Sailajamehra.. writes for u…

Just another WordPress.com weblog

Archive for the ‘Know About Non-destructive Ultrasonic Techniques’ Category

Know About Non-destructive Ultrasonic Techniques

Posted by sailjamehra on December 21, 2013

Introduction

Ultrasound is simply sound pitched above human hearing. It has found many uses in many areas. At home, we use ultrasound for dog whistles, burglar alarms, and jewelry cleaners. In hospitals, doctors use ultrasound to remove kidney stones without surgery  and to image fetal development during pregnancy. In industry, ultrasound is important for emulsifying cosmetics and foods, welding plastics, cutting alloys, and large-scale cleaning. Ultrasonic irradiation differs from traditional energy sources (such as heat, light, or ionizing radiation) in duration, pressure, and energy per molecule.

Sound & Ultrasound

Sound is nothing more than waves of compression and expansion passing through gases, liquids or solids. We can sense these waves directly through our ears if they have frequencies from about Hertz to 16 kHz. These frequencies are similar to low frequency radio waves, but sound is intrinsically different from radio or other electromagnetic radiation. For example, electromagnetic radiation (radio waves, infrared, visible light, ultraviolet, x-rays, gamma rays) can pass through a vacuum without difficulty; on the other hand, sound cannot because the compression and expansion waves of sound must be contained in some form of matter.

High intensity sound and ultrasound are generally produced in a similar fashion: electric energy is used to cause the motion of a solid surface, such as a speaker coil or a piezoelectric ceramic. Piezoelectric materials expand and contract when an electric field is applied. For ultrasound a high frequency alternating electric current is applied to a piezoelectric attached to the wall of a metal container.

Ultrasound has frequencies pitched above human hearing (above roughly 16 kHz). Scientists can make narrow beams of “silent” ultrasound far more intense than the roar of a jet engine, but completely unheard by our ears. Ultrasound has wavelengths between succession compression waves measuring roughly 10 cm to 10-3 centimeters. These are not comparable to molecular dimensions. Because of this mismatch, the chemical effects of ultrasound cannot result from a direct interaction of sound with molecular species.

It would be greatly advantageous to be able to quickly and accurately discern texture in relatively thin plate and sheet material during its production. With such information, the manufacturing process could be efficiently controlled to produce a product of desired texture characteristics, or to correct texture deficiencies within a short time of when they occur. Knowledge of this texture is important, for example, in predicting the capability of the metal to be formed into parts of complex shape during formation.

Utilization of x-ray or neutron diffraction techniques was used for texture analysis earlier that required periodic sampling of the continuously produced sheet or plate material or metal rods. This involve  destructive analysis from the periodically existing samples and also such processes are time consuming and therefore cannot be used to immediately correct or change the production processes. Furthermore, x-ray processes give data only regarding the near surface of the material. Even though the material being analyzed is relatively thin sheet or plate, the texture characteristics can change drastically through its cross-section. The neutron processes provide information about the entire thickness of the material. However, the samples must be taken to a neutron source to perform the analysis. Thus, the accuracy of these processes is not as reliable or convenient as is desired. Therefore, a real need exists for improvement regarding the monitoring and estimation of texture in sheet or plate or thick metal pieces.

It has previously been known that certain properties and inferences of texture can be derived from the analysis of received ultrasonic energy after it has been passed through the material. In particular, it has been discovered that texture might be inferred from measuring the differences in speed of ultrasonic energy in different directions through the material. The speed of wave propagation and energy loss by interactions with material microstructure is key factors in ultrasonic determination of material properties including imperfections. Relatively small variations of velocity and attenuation can indicate significant property variations. Ultrasonic methods can be used to determine micro structural differences in metals. The testing can be either the measurement of ultrasonic attenuation or the measurement of bulk sound velocity.

Applications of ultrasonics has made possible not only the evaluation of physico-chemical properties of the mixtures/solutions but also more reliability on the interpretation of molecular interactions. Due to low cost easy operational procedure and spontaneous result, the molecular interactions studies through ultrasonics have gained importance all over the world. Thus I am motivated towards working in the areas of ultrasonic and have taken up this study.

The objective of present study therefore is to measure ultrasonic velocity of ultrasonic energy waves through different samples in a particular direction and make a comparative study of some physical parameters like density, adiabatic compressibility young’s modulus, reflection and transmission coefficients of the samples which in turn can reveal the quality of samples, using non-destructive ultrasonic investigation of the material. The main purpose of this study is to search for the correlations between ultrasonic properties and the material properties in the light of metallographic examinations. The results of these measurements are used to predict properties of the material.

Basic Principles of Ultrasonics

           Ultrasonic Testing (UT) uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional measurements, material characterization, and more. A typical UT inspection system consists of several functional units, such as the pulser/receiver, transducer, and display devices. A pulser/receiver is an electronic device that can produce high voltage electrical pulses. Driven by the pulser, the transducer generates high frequency ultrasonic energy. The sound energy is introduced and propagates through the materials in the form of waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface. The reflected wave signal is transformed into an electrical signal by the transducer and is displayed on a screen. Ultrasonic Inspection is a very useful and versatile NDT method. Some of the advantages of ultrasonic inspection that are often cited include:

  • It is sensitive to both surface and subsurface discontinuities.
  • The depth of penetration for flaw detection is superior.
  • Minimal part preparation is required.
  • Electronic equipment provides instantaneous results.
  • It has other uses, such as thickness measurement, in addition to flaw detection.

As with all NDT methods, ultrasonic inspection also has its limitations, which include:

  • Surface must be accessible to transmit ultrasound.
  • Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect.
  • Linear defects oriented parallel to the sound beam may go undetected.
  • Reference standards are required for both equipment calibration and the characterization of flaws.

The above introduction provides a simplified introduction to the NDT method of ultrasonic testing.  However, to effectively perform an inspection using ultrasonics, much more about the method needs to be known. The following pages present the information on the science involved in ultrasonic inspection, the equipment that is commonly used, some of the measurement techniques used, as well as other information.

In solids, sound waves can propagate in four principle modes that are based on the way the particles oscillate. Sound can propagate as longitudinal waves, shear waves, surface waves, and in thin materials as plate waves. Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic testing. The particle movement responsible for the propagation of longitudinal and shear waves is illustrated below.

 

sonic1

In longitudinal waves, the oscillations occur in the direction of wave propagation. Since compressional and dilational forces are active in these waves, they are also called pressure or compressional waves. They are also sometimes called density waves because their particle density fluctuates as they move. Compression waves can be generated in liquids, as well as solids because the energy travels through the atomic structure by a series of compression and expansion (rarefaction) movements.

In the transverse or shear wave, the particles oscillate at a right angle or transverse to the direction of propagation. Shear waves require an acoustically solid material for effective propagation, and therefore, are not effectively propagated in materials such as liquids or gasses. Shear waves are relatively weak when compared to longitudinal waves. In fact, shear waves are usually generated in materials using some of the energy from longitudinal waves.

Longitudinal and transverse (shear) waves are most often used in ultrasonic inspection. Surface (or Rayleigh) waves travel the surface of a relatively thick solid material penetrating to a depth of one wavelength. Surface waves combine both a longitudinal and transverse motion to create an elliptic orbit motion, the major axis of the ellipse is perpendicular to the surface of the solid. As the depth of an individual atom from the surface increases the width of its elliptical motion decreases. Rayleigh waves are useful because they are very sensitive to surface defects.

Plate waves are similar to surface waves except they can only be generated in materials a few wavelengths thick.  Lamb waves are the most commonly used plate waves in NDT.  Lamb waves are complex vibrational waves that propagate parallel to the test surface throughout the thickness of the material. Propagation of Lamb waves depends on the density and the elastic material properties of a component.  They are also influenced a great deal by the test frequency and material thickness. Lamb waves are generated at an incident angle in which the parallel component of the velocity of the wave in the source is equal to the velocity of the wave in the test material. Lamb waves will travel several meters in steel and so are useful to scan plate, wire, and tubes.

Wavelength, Frequency and Velocity

The properties of waves propagating in isotropic solid materials are wavelength, frequency, and velocity. The wavelength is directly proportional to the velocity of the wave and inversely proportional to the frequency of the wave. This relationship is shown by the following equation

sonic2

In ultrasonic testing, the shorter wavelength resulting from an increase in frequency will usually provide for the detection of smaller discontinuities. The wavelength of the ultrasound used has a significant effect on the probability of detecting a discontinuity. A general rule is that a discontinuity must be larger than one-half the wavelength to stand a reasonable chance of being detected.

Sensitivity and resolution are two terms that are often used in ultrasonic inspection to describe a technique’s ability to locate flaws. Sensitivity is the ability to locate small discontinuities. Sensitivity generally increases with higher frequency (shorter wavelengths). Resolution is the ability of the system to locate discontinuities that are close together within the material or located near the part surface. Resolution also generally increases as the frequency increases.

As frequency increases, sound tends to scatter from large or course grain structure and from small imperfections within a material. Cast materials often have coarse grains and other sound scatters that require lower frequencies to be used for evaluations of these products. Wrought and forged products with directional and refined grain structure can usually be inspected with higher frequency transducers.

Since more things in a material are likely to scatter a portion of the sound energy at higher frequencies, the penetrating power (or the maximum depth in a material that flaws can be located) is also reduced. It should be mentioned, so as not to be misleading, that a number of other variables will also affect the ability of ultrasound to locate defects. These include the pulse length, type and voltage applied to the crystal, properties of the crystal, backing material, transducer diameter, and the receiver circuitry of the instrument.

The Speed of Sound

Hooke’s Law, when used along with Newton’s Second Law, can explain a few things about the speed of sound. The speed of sound within a material is a function of the properties of the material and is independent of the amplitude of the sound wave. Newton’s Second Law says that the force applied to a particle will be balanced by the particle’s mass and the acceleration of the the particle. Mathematically, Newton’s Second Law is written as F = ma. Hooke’s Law then says that this force will be balanced by a force in the opposite direction that is dependent on the amount of displacement and the spring constant (F = -kx). Therefore, since the applied force and the restoring force are equal, ma = -kx can be written. The negative sign indicates that the force is in the opposite direction.

Since the mass m and the spring constant k are constants for any given material, it can be seen that the acceleration a and the displacement x are the only variables. It can also be seen that they are directly proportional. For instance, if the displacement of the particle increases, so does its acceleration. It turns out that the time that it takes a particle to move and return to its equilibrium position is independent of the force applied. So, within a given material, ultrasonic wave always travels at the same speed no matter how much force is applied when other variables, such as temperature, are held constant.

Properties of material that affect speed of sound

Ultrasonic wave does travel at different speeds in different materials. This is because the mass of the atomic particles and the spring constants are different for different materials. The mass of the particles is related to the density of the material, and the spring constant is related to the elastic constants of a material. The general relationship between the speed of sound in a solid and its density and elastic constants is given by the following equation:

sonic3

Where V is the speed of sound, C is the elastic constant, and  is the material density. This equation may take a number of different forms depending on the type of wave (longitudinal or shear) and which of the elastic constants that are used. The typical elastic constants of materials include:

  • Young’s Modulus, E: a proportionality constant between uniaxial stress and strain.
  • Poisson’s Ratio, n: the ratio of radial strain to axial strain
  • Bulk modulus, K: a measure of the incompressibility of a body subjected to hydrostatic pressure.
  • Shear Modulus, G: also called rigidity, a measure of a substance’s resistance to shear.
  • Lame’s Constants, l and m: material constants that are derived from Young’s Modulus and Poisson’s Ratio.

When calculating the velocity of a longitudinal wave, Young’s Modulus and Poisson’s Ratio are commonly used. When calculating the velocity of a shear wave, the shear modulus is used. The subscript ij attached to C in the above equation is used to indicate the directionality of the elastic constants with respect to the wave type and direction of wave travel. In isotropic materials, the elastic constants are the same for all directions within the material. However, most materials are anisotropic and the elastic constants differ with each direction. For example, in a piece of rolled aluminum plate, the grains are elongated in one direction and compressed in the others and the elastic constants for the longitudinal direction are different than those for the transverse or short transverse directions.

Examples of approximate compressional sound velocities in materials are:

  • Aluminum – 0.632 cm/microsecond
  • 1020 steel – 0.589 cm/microsecond
  • Cast iron – 0.480 cm/microsecond.

Examples of approximate shear sound velocities in materials are:

  • Aluminum – 0.313 cm/microsecond
  • 1020 steel – 0.324 cm/microsecond
  • Cast iron – 0.240 cm/microsecond.

When comparing compressional and shear velocities, it can be noted that shear velocity is approximately one half that of compressional velocity.

Attenuation of Sound Waves

When sound travels through a medium, its intensity diminishes with distance. In idealized materials, sound pressure (signal amplitude) is only reduced by the spreading of the wave. Natural materials, however, all produce an effect which further weakens the sound. This further weakening results from scattering and absorption. Scattering is the reflection of the sound in directions other than its original direction of propagation.  Absorption is the conversion of the sound energy to other forms of energy.  The combined effect of scattering and absorption is called attenuation.  Ultrasonic attenuation is the decay rate of the wave as it propagates through material. Attenuation is generally proportional to the square of sound frequency. Also, the actual value of the attenuation coefficient for a given material is highly dependent on the way in which the material was manufactured.

Acoustic Impedance

Sound travels through materials under the influence of sound pressure. Because molecules or atoms of a solid are bound elastically to one another, the excess pressure results in a wave propagating through the solid.

The acoustic impedance Z of a material is defined as the product of its density and acoustic velocity V.

Z = V

Acoustic impedance is important in

  • the determination of acoustic transmission and reflection at the boundary of two materials having different acoustic impedances.
  • the design of ultrasonic transducers.
  • assessing absorption of sound in a medium.

The acoustic impedance can be calculated for any material, if  its density and acoustic velocity V are known.

Reflection and Transmission Coefficients (Pressure)

Ultrasonic waves are reflected at boundaries where there is a difference in acoustic impedances (Z) of the materials on each side of the boundary.  This difference in Z is commonly referred to as the impedance mismatch.  The greater the impedance mismatch, the greater the percentage of energy that will be reflected at the interface or boundary between one medium and another.

The fraction of the incident wave intensity that is refracted can be derived because particle velocity and local particle pressures must be continuous across the boundary.  When the acoustic impedances of the materials on both sides of the boundary are known, the fraction of the incident wave intensity that is reflected can be calculated with the equation below.  The value produced is known as the reflection coefficient.  Multiplying the reflection coefficient by 100 yields the amount of energy reflected as a percentage of the original energy.

Since the amount of reflected energy plus the transmitted energy must equal the total amount of incident energy, the transmission coefficient is calculated by simply subtracting the reflection coefficient from one.

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. The active element is the heart of the transducer as it converts the electrical energy to acoustic energy, and vice versa. The active element is basically a piece of polarized material (i.e. some parts of the molecule are 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, resulting in induced dipoles within the molecular or crystal structure of the material. This alignment of molecules will cause the material to change dimensions. This phenomenon is known as electrostriction. 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.

The active element of most acoustic transducers used today is a piezoelectric ceramic, which can be cut in various ways to produce different wave modes. Piezoelectric crystals made from quartz crystals and magnetostrictive materials were primarily used. When piezoelectric ceramics were introduced, they soon became the dominant material for transducers due to their good piezoelectric properties and their ease of manufacture into a variety of shapes and sizes. They also operate at low voltage and are usable up to about 300oC. The first piezoceramic in general use was barium titanate, and that was followed during the 1960’s by lead zirconate titanate compositions, which are now the most commonly employed ceramic for making transducers. New materials such as piezo-polymers and composites are also being used in some applications.

The thickness of the active element is determined by the desired frequency of the transducer. A thin wafer element vibrates with a wavelength that is twice its thickness. Therefore, piezoelectric crystals are cut to a thickness that is 1/2 the desired radiated wavelength. The higher the frequency of the transducer, the thinner the active element. The primary reason that high frequency contact transducers are not produced is because the element is very thin and too fragile.

Characteristics of Piezoelectric Transducers

The transducer is a very important part of the ultrasonic instrumentation system. The transducer incorporates a piezoelectric element, which converts electrical signals into mechanical vibrations (transmit mode) and mechanical vibrations into electrical signals (receive mode). Many factors, including material, mechanical and electrical construction, and the external mechanical and electrical load conditions, influence the behavior of a transducer. Mechanical construction includes parameters such as the radiation surface area, mechanical damping, housing, connector type and other variables of physical construction. Transducer manufacturers are hard pressed when constructing two transducers that have identical performance characteristics.

A cut away of a typical contact transducer is shown above. It was previously learned that the piezoelectric element is cut to 1/2 the desired wavelength. To get as much energy out of the transducer as possible, an impedance matching is placed between the active element and the face of the transducer. Optimal impedance matching is achieved by sizing the matching layer so that its thickness is 1/4 of the desired wavelength This keeps waves that were reflected within the matching layer in phase when they exit the layer(as illustrated above). For contact transducers, the matching layer is made from a material that has an acoustical impedance between the active element and steel.

sonic4

Immersion transducers have a matching layer with an acoustical impedance between the active element and water. Contact transducers also incorporate a wear plate to protect the matching layer and active element from scratching.

The backing material supporting the crystal has a great influence on the damping characteristics of a transducer. Using a backing material with impedance similar to that of the active element will produce the most effective damping. Such a transducer will have a wider bandwidth resulting in higher sensitivity. As the mismatch in impedance between the active element and the backing material increases, material penetration increases but transducer sensitivity is reduced.

sonic5

The backing material supporting the crystal has a great influence on the damping characteristics of a transducer. Using a backing material with impedance similar to that of the active element will produce the most effective damping. Such a transducer will have a wider bandwidth resulting in higher sensitivity. As the mismatch in impedance between the active element and the backing material increases, material penetration increases but transducer sensitivity is reduced.

                             

Transducer Efficiency, Bandwidth and Frequency

Some transducers are specially fabricated to be more efficient transmitters and others to be more efficient receivers. A transducer that performs well in one application will not always produce the desired results in a different application. For example, sensitivity to small defects is proportional to the product of the efficiency of the transducer as a transmitter and a receiver. Resolution, the ability to locate defects near the surface or in close proximity in the material, requires a highly damped transducer.

It is also important to understand the concept of bandwidth, or range of frequencies, associated with a transducer. The frequency noted on a transducer is the central or center frequency and depends primarily on the backing material. Highly damped transducers will respond to frequencies above and below the central frequency. The broad frequency range provides a transducer with high resolving power. Less damped transducers will exhibit a narrower frequency range and poorer resolving power, but greater penetration. The central frequency will also define the capabilities of a transducer. Lower frequencies (0.5MHz-2.25MHz) provide greater energy and penetration in a material, while high frequency crystals (15.0MHz-25.0MHz) provide reduced penetration but greater sensitivity to small discontinuities. High frequency transducers, when used with the proper instrumentation, can improve flaw resolution and thickness measurement capabilities dramatically. Broadband transducers with frequencies up to 150 MHz are commercially available. Transducers are constructed to withstand some abuse, but they should be handled carefully. Misuse, such as dropping, can cause cracking of the wear plate, element, or the backing material.

Couplant

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 (i.e. such as the test specimen) 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.

Pulse-echo ultrasonic measurements can determine the location of a discontinuity in a part or structure by accurately measuring the time required for a short ultrasonic pulse generated by a transducer to travel through a thickness of material, reflect from the back or the surface of a discontinuity, and be returned to the transducer. The transducer employed is a 5 MHz broadband transducer 0.25 inches in diameter. In most applications, this time interval is a few microseconds or less. The two-way transit time measured is divided by two to account for the down-and-back travel path and multiplied by the velocity of sound in the test material. The result is expressed in the well-known relationship

                           d = vt/2 or v = 2d/t

where d is the distance from the surface to the discontinuity in the test piece, v is the velocity of sound waves in the material, and t is the measured round-trip transit time. A wide variety of transducers with various acoustic characteristics have been developed to meet the needs of industrial applications. Typically, lower frequencies are used to optimize penetration when measuring thick, highly attenuating or highly scattering materials, while higher frequencies will be recommended to optimize resolution in thinner, non-attenuating, non-scattering materials. It is possible to measure most engineering materials ultrasonically, including metals, plastic, ceramics, composites, epoxies, and glass as well as liquid levels and the thickness of certain biological specimens.

sonic6

Experimental techniques

Ultrasonic Interferometer For Solids

sonic7

           Non-Destructive Testing of Material is an important part of Engineering Education as it gives information without deformation in the shape and size of the material. One of the NDT techniques, Piezoelectric Technique is widely used for the measurement of composition dependent properties such as ultrasonic velocity, compressibility, elastic constant, Young’s modulus and Bulk modulus. Its suitability for metals, plastics, polymers and crystals etc makes it versatile tool for Engineering Physics, Material Science and Polymer Science. This low cost NDT apparatus is being used in several I.I.T.s/Universities/Engineering Colleges for laboratory experiments and Research work.

Instrument: It consists of Piezoelectric Oscillator, power supply, quartz rod, holder, quartz rod with sample, connecting cables and R.F. meter. When a bar, plate or sheet of metal is manufactured a number of working processes are involved that impart a crystallographic texture to the metal component. It is crucial that the final properties of the metal are suited to the process that would be applied in the next stage of product manufacture. As an example, one of the major users of sheet aluminium and steel is the canning industry, where the can bodies are drawn from the sheet metal. Incorrect texture results in increased wastage and production costs to the point where the metal may actually tear in the manufacturing process. Sheet metal manufacturers do not routinely use online inspection techniques to monitor the texture of the metal as it is produced, and most tend to perform X-ray  for ‘spot checks’ on the final product.

The electric circuit is housed in a metal cabinet. Connections to the crystal holder and CRO are made using cables. The setup assembly is shown in the above figure.

Working Principle:

This figure illustrates the principle of desonic9termining the resonant frequency. A is a specially designed amplifier having gain of 39(G=38) in the frequency range of operation. The output of the amplifier is applied to the quartz rod in series with Rf, a 10-turn potentiometer provided with a calibrated dial to facilitate direct reading of the value of Rf .The voltage across Rf is fed back to the input of the amplifier. The system oscillates at the resonant frequency of the quartz when

sonic10

Where Rc is the resistance of the quartz at resonance. By varying Rf the system can be made to oscillate .The oscillations are detected using CRO and the frequency measured by digital frequency meter.

Experimental study

For the experiment, different samples like brass, iron, aluminium and marble were taken. Samples can also have low thermal and electrical conductivity and low sensitivity to temperature variation. The many factors related to the properties of samples include the properties of the base material, the type, shape, dimensions, geometric arrangement and volume fraction of the samples and the presence/absence of voids. The measurements are frequency dependent. The method for measuring quality of samples using non-destructive ultrasonic investigation, comprising the steps of:

  • imposing ultrasonic energy waves through the material
  • measuring the velocity of ultrasonic energy waves in the material in a particular direction.

The masses of the quartz rod and the specimens are measured. The length of the specimens (L) using a traveling microscope are measured. The density of the specimens is calculated. The longitudinal crystal holder with the main circuit is connected using cords supplied with the instrument. The CRO is connected to the main unit using co-axial cable supplied. The longitudinal quartz rod R is inserted in the crystal holder such that the holder pins are approximately at the center of the plated electrode face of the crystal. Cement the quartz rod to the specimen, in the form of rectangular rod of identical cross-section, using cementing glue provided. Rf (10 turn potentiometer) is varied and the system oscillating (as indicated by the C.R.O. pattern) above a particular value of Rf  isobserved. The resonant frequency of the quartz rod (fq) is measured.

RESULTS AND DISCUSSION

 Ultrasonic Measurements
Four samples of different nature were taken. The specimens were machined and ground to obtain the parallel surfaces. The dimensions of the specimens were measured by a travelling microscope having 0.001 cm accuracy and are tabulated in Table 1. The longitudinal ultrasonic waves were generated and applied. The wave was transmitted into the specimen. Sound travels through materials under the influence of sound pressure. Because molecules or atoms of a solid are bound elastically to one another, the excess pressure results in a wave propagating through the solid. The propagation velocity of the sound was determined using the formula given below. Within a given material, we find that ultrasonic wave travels at the same speed.  However, ultrasonic wave does travel at different speeds in different materials. There is a direct relation between the acoustic velocity and the molecular packing of the samples. This is because the mass of the atomic particles and the spring constants are different for different materials. The mass of the particles is related to the density of the material, and the spring constant is related to the elastic constants of a material. Therefore ultrasonic velocity in samples, to a great extent, reflects the sample morphology.

 Samples

Length

(m)

Breadth

(m)

Thickness

(m)

Volume

(m3)x10-7

Mass

(Kg)

Density

ρ(Kg/m3)

Brass 0.01402 0.00388 0.00436 2.37 0.002159 9103.04
Iron 0.0142 0.00412 0.00413 2.42 0.002016 8343.63
Aluminium 0.01388 0.00369 0.00342 1.75 0.000536 3060.01
Marble 0.01454 0.00502 0.00454 3.31 0.000826 2492.62

Table 1: Physical parameters of the samples

The results of ultrasonic measurements are given at Table 2 and 3. In this technique, the specimen in the form of rectangular rod for longitudinal oscillations is cemented to a quartz rod of identical cross section and resonant frequency of the composite system fc is determined using the apparatus. The resonant frequency of the quartz crystal fq is also determined. From the knowledge of fq, fc and the masses of the quartz mqand the specimen ms, the resonant frequency of the specimen fs is evaluated using the relation

Samples

fc

(kHz)

fq

(kHz)

(fc-fq)

kHz

mq

(Kg)

ms

(Kg)

mq /ms

fs

(kHz)

Brass 124.1878 124.078 0.1098 0.937 2.159 0.433997 124.24
Iron 151.136 124.078 27.058 0.937 2.016 0.464782 163.71
Aluminium 139.48 124.078 15.402 0.937 0.536 1.748134 166.40
Marble 128.742 124.078 4.664 0.937 0.826 1.134383 134.03

Table 2: Resonant frequencies of the samples

Using the value of fs, the length of the specimen and the density of the specimen, the velocity of the ultrasonic waves in the specimen v and compressibility  can be calculated using relations

where is density of specimen .
Young’s Modulus of specimen is calculated using relation

 

The acoustic impedance Z of a material is calculated using the relation

Z = V

where  is the density of the sample and V is the acoustic velocity in that sample and are tabulated in Table 3.

Samples

velocity

(m/s)

Density

(Kg/m3)

Impedance

(Z) x 106

Young’s modulus Y     (N/m2)

Compressibility

β

Brass 3483.6 9103.04 31.711

  1.1047×10 11

0.9052 x10-11
Iron 4649.4 8343.63 38.793

1.804×10 11

0.5544×10-11
Aluminium 4619.4 3060.01 14.135

0.653×10 11

1.5315×10-11
Marble 3897.7 2492.62 9.715

0.379×10 11

2.6407×10-11

Table 3: Acoustical parameters of the samples

 

A graph is drawn between impedance and density Vs various samples and is shown in Fig.1. We can understand from Fig.1 that impedance and density are

directly proportional to each other and are high for brass and least for marble. It is evident from the fact that as the density of brass is more, the number of particles available to convey the acoustic energy through it is more and so ultrasonic wave velocity in brass must be more. But from Table 3 we find that ultrasonic wave velocity in brass is the least compared to other samples indicating that it is inversely proportional to density. The mass of particles of brass is more and the elastic constant of brass may also be more and therefore the acoustic energy supplied may displace the particles to lesser extent. As acceleration is directly proportional to the displacement of the particles, this situation naturally leads to lesser ultrasonic wave velocity in brass. Fig. 2 shows the variation of longitudinal velocity and density Vs various samples. We can ascertain from this figure that they are inversely proportional. We can also find that ultrasonic velocity in iron sample is comparatively more than other samples. Fig.3 shows the variation of Young’s modulus and adiabatic compressibility Vs various samples. From this graph we can infer that Young’s

 

 

modulus will be more for the samples in which ultrasonic velocity is more i.e in the sample iron. When the Young’s modulus is more, automatically adiabatic compressibility will be less.  This indicates that the pressure due to the sound energy delivered at the sample, causes the grains of the sample to be elongated or stretched in the longitudinal direction and therefore compression in that direction will be less which is the reason for the reduction in compressibility. Hereby, we come across a strange fact that though the ultrasonic wave velocity in the marble sample is more when compared to the brass sample and so we expect more Young’s modulus in marble but we get a contrary situation as can be seen from Table.3  & Fig.3. This may be due to the fact that the porosity or the void spaces of marble sample may be more than the brass sample. The sharp edges of samples cause critical stress concentrations which results in a decrease in Young’s Modulus. When the acoustic energy is delivered to the marble sample, the grains are getting adiabatically compressed rather elongation. The compressibility of marble is nearly double that of aluminium.

 

 

The velocity of ultrasound that propagates in an inhomogeneous medium depends upon the overall effective stiffness and density of the medium. Table.3 confirms that the ultrasonic velocity and the Young’s modulus are the highest and adiabatic compressibility is the least for the iron sample. Therefore, the porosity or the void spaces between the atoms will be least in iron. Thus, the study of ultrasonic velocities and Young’s modulus allows determining quickly and accurately the quality of ductile irons. The influence of matrix structure on ultrasonic velocity was relatively unapparent in the samples. Thus, the effect of the matrix structure on the modulus of elasticity and hence ultrasonic velocity would not be significant.

 

 

 

 

 

 

Samples

 

Density

measured

(Kg/m3)

Density

reported

(Kg/m3)

Impedance

measured

(Z) x 106

Impedance

reported

 (Z) x 106

Young’s modulus

 Y

measured    (N/m2) x 1011

Young’s modulus Y

reported

    (N/m2)

Compressibility measured

 β x 10-11

Compressibility reported

 β x 10-11

Brass

9103.04

8100

31.711

35.88

1.105

0.9052

Iron

8343.63

6900

38.793

24.15

1.804

0.5544

Aluminium

3060.01

2700

14.135

17.06

0.653

1.5315

Marble

2492.62

9.715

0.379

2.6407

 

 

 

 

 

 

 

 

 

 

 

 

Table 4: Reflected and transmitted power in the samples

Sample Impedance

(Z1)x106

Pasm-1

Impedance

(Z2) x10 6

Pasm-1

ar

=[z2-z1/z1+z2]2

at =

1-ar

Pr=

[Z2-Z1/Z1+Z2]2

Pt =

2Z2/Z1+Z2

Brass 14.29 31.71 0.413 0.857 0.379 1.379
Iron 14.29 38.79 0.213 0.787 0.462 1.462
Aluminium 14.29 14.14 0.0000317 0.999968 -0.0056 0.994
Marble 14.29 9.72 0.036 0.964 -0.19 0.8096

For SiC reinforced Al composites the reinforcement is much stiffer than the matrix material, while the density of the reinforcemnet is comparable to that of the matrix material. Thus, ultrasound propagates faster in SiC reinforced composites than in unreinforced ones. Consequently, when SiC % increases, stiffness so the elastic modulus of the composite increases. This will cause increase in the longitudinal wave velocity in accordance with the following equation (see Figure 2 ).

 

VL = { ( E/ ) (1- ) / [(1+ ) (1-2 )]} 1/2   (3)

where  , E and  are density, Young’s Modulus and Poisson’s ratio, respectively

 

2 Effect of SiC Powder Size on Ultrasonic Properties
For constant % SiC, as particle size decreases the number of particles increases, therefore stiffness of the composite increases. Thus, velocity increases in accordance with equation 3.

In this study, material properties of Al-SiC composites produced by hot pressing were investigated by ultrasonic methods. The main distinguishing point is the possibility of controlling the size and distribution of SiC particles during production. Since the process parameters are going to be under control, it will be also possible to control production quality. Consequently, without specimen preparation and metallographic examination or mechanical testing, material properties of metal-matrix composites can be examined ultrasonically during or at the end of any production route. The conclusions for the present study can be summarized as follows:

    • For constant powder sizes, as SiC % in the composite increases, sound velocity and apparent attenuation also increase.
    • For constant composition, while reinforcement particle size decreases, sound velocity increases. As matrix particle size decreases, sound velocity increases.
    • For 25 and 100 mm Al powder, as the size of SiC powder increases, apparent attenuation increases; however, for 180 mm Al powder the behaviour is reversed. In order to analyse the effect of Al-powder size, further research is necessary.
    • Although frequency does not affect velocity, the apparent attenuation increases with decreasing frequency.

Advantages of this technique

  • This method is a non-destructive to the material being analyzed.
  •  It can be easily adapted to be used in the area of and during production of the material.
  • It does not require complex, time-consuming, or impractical processes or equipment to derive accurate texture estimations.
  • It utilizes ultrasound as an interrogating medium.

Properties of material that affect speed of sound

Sound travels through materials under the influence of sound pressure. Because molecules or atoms of a solid are bound elastically to one another, the excess pressure results in a wave propagating through the solid.

Conclusion

It was concluded that  as ultrasonic techniques are sensitive to the changes in the graphite morphology, they can also be used to predict the mechanical properties. The ultrasonic techniques eliminate the need for preparation and destructive testing of specimens, furthermore, they can be carried out in a few minutes, and are cost effective.

While there are no discrepancies about the predominant role of sample morphology on ultrasonic velocity most of the researchers believe that the sample structure does not affect the ultrasonic velocity measurements. For metallic matrix evaluation, it is recommended to combine the ultrasonic velocity with a second measurement that is sensitive to changes in the metallic matrix, such as hardness. The investigation  therefore allows accurate estimation of texture without having to utilize the commonly used destructive x-ray or neutron diffraction techniques. Additionally, the method can be in continuous operation and give quick “in-process” feedback. The manufacturing process can therefore be adjusted to change or correct texture characteristics according to desire. The value of such an in-process method is enormous to the rolled metal-plate industry. The application of the investigation can also be applied to similar materials. With such knowledge, characteristics such as hardness, grain size, ductility, strength and formability can be estimated. It is important to understand that the present invention allows the determination of these texture characteristics “in-process”; that is, contrary to conventional methods of destructive x-ray or neutron diffraction, the present invention allows non-destructive evaluation as the plate is being produced, and directly on the plate production line. The advantages of this much quicker, local, non-destructive method are many.

In one embodiment, one observes the frequency of waves which can be excited by a transducer having a fixed periodicity, which defines the wavelength of the waves at that frequency. However, thickness must be precisely known for determining ultrasonic velocity, as these methods average the elastic properties through the thickness of the samples over the path transversed by the ultrasonic waves.

Such investigation can be advantageous in many applications. It can be used in process control of sheet metal production and quality control of incoming materials for aerospace, automotive, packaging, and many other processes and industries.

Quality of samples can be determined by longitudinal ultrasonic velocity and attenuation measurements. As the sample varies, the ultrasonic velocity also varies as the density among the specimens varies. The main reason for velocity change is the elastic modulus. Changing the probe frequency did not affect the velocity values. If surfaces of samples according to the requirements are obtained, and if there is no side wall effects, the ultrasonic velocity measurements can be used as a quality control tool in a foundry (industries). For the evaluation of the metallic matrix, it is recommended to combine the ultrasonic measurements with another technique that is more sensitive to the changes in the matrix structure.

Cast iron has markedly greater strength and greater ductility than gray iron of similar composition. The relatively high strength and toughness of ductile irons give it an advantage in many structural applications. Some typical applications of cast iron are automotive and diesel-crankshafts, pistons, and cylinder heads, electrical fittings, switch boxes, motor frames and circuit breaker parts, mining-hoist drums, drive pulleys, flywheels, and elevator buckets, steel mill-work rolls, furnace doors, table rolls, and bearings, tool and die-wrenches, levers, handles, clamp frames, and miscellaneous dies for shaping steel, aluminium, brass, bronze, and titanium.

Most of the specifications for standard grades of ductile iron are based on properties-that is, strength and/of hardness is specified for each grade of ductile iron, and composition is either loosely specified or made subordinate to the mechanical properties. Many ductile iron castings are used as cast, but in some foundries, 50% or more are heat treated. Heat-treated ductile iron usually has more uniform mechanical properties than as-cast ductile iron, particularly in castings with wide variations in section thickness. As the matrix structure is varied progressively, hardness, strength and wear resistance increase, but impact resistance, ductile and machinability decrease.

 

So, within a given material, sound always travels at the same speed no matter how much force is applied when other variables, such as temperature, are held constant.

 

 

 

Applications

The application of ultrasonic waves to the joint between ceramic and metal during the joining process has improved joint strength by removing bubbles from the interface.

Construction and Building Materials ; Abstract An experimental study was conducted to evaluate the effect of concrete aggregate gradation, water-cement ratio, and curing time on measured ultrasonic wave velocity (UPV). 30 x 30 x 10 cm Portland cement concrete slabs were cast for ultrasonic evaluation, while 10 cm diameter by 20 cm

No soap, will wash!
Coventry Evening Telegraph (England) ; JAPANESE electrical giant Sanyo has unveiled what it says is the world’s first washing machine to use ultrasonic waves and electrolysis instead of liquids or powders.

 

Advertisements

Posted in Know About Non-destructive Ultrasonic Techniques | Leave a Comment »