Text of Introduction to the Physics of Ultrasound Sound? Sound is a longitudinal wave that travels in a...
Introduction to the Physics of Ultrasound
Sound? Sound is a longitudinal wave that travels in a straight line Sound requires a medium for traveling Range of audible sound 20Hz -20kHz Range of audible sound 20Hz -20kHz
When a longitudinal wave moves through a material, the particles of the material move backwards and forwards along the direction in which the wave is travelling. particles When a longitudinal wave moves through a material, the particles of the material move backwards and forwards along the direction in which the wave is travelling. particles The wavelength of a longitudinal wave can be measured as the distance between the centre of two compressions wavelength The wavelength of a longitudinal wave can be measured as the distance between the centre of two compressions wavelength
Below is a picture of a longitudinal wave travelling along a spring.
Ultrasonic wave is a longitudinal wave with a frequency exceeding the upper limit of human hearing, which is 20 kHz. Ultrasonic wave
ULTRASONIC WAVE Frequency greater than audible sound (f>20kHz).
ULTRASONICS - Frequency greater than audible sound. INFRASONICS -Frequency lesser than audible sound SUPERSONIC Speed greater than sound.
General Properties of Ultrasonics Acoustic waves Frequency > 20kHz High energy waves Speed in a thin rod v = (Y/) 1/2, in liquid (K/) 1/2 in gas (P/ ) 1/2 Speed depends on frequency. Greater the frequency, higher the velocity Modes of propagation- longitudinal & transverse Can be reflected, refracted & diffracted
Heating effect Stirring effect. Attenuation A=A 0 e x Can be transmitted through large distances Stationary waves are produced
Piezo-electric effect When crystals like quartz, tourmaline etc. are subjected to stress along the mechanical axis, a P.D is developed across the perpendicular electrical axis. This is called Piezo electric effect Discovered by J & P. Curie Converse of this effect is also possible.
Converse piezo electric effect When a P.D is applied between the two opposite faces of a crystal, then stress or strain is induced along the perpendicular faces. If frequency of osculation is equal to the natural frequency of the crystal, resonance takes place and ultrasonics are produced.
Krautkramer NDT Ultrasonic Systems + The crystal gets thicker, due to a distortion of the crystal lattice Piezoelectric Effect
Krautkramer NDT Ultrasonic Systems + The effect inverses with polarity change Piezoelectric Effect
Krautkramer NDT Ultrasonic Systems An alternating voltage generates crystal oscillations at the frequency f U(f) Sound wave with frequency f Piezoelectric Effect
Piezo electric crystals Piezoelectric crystal: a crystal that exhibits piezo electric effect is called piezo electric crystals Eg:- Quartz, tourmaline, Rochelle salt etc
Piezoelectric Crystals The thickness of the crystal determines the frequency of the scanhead Low Frequency 3 MHz High Frequency 10 MHz
The circuit is basically a Hartley oscillator. The tank circuit produces high frequency alternating potential which is fed to the quartz crystal.
Tank circuit To produce high frequency alternating potential. The frequency
Piezoelectric generator This is based on the converse Piezo electric effect. When the frequency of oscillation coincides with the natural frequency of the crystal, resonance will take place. This principle is used for the production of ultrasonics.
Frequency up to 15 MHz can be produced by this method.
Magnetostriction Magnetostriction is a property of magnetic materials that causes them to change their shape when subjected to a magnetic field. The effect was first identified bby James Joule James Prescott Joule, (1818 1889)
When a ferromagnetic rod is placed in an alternating M.F, with its length parallel to the field, the length of the rod increases and decreases rapidly (or the rod vibrates) This phenomena is known as Magnetostriction. This principle is used to produce ultrasonic waves Magnetostriction
FREQUENCY The frequency generated is given by
DEPTH OF SEA
Bats navigate using ultrasound
SOLDERING & METAL CUTTING
Ultrasound imaging: How does it work? An ultrasound element acts like a bat. Emit ultrasound and detect echoes Map out boundary of object
More about how it works The probe contains a transmitter and a receiver. A pulse of ultrasound is sent out by the transmitter. The pulse is reflected from a surface and returns to the receiver. The ultrasound machine measures how long it takes for the pulse to return Ultrasound probe Body tissue (muscle etc) skin
Ultrasound imaging: foetus feet This is a 2D ultrasound scan through the foot of a foetus. You can see some of the bones of the foot.
Ultrasound imaging: more surface rendering
Ultrasound imaging: imaging the heart heart valves atrium ventricle
Why Use Ultrasound? Ultrasound is very safe. There is no firm evidence that it does any harm to the body (or the baby in the case of pregnancy scans). X-rays are potentially dangerous, particularly to young children and pregnant women (they damage the unborn baby).
COAGULATION & CRYSTALLIZATION
FORMATION OF ALLOYS
TO FIND VELOCITY OF SOUND IN GASES & LIQUID
NON DESTRUCTIVE TESTING
Non destructive testing is a new method of testing of material Without destruction of the material This can be used to detect the imperfections like flaws, cracks, breakings, cavity, airpockets, discontinuities etc in material
Aircraft Inspection Nondestructive testing is used extensively during the manufacturing of aircraft.
Pulse echo system Pulse transmission system Pulse resonance system
Pulse echo system
back surface echo
High frequency sound waves are introduced into a material and they are reflected back from surfaces or flaws. Reflected sound energy is displayed versus time, and inspector can visualize a cross section of the specimen showing the depth of features that reflect sound. f plate crack 0246810 initial pulse crack echo Oscilloscope, or flaw detector screen Ultrasonic Inspection (Pulse- Echo)
Pulse transmission system
Advantages of Ultrasonic testing 1.Simple and accurate 2.Very minute flows can be detected 3.Nature, size and location of defect can be accurately determined 4.Cheap method 5.Used as high speed testing 6.Large specimen can be inspected in a short time.
Ultrasonic diffractometer To find the velocity and wavelength of ultrasonics a quartz crystal is set into vibrations in a liquid using an R.F oscillator. Ultasonics produced This makes the liquid an aquastic grating Collimated sodium light allowed to fall normally on the grating
Diffraction takes place d sin = n--------(1) where d= the distance between 2 nodal or antinodal planes d= a /2 or a = 2d V= a