ARTICLES RESEARCH & TECHNOLOGY

Application of Ultrasonic Technology in Engine Flaw Detection: Fundamentals of Ultrasound

aircraft-engine-ultrasonic-test

CHAPTER 1


Discovery of piezoelectric effect and development of SONAR

In 1880, French physicists Pierre and Jacques Curie discovered the piezoelectric effect. French physicist Paul Langevin attempted to develop piezoelectric materials as senders and receivers of high-frequency mechanical disturbances (ultrasound waves) through materials. His specific application was the use of ultrasound to detect submarines during World War I.

This technique, sound navigation and ranging (SONAR), finally became practical during World War II. Industrial uses of ultra-sound began in 1928 with the suggestion of Soviet Physicist Sokolov that it could be used to detect hidden flaws in materials.

Medical uses of ultrasound through the 1930s were confined to therapeutic applications such as cancer treatments and physical therapy for various ailments. Diagnostic applications of ultrasound began in the late 1940s through collaboration between physicians and engineers familiar with SONAR.

Non-destructive testing (NDT)

The application of physical principles for detecting inhomogeneities in materials without impairing the usefulness of the materials has brought into being a technique known as “nondestructive testing”-(NDT). These methods and techniques can be used to determine what variations or non-uniformities in properties can be tolerated in the anticipated service.

The rapid growth in the use of NDT methods and techniques these days has resulted from demands by industry for improved quality and especially by the critical requirements in the fields of jet aircraft, missiles, and nuclear energy etc.

A number of other NDT methods exist. These are used only for specialized applications and consequently are limited in use. Some of these methods are as follows:

  1. Visual f. Acoustic (sonic and ultrasonic)
  2. Pressure and leak
  3. Penetrate
  4. Thermal
  5. Radiography (X-ray and gamma ray)
  6. Acoustic (sonic and ultrasonic)
  7. Magnetic
  8. Electrical and electrostatic
  9. Electromagnetic induction
  10. Others

Penetrant testing

The dye penetrant testing can be used to locate discontinuities on material surfaces. A highly penetrating dye on the surface will enter discontinuities after a sufficient penetration time, and after removing the excess dye with a developing agent, the defects on the surface will be visible.

Four stages of liquid penetrant process:

(a) Penetrant application and seepage into the discontinuity.
(b) Removal of excess penetrant.
(c) Application of developer, and
(d) Inspection for the presence of discontinuities.

Magnetic particle testing

Magnetic particle testing is accomplished by inducing a magnetic field in a ferromagnetic material and then dusting the surface with iron particles. The surface will produce magnetic poles and distort the magnetic field in such a way that the iron particles are attracted and concentrated making defects on the surface of the material visible.

Eddy current testing

It uses electromagnetic induction to detect flaws in conductive materials. There are several limitations, among them: only conductive materials can be tested, the surface of the material must be accessible, the finish of the material may cause bad readings, the depth of penetration into the material is limited by the materials’ conductivity, and flaws that lie parallel to the probe may be undetectable.

Radiographic testing method

Radiographic testing can be used to detect internal defects in castings, welds or forgings by exposure the construction to x-ray or gamma ray radiation. Defects are detected by differences in radiation absorption in the material as seen on a shadow graph displayed on photographic film or a fluorescent screen.

Ultrasonic testing

Ultrasonic Testing (UT) uses high-frequency sound waves (typically in the range between 0.5 and 15 MHz) to conduct examinations and make measurements. Besides its wide use in engineering applications (such as flaw detection/evaluation, dimensional measurements, material characterization, etc.), ultrasonic are also used in the medical field (such as sonography, therapeutic ultrasound, etc.).

In general, ultrasonic testing is based on the capture and quantification of either the reflected waves (pulse-echo) or the transmitted waves (through-transmission). Each of the two types is used in certain applications, but generally, pulse echo systems are more useful since they require one-sided access to the object being inspected.

 

Introduction to Ultrasound

Before defining ultrasonic waves it is better to discuss the propagation of sound in a medium, and this is demonstrated below in Fig.1. The vibration of the air by the voice in one of the compartments either left or right vibrates the thin paper right and left.

This vibration is carried in a string tensioned tightly to the left and reaches another thin paper of the left pipe, and the paper begins to vibrate right and left. Then, the air in the left pipe vibrates and the vibration reaches the ear and we can hear the voice.

principle-sound-propagation

Figure1.1 Principle of sound propagation

We human beings can hear a sound of a frequency ranging from about 20 to 20000 HZ. When we become older, it becomes difficult for us to hear a sound of higher frequency.

All sounds are produced by vibrations in bodies. In musical instruments, the sound is emitted by vibrating strings or a reed. Our voice is the result of vibrations of our vocal cords. The number of vibrations a body makes per second is called its frequency and is commonly referred to as hertz.

Ultrasound is defined as “…sound waves having a frequency above the limits of human hearing, or in excess of 20,000 cycles per second (hertz).” So by definition, ultrasound is totally undetectable by human ears unless aided by instruments capable of translating ultrasound to audible sound.

frequency-ranges-ultrasound

Figure1.2 Frequency ranges corresponding to ultrasound, with rough guide of some applications

Characteristics of wave propagation

Frequency
The number of cycles per unit of time is called the frequency. For convenience, the frequency is most often measured in cycles per second (cps) or the interchangeable Hertz (Hz) (60 cps = 60 Hz). It is denoted by letter f.

1 H z =1 cycle per second

1 KHz = 1000 Hz = 1000 cycles per second

1 MHz = 1000000 Hz = 1000000 cycles per second

frequency-different-hertz

Figure1.3 Frequency of different hertz

Wave Length

The distance between one peak or crest of a wave and the next peak or crest. It is equal to the speed of the wave divided by its frequency, and to the speed of a wave times its period. It is denoted by Greek letter λ.

Velocity

The speed with which energy is transported between two points in a medium by the motion of waves is known as the velocity of the waves. It is usually denoted by the letter V.

Fundamental wave equations

When a mechanical wave traverses a medium, the displacement of a particle of the medium from its equilibrium position at any time t is given by :
a = a0 sin 2 π f t ———– (1.1)

Where,

 

a = Displacement of the particle at time t.
a0 = Amplitude of vibration of the particle,
f = Frequency of vibration of the particle.
A graphical representation of equation 1.1 is given in figure 1.3

graphical-representation-equation

Figure1.4 Graphical representation of equation 1.1

Equation 1.1 is the equation of motion of a mechanical wave through a medium. It gives the state of the particles (i.e. the phase) at various distances from the particle first excited at a certain time t.
a = a0 sin 2 π T f (t – X/V) ————- (1.2)

Where,

a = Displacement (at a time t and distance X from the first excited particle) of a particle of the medium in which mechanical wave is travelling.
a0 = Amplitude of the wave which is the same as that of the amplitude of vibration of the particles of the medium.
V= Velocity of propagation of the wave.
f= Frequency of the wave

the-graphical-representation-equation-1.2

Figure1.5 The graphical representation of equation 1.2

Since in the time period T, a mechanical wave of velocity V travels a distance λ in a medium, therefore we have:-

λ = V T ——— (1.3)

But the time period T is related to the frequency f by

f =1/T ——— (1.4)

Combining equations we have the fundamental equation of all wave motion i.e.

V = λ f ——– (1.5)

Where,

f = frequency
λ= Distance covered in one cycle is wavelength
V= Velocity of Ultrasonic wave inside the medium in ‘mm/s’

Ultrasonic waves

Ultrasonic waves consist of frequencies greater than 20 kHz and exist in excess of 25 MHz. The most common range for testing metals is from 2 MHz to 5 MHz.

Acoustic impedance

The resistance offered to the propagation of an ultrasonic wave by a material is known as the acoustic impedance. It is denoted by the letter Z and is determined by multiplying the density ρ of the material by the velocity V of the ultrasonic wave in the material i.e.

Z= ρ V ———– (1.6)

Where,

Z = Acoustic Impedance
ρ= Density
V = Velocity

Acoustic pressure and intensity

Acoustic pressure is the term most often used to denote the amplitude of alternating stresses on a material by a propagating ultrasonic wave. Acoustic pressure P is related to the acoustic impedance Z and the amplitude of particle vibration as:

P = Z a ——– (1.7)

Where P = acoustic pressure.

Z = acoustic impedance.

a = amplitude of particle vibration.

The transmission of mechanical energy by ultrasonic waves through a unit cross-section area, which is perpendicular to the direction of propagation of the waves, is called the intensity of the ultrasonic waves. Intensity of the ultrasonic waves is commonly denoted by the letter I.

Intensity I of ultrasonic waves is related to the acoustic pressure P, acoustic impedance Z and the amplitude of vibration of the particle as:

I=P2 / 2Z ——– (1.8)

And

I= P a /2 ——— (1.9)

Where,

I = intensity
P = acoustic pressure
Z = acoustic impedance.
a = amplitude of vibration of the particle.

Types of ultrasonic waves

Longitudinal waves

In longitudinal waves, the oscillations occur in the longitudinal direction or the direction of wave propagation. Since compression and expansion forces are active in these waves, they are also called pressure or compression 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 movements.

direction-propagation-longitudinal-wave-parallel-displacement-particle

Figure1.6 Direction of propagation longitudinal wave and parallel displacement of particle

Transverse or shear waves

In the transverse or shear waves, 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.

direction-propagation-transverse-wave-perpendicular-displacement-particle

Figure1.7 Direction of propagation transverse wave and perpendicular displacement of particle

Surface or Rayleigh waves

Surface waves were first described by Lord Rayleigh and that is why they are also called Rayleigh waves. These type of waves can only travel along a surface bounded on one side by the strong elastic forces of the solid and on the other side by the nearly nonexistent elastic forces between gas molecules. Surface waves, therefore, are essentially nonexistent in a solid immersed in a liquid, unless the liquid covers the solid surface only as a very thin layer.

direction-propagation-surface-wave-elliptical-displacement-particle

Figure1.8 Direction of propagation surface wave and elliptical displacement of particle

Surface waves are useful for testing purposes because the attenuation they suffer for a given material is lower than for an equivalent shear or longitudinal wave and because they can flow around corners and thus be used for testing quite complicated shapes. Only surface or near surface cracks or defects can be detected, of course.

Lamb or plate waves

With Lamb waves, a number of modes of particle vibration are possible, but the two most common are symmetrical and asymmetrical. The complex motion of the particles is similar to the elliptical orbits for surface waves. Symmetrical Lamb waves move in a symmetrical fashion about the median plane of the plate.

This is sometimes called the extensional mode because the wave is “stretching and compressing” the plate in the wave motion direction. Wave motion in the symmetrical mode is most efficiently produced when the exciting force is parallel to the plate.

The asymmetrical Lamb wave mode is often called the “flexural mode” because a large portion of the motion moves in a normal direction to the plate, and a little motion occurs in the direction parallel to the plate. In this mode, the body of the plate bends as the two surfaces move in the same direction.

direction-propagation-lamb-wave-displacement-particle

Figure1.9 Direction of propagation lamb wave and displacement of particle

Velocity of ultrasonic waves

The velocity of propagation of longitudinal, transverse, and surface waves depends on the density of the material, and in the same material, it is independent of the frequency of the waves and the material dimensions.
Velocities of longitudinal and transverse are given by the following equations.

euation-1-10

 

 

 

Where,

VL = Longitudinal Wave Velocity
E = Modulus of Elasticity
ρ = Density
µ= Poisson’s Ratio
equation-1-11

 

 

Where:

Vs = Shear Wave Velocity
E = Modulus of Elasticity
r = Density
m = Poisson’s Ratio
G = Shear Modulus

Ultrasonic Testing for Nondestructive material testing

The ultrasonic principle is based on the fact that solid materials are good conductors of sound waves. The waves are not only reflected at the interfaces but also reflected by internal flaws. The interaction effect of sound waves with the material is stronger the smaller the wavelength, this means the higher the frequency of the wave.

This means that ultrasonic waves must be used in a frequency range between about 0.5 MHz and 25 MHz and that the resulting wavelength is in or mm. With lower frequencies, the interaction effect of the waves with internal flaws would be so small that detection becomes questionable. Both test methods, radiography, and ultrasonic testing  are the most frequently used methods of testing different test pieces for internal flaws, partly covering the application range and partly extending it.

This means that today many volume tests are possible with the more economical and non-risk ultrasonic test method, on the other hand, special test problems are solved, the same as before, using radiography. In cases where the highest safety requirements are demanded (e.g. nuclear power plants, aerospace industry) both methods are used.

Listed below are the primary advantages and disadvantages when compared to other NDT methods.

Advantages of Ultrasonic Testing

1. It is sensitive to both surface and subsurface discontinuities.
2. The depth of penetration for flaw detection or measurement is superior to other NDT methods.
3. Only single-sided access is needed when the pulse-echo technique is used.
4. It is highly accurate in determining reflector position and estimating size and shape.
5. Minimal part preparation is required.
6. It provides instantaneous results.
7. Detailed images can be produced with automated systems.
8. It is nonhazardous to operators or nearby personnel and does not affect the material being tested.
9. It has other uses, such as thickness measurement, in addition to flaw detection.
10. Its equipment can be highly portable or highly automated.

Disadvantages of Ultrasonic Testing

1. Surface must be accessible to transmit ultrasound.
2. Skill and training are more extensive than with some other methods.
3. It normally requires a coupling medium to promote the transfer of sound energy into the test specimen.
4. Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect.
5. Cast iron and other coarse-grained materials are difficult to inspect due to low sound transmission and high signal noise.
6. Linear defects oriented parallel to the sound beam may go undetected.
7. Reference standards are required for both equipment calibration and the characterization of flaws.

 

Next: CHAPTER 2