions in the scan direction can be determined.

The B-scan is typically produced by establishing a trigger gate on the A-scan.

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*C Scan

The C-scan presentation is a type of presentation that is possible for automated two-dimensional scanning systems that provides a plan-type view of the location and size of test specimen features.

The plane of the image is parallel to the scan pattern of the transducer.

C-scan presentations are typically produced with an automated data acquisition system, such as a computer controlled immersion scanning system.

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* Weld Defects

PorosityCracksSlagInclusionsUndercuttingLongitudinal or transverse cracks etc..

05/01/2023 Jahangirabad institute of technology 18*Applications

Quality control & material inspection.Detection of failure of rail rolling stock axes, pressure columns,

earthmoving equipment, mill rolls, mixing equipment, etc.Measurement of metal section thickness.Thickness measurements – refinery & chemical processing

equipment, submarine hulls, aircraft sections, pressure vessels, etc.Inspect pipe & plate welds.Inspect pins, bolts & shafts for cracks.Detect internal corrosion.

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* Advantages

Sensitive to surface & subsurface discontinuitiesSuperior depth of penetration for flaw detectionHigh accuracy – position, size & shape of defectMinimal part preparation

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* 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.