Ultrasonics & Non Destructive Testing

Ultrasonic Non Destructive Testing Mahesh Lohith K.S,

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VVIET,Mysore

Introduction

1.1

Wave Modes

The waves are characterized in a medium by oscillatory patterns that are capable of maintaining their shape and propagating in a stable manner. Thus the proagation of waves can occur through different modes in a solid medium. The different 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 solid material pertaining to a depth of one wavelength. It combines both longitudinal and transverse wave motions to create an elliptical wave motion on the surface. The direction of propagation is perpendicular to the plane of ellipse. Plate waves are similar to Rayleigh waves but they can only be generated in a material of thickness of few wavelengths. They are the complex vibrational waves that propagate 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 microcavities 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 of testing may not help for machine parts due to structural damage occuring with it. Thus, Non Destructive Testing,is a method used to test a part for the flaws without affecting the physical properties and causing no structural damage to it. There are many methods of NDT techniques available for testing. Some of them are 1. Liquid Penetration Test 2. Eddy Current Test 3. Magnetic Particle Test 4. X-ray and Gamma ray Radiography Test 5. Ultrasonic Test 1

Figure 1: Propagation of sound energy

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Ultrasonic Testing Principle

Ultrasonics are the sound waves whose frequency is greaterthan 20kHz. Due to the high frequency they have a very good penetrating power. When sound waves propagate from one medium to another a part of the sound energy is reflected and the rest is transmitted at the interface seperating the two media [see fig 1]. This property is made use to detect flaws because not only intefaces also the flaws can reflect the ultrasonic sound energy. The interaction of the sound energy is stronger for higher frequencies. Hence high frequency ultrasound in the frequency range 0.5 MHz to 25MHz are found suitable for the testing.The waves are generated by using either a Piezo-electric energised crystal cut in a particular fashion to generate the desired wave mode or an Electromagnetic accoustic transducer. The relation among the intensities of the incident and n o2 2 reflected sound energy is given by I2 = I1 ρρ11 −ρ +ρ2 The intensity of the sound wave reflected from the interface generally depends upon the difference in the densities of the pair of media (ρ1 − ρ2 ) for the given incident wave intensity. Here ρ1 and ρ2 are the densities of the two media 1 and 2 respectively through which the sound wave is propagating. Thus, if the ultrasonic wave propagates from a medium of higher density into a medium of lower density then maxium reflection of intensity takes place at the interface seperating the two media. The flaw in the medium results in the reflection of sound energy due to the variation of density and hence thier detection is made passible. reflections are analysed electrically and the reflection is called echo.

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Velocity of ultrasonics in solids Ultrasonic testing setup and method

The ultrasonic testing device consists of a probe using which the pulses of sound energy can be generated , the part which is to be tested and an oscilloscope to analyse the echoes electrically. In order to transfer the sound waves effectively into the part a layer coupling material like gel is used between the probe and the 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 each generated pulse the echoes are observed on the oscilloscope as shown in the figure 2. The first echo corresponds to the reflection from the upper surface of the part. If there exists a flaw, a second echo is observed with a lower pulse height due to smaller reflection intensity. A third echo is observed due to the reflection from th back surface. The intensity of the echo from the back surface reflection is less due to attenuation of sound energy in the medium. The Analyis of the echoes using CRO provides the time taken by the sound to travel too and fro distance from the surface to the flaw. Thus the flaw detection is achieved and the depth of the flaw can be calculate using the formula S = Ct 2 Here ’S’ is the depth of the flaw, ’C’ is the velocity of ultrasonics in the medium and ’t’ is the 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 determine the velocity of ultrasonics in solids. Using the pulse echo method the time duration 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 elastic constants are determined using the following formulae.

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The Poisson’s Ratio is given by σ =

1−2 2−2

CS CL CS CL


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The Young’s Modulus is given by E = 2ρCS2 (1 + σ) The Rigidity Modulus is given by n = ρCS2 Here ρ is the density of the material

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Velocity of ultrasonics in liquids Determination by forming the Acoustic Grating

The velocity of Ultrasonics in a liquid can be determined by using the method of diffraction of light by the Aqua-Grating. The given liquid in filled into a small glass chamber.A quartz crystal is mounted in between two metal plates and is immersed in the given liquid. These plates are connected to an oscillator which can generate audio frequencies. The frequency is so adjusted that the crystal vibrates in resonance with the oscillator.The vibration of crystal produces ultrasonics in the medium which undergo reflection from the walls of the container as shown in the Fig.3.

Figure 3: Acoustic Grating Due to superposition of forward and reflected waves, longitudinal stationary waves are formed. The density is maximum at nodes and minimum at antinodes. The arrangement is called acoustic grating. The acoustic grating is mounted on the prism table of a spectrometer. A parallel beam of sodium light (S) is allowed to incident normally on acoustic grating and the diffracted light is viewed through the telescope and diffraction pattern consisting of many principal maxima is observed. The position of principal maxima is given by d sin θn = nλ Where 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 = λ2u Thus λ2u sin θn = nλ 2nλ ⇒ λu = sin θn If the resonant frequency of piezoelectric crystal is ν, the velocity of ultrasonic wave is given by 2νnλ v = νλu or v = sinθ n

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