Ultrasonic Non Destructive Testing

Mahesh Lohith K.S, VVIET,Mysore

1 Introduction

1.1 Wave Modes

The waves are characterized in a medium by oscillatory patterns that are capa-ble of maintaining their shape and propagating in a stable manner. Thus theproagation of waves can occur through different modes in a solid medium. Thedifferent types of wave propagation in a solid medium are

1. Longitudinal

2. Transverse of Shear

3. Surface Waves or Rayleigh Waves

4. Plate Waves (Lamb waves)

The Surface or Rayleigh waves travel on the sufrace of the relatively thick solidmaterial pertaining to a depth of one wavelength. It combines both longitudinaland transverse wave motions to create an elliptical wave motion on the surface.The direction of propagation is perpendicular to the plane of ellipse. Plate wavesare similar to Rayleigh waves but they can only be generated in a material ofthickness of few wavelengths. They are the complex vibrational waves thatpropagate parallel to the test sufrace throughout the thickness of the material.

1.2 Non Destructive Testing(NDT)

Inherent flaws in the work piece of a machine such as cracks, pores and microcav-ities may result is a fatal failure of the machine, thus affecting the production.Hence it is very important to detect the flaws in the part. Destructive method oftesting may not help for machine parts due to structural damage occuring withit. Thus, Non Destructive Testing,is a method used to test a part for the flawswithout affecting the physical properties and causing no structural damage toit. There are many methods of NDT techniques available for testing. Some ofthem are

1. Liquid Penetration Test

2. Eddy Current Test

3. Magnetic Particle Test

4. X-ray and Gamma ray Radiography Test

5. Ultrasonic Test

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Figure 1: Propagation of sound energy

2 Ultrasonic Testing Principle

Ultrasonics are the sound waves whose frequency is greaterthan 20kHz. Due tothe high frequency they have a very good penetrating power. When sound wavespropagate from one medium to another a part of the sound energy is reflectedand the rest is transmitted at the interface seperating the two media [see fig1]. This property is made use to detect flaws because not only intefaces alsothe flaws can reflect the ultrasonic sound energy. The interaction of the soundenergy is stronger for higher frequencies. Hence high frequency ultrasound inthe frequency range 0.5 MHz to 25MHz are found suitable for the testing.Thewaves are generated by using either a Piezo-electric energised crystal cut in aparticular fashion to generate the desired wave mode or an Electromagneticaccoustic transducer. The relation among the intensities of the incident and

reflected sound energy is given by I2 = I1

{ρ1−ρ2ρ1+ρ2

}2

The intensity of the sound wave reflected from the interface generally de-pends upon the difference in the densities of the pair of media (ρ1 − ρ2) for thegiven incident wave intensity. Here ρ1 and ρ2 are the densities of the two media1 and 2 respectively through which the sound wave is propagating. Thus, if theultrasonic wave propagates from a medium of higher density into a medium oflower density then maxium reflection of intensity takes place at the interfaceseperating the two media. The flaw in the medium results in the reflection ofsound energy due to the variation of density and hence thier detection is madepassible. reflections are analysed electrically and the reflection is called echo.

3 Velocity of ultrasonics in solids

3.1 Ultrasonic testing setup and method

The ultrasonic testing device consists of a probe using which the pulses of soundenergy can be generated , the part which is to be tested and an oscilloscope toanalyse the echoes electrically. In order to transfer the sound waves effectivelyinto the part a layer coupling material like gel is used between the probe andthe part.The probe performs both transmission and reception of soundwaves.

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Figure 2: Ultrasonic testing setup

Its transduction converts electrical pulses into sound pulses and vice versa.

3.2 Pulse-Echo Method of Detection

The longitudinal ultrasonic pulses are generated using the probe. For eachgenerated pulse the echoes are observed on the oscilloscope as shown in thefigure 2. The first echo corresponds to the reflection from the upper surface ofthe part. If there exists a flaw, a second echo is observed with a lower pulseheight due to smaller reflection intensity. A third echo is observed due to thereflection from th back surface. The intensity of the echo from the back surfacereflection is less due to attenuation of sound energy in the medium. The Analyisof the echoes using CRO provides the time taken by the sound to travel too andfro distance from the surface to the flaw. Thus the flaw detection is achievedand the depth of the flaw can be calculate using the formula S = Ct

2 Here ’S’ isthe depth of the flaw, ’C’ is the velocity of ultrasonics in the medium and ’t’ isthe time elapsed between too and fro journey of ultrasonic energy.

3.3 Determination of velocity of ultrasonics in solids

A standard specimen with accurate demensions(Thickness) can be used to deter-mine the velocity of ultrasonics in solids. Using the pulse echo method the timeduration for the too and fro journey of the back reflection can be determined.Thus the Velocity of ultrasonic is determined using the formula C = 2S

t

3.4 Determination of Elastic constants

The velocites of longitudinal ultrasonic waves(CL) and shear ultrasonic waves(CS) are determined for the given solid using pulse echo method. The elasticconstants are determined using the following formulae.

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The Poisson’s Ratio is given by σ =1−2(

CSCL

)22−2(

CSCL

)2The Young’s Modulus is given by E = 2ρC2

S (1 + σ)The Rigidity Modulus is given by n = ρC2

S

Here ρ is the density of the material

4 Velocity of ultrasonics in liquids

4.1 Determination by forming the Acoustic Grating

The velocity of Ultrasonics in a liquid can be determined by using the method ofdiffraction of light by the Aqua-Grating. The given liquid in filled into a smallglass chamber.A quartz crystal is mounted in between two metal plates and isimmersed in the given liquid. These plates are connected to an oscillator whichcan generate audio frequencies. The frequency is so adjusted that the crystalvibrates in resonance with the oscillator.The vibration of crystal produces ultra-sonics in the medium which undergo reflection from the walls of the containeras shown in the Fig.3.

Figure 3: Acoustic Grating

Due to superposition of forward and reflected waves, longitudinal station-ary waves are formed. The density is maximum at nodes and minimum atantinodes. The arrangement is called acoustic grating. The acoustic gratingis mounted on the prism table of a spectrometer. A parallel beam of sodiumlight (S) is allowed to incident normally on acoustic grating and the diffractedlight is viewed through the telescope and diffraction pattern consisting of manyprincipal maxima is observed. The position of principal maxima is given by

d sin θn = nλWhere λ = wavelength of sodium light,d = Grating Constant,θn = angle of

diffraction for nth order, n = order of spectrum.It λu be the wavelength of ultrasonic through the medium, then d = λu

2

Thus λu

2 sin θn = nλ

⇒ λu = 2nλsin θn

If the resonant frequency of piezoelectric crystal is ν, the velocity of ultra-sonic wave is given by

v = νλu or v = 2νnλsinθn

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