Ultrasonic Testing Chris Wu

8/2/2019 Ultrasonic Testing Chris Wu

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Ultrasonic Testing

ASHISH KUMAR KHICHAR

08BEI013


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Introduction

High-frequency sound waves are sent out at a materialto find material changes

A pulser produces an electrical pulse that causes apiezoelectric transducer to send out a sound wave

Reflected waves are transformed back into electricalsignals by the transducer and analyzed

Its main applications are in thickness gauging and flawdetection


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Background

Originated from sonar technology, which was developedjust before World War II

Sonar uses technique of bouncing echoes off ofsubmerged objects to detect them

Ultrasonics, a form of nondestructive testing, wasapplied to safe life design, which ensures that

structures dont develop macroscopic defects during its

life, and any detection led to its removal


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Background (contd)

In the early 1970s, ultrasonic testing had made largestrides and could detect extremely small defects ordiscontinuities in metals

An unforeseen downside was that many manufacturerswere now discarding more metal parts that were deemedsatisfactory earlier

This in turn led to fracture mechanics, which allowed thedetermination of whether a crack of a given size wouldfail under a certain load if the fracture toughness wereknown.


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Background (contd)

Fracture mechanics allowed for the concept of fail safedesign, which stated that structures could have defectsas long as they would not grow to cause failure.

Over the past few years, ultrasonic testing has becomemore popular due to advances in both computer andinformation technology


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Theory and Technique

High frequency sound energy is used to find suchthings as material flaws and dimensionalmeasurement

Items needed to run an ultrasonic test:

Pulser/receiver unit

Piezoelectric transducer

Display devices


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The Role of Each Device

A pulser/receiver creates a high voltageelectrical pulse, which is sent throught thematerial in the form of propagating sound wavesby the transducer

Any discontinuity or flaw in the material willcause some energy to reflect back

The reflected waves are converted into anelectrical signal by the transducer and amplifiedby the receiver for signal processing


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The Role of Each Device (contd)

The amplified electrical signal is then displayedon a screen

The reflected signal strength is often displayedvs. time b/w signal generation and echoreception

D= vt/2 for normal beam inspection ofdiscontinuities, different for angle beams

This can lead to info on the flaws size, location,

and orientation among other things


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Picture Gallery

Test Specimen

Transducer

Pulser/Receiver

Display Device


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Pros

Advantages Sensitive to both surface and subsurface

discontinuities Penetration depth is better than other NDT methods

With pulse-echo, access to only one side is needed

Highly accurate in regards to reflector size, shape,

and locationMinimal part preparation


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Cons

Disadvantages Surface must be accessible to transmit ultrasound

More training required relative to other methods Coupling medium is normally required to promote

transfer of sound

Has difficulty inspecting rough, small, or irregularly

shaped objects Linear defects parallel to sound beam may go

undetected


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Piezoelectric Transducers

PTs contain polarized material

When electric charge isapplied, dipoles are induced

and dimensions change If a force is placed on the

material, it will changedimensions and create anelectric field


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The Piezoelectric Effect

Crystal

Current Meter= 0

+ - + - + -

+ - + - + -Charges cancel

each other, so

no current flow

Crystal material at rest: No forces applied,so net current flow is 0


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Crystal

Current Meter

deflects in +

direction

- - - - -

+ + + + +

Due to properties of symmetry,

charges are net + on one side &

net - on the opposite side: crystal gets

thinner and longer

Crystal material with forces applied

in direction of arrows..

Force


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Crystal

Current Meter

deflects in -

direction

+ + + +

- - - - -

. Changes the direction of

current flow, and the crystal gets

shorter and fatter.

Changing the direction of the

applied force..

Force


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The electromechanical effect

Crystal

. and, the crystal should get shorter and fatter.

When the switch is closed, and you apply the exact amountof power to get the same current that resulted when you squeezed

the crystal, the crystal should deform by the same amount!!

power source

(battery)

+ side

- side

+ + + +

- - - - -


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Transducer types

Contact Direct contact w/ specimen

Rugged casing and plates

Uses coupling material toremove air gaps

Immersion Non-contact

Operates in liquid


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Documents

Ultrasonic Testing Chris Wu