Equipment and Accessories

Ultrasonic Equipment and Accessories

TransducersThe term transducer refers to the main probing element which acts both as,a transmitter converting electrical oscillation into mechanical vibrationsand, conversely, as a receiverTransducers capable of producing ultrasound within the standard testing frequencies can be grouped into two categories, depending on the physical principle they are based on,Piezoelectric transducersElectrostrictive transducersPiezoelectric TransducersPiezoelectric transducers are made up of a material (quartz, for example) characterized by piezoelectricity, which is the ability that certain crystalline structures have to charge themselves electrically when they are mechanically deformed. (Direct piezoelectric effect)

Vice versa if we apply voltage to opposite surfaces of the material, we obtain a mechanical deformation of the crystal. (Converse piezoelectric effect)

Electrostrictive TransducersElectrostrictive transducers are composed of ferroelectric materials, usually plates of BaTiO3, LiSO4, PbNb2O6 and are based on the physical principle of electrostriction.

At a macroscopic level this phenomenon is similar to piezoelectricity.

Types of Ultrasonic TransducersStraight beam transducersStraight beam ultrasonic probe generates a beam perpendicular to the incidence surface of the test piece.Diameter of these probes can vary from 4 to 30 mm, and the most usual frequencies range between 1 and 25 MHz , the waves emitted are Longitudinal.Piece

Types of Ultrasonic TransducersAngle beam transducersIn angled transducers the refracted beam consists of transverse waves.In facts, the transducer emits a beam of longitudinal waves which divides into reflected and refracted beams following the reflection and refraction laws.For industrial use normally have frequencies ranging between 2 and 5 MHz

PieceTwin crystal transducersDue to the presence of the dead zone the flaws which are very close to the beam entry surface cannot be detected;However, this dead zone can be almost completely eliminated by means of two coupled transducers.

Types of Ultrasonic TransducersWheel-type transducersWheel-type transducers allow large flat surfaces to be tested in a short time.Generally used to test sheet steel.Around the central axis, where the probe is fixed, a cylindrical chamber with a double rubber membrane is free to rotate on two ball bearings.The lowest part of the chamber is usually filled with liquid through which the ultrasonic beam is transmitted to the piece being tested.

Types of Ultrasonic Transducers

Water-column transducersTo carry out rapid testing of pipes, sheet steel etc., water-column transducers are sometimes used.Acoustic coupling with the test pieces is carried out by means of a water column. Types of Ultrasonic TransducersPiece being tested

Immersion transducersImmersion transducers are practically same as contact transducers, with the two transducer elements on the sides of the plate, except that they do not have a protective layer and are perfectly water tight.There are two types;Flat transducers,Focused transducers.Types of Ultrasonic Transducers

PieceWaterPieceWaterFlat transducerFocused transducerUltrasonic EquipmentFollowing functions are carried out by non-destructive ultrasonic testing equipment.

It provides the electrical signal that the transmitting probe is to transform into sound wave;It receives the electrical signal from the receiving probe and amplifies it;It displays the signals, both emitted and received, on a screen

Ultrasonic EquipmentThe screen can be:A Cathode Ray Tube (CRT), as found in traditional equipment,A Liquid Crystal Screen, as found in modern digital equipment.

Block Diagram of a Traditional Unit for Non-destructive Ultrasonic TestingIt comprises the following elements:Synchronizer/ TimerTransmitter/ PulserDelaySweep GeneratorReceiverCathode Ray Tube

Cathode Ray TubeCathode ray tubes consist of a glass case, in high vacuum, containing the following parts.CathodeAdjustment gridFocusing deviceAnodeHorizontal deflection platesVertical deflection platesFluorescent screen

CRTThe cathode, the adjustment grid, the focusing device and the anode make up the electron gun.

CRTCathodeThe (negatively charged) cathode undergoes strong heating due to the Joule effect.It then releases electrons that are sent to the anode (positive) from the electric field created between the cathode and the anode.Where the luminous beam meets the screen a light spot will be produced.

CRTAdjustment GridThe brightness of the display is controlled by the adjustment grid, which allows more or less electrons to pass.Focusing DeviceThis constricts the electron flow into a narrow beam.Horizontal and Vertical (X & Y) Deflection PlatesThe horizontal and vertical movements of the electron beam are controlled by the X and Y plates respectively.

Synchronizer/TimerThe synchronizer emits, at a fixed frequency, pulses that control both the pulser and the scanning voltage generator.This controls the PRF (Pulse Repetition Frequency)The number of pulses transmitted in the unit of time (expressed in Hz) is called PRF.PRF is automatically set by the equipment when the test range is set.PRF depends on thickness of the material.Transmitter/ PulserAlso called signal generator.The transmitter emits a very short voltage pulse with a peak value.This is sent to the probe on one side and directly to the receiver on the other and also triggers the sweep generator.This will display the initial pulse on the screen.

Sweep / Time-Base GeneratorWith every synchronizer command pulse, the sweep generator produces a saw-tooth voltage signal.This voltage, if applied to the X-plates of CRT, deflects the electron beam spot from left to right across the CRT screen with a uniform speed.The time it takes for the saw-tooth voltage to rise determines the deflection speed of the electronic beam and, with it, the full scale depth of the instrument: steeper the signal rise, smaller the full scale depth.Delay CircuitThe delay circuit creates a phase difference between the moments the saw-tooth generator and the pulse generator are activated.In testing with transducers, the beam generating plane does not coincide with the beam entry surface. By delaying scanning in relation to transmission time, it is possible to compensate for the time delay with which the beam penetrates the test piece.Delay adjustment affects position of the signal on the screen but not its shape.

ReceiverThe receiver basically comprises:An amplifier, with a series of attenuators to adjust the amplification level.A detector circuit which rectifies and integrates the amplified signal.The following enter the receiver, in sequence:The pulse coming directly from the transmitter;The pulse supplied by the transducer in reception.Gain Control and Reject are also associated with the receiver circuit.ReceiverThe receiver cannot receive signals until the emission pulse is finished. During this time it is therefore impossible to detect reflecting discontinuities located near the surface of the test piece.The dead zone is the length, in mm, of the section of the time axis line reached by the output pulse where reflecting discontinuities of the material being tested cannot be detected.The resolving power of the equipment-transducer combination, or rather its capacity to clearly detect two or more discontinuities close to each other, depends on the length of the echoes.ECHO PRESENTATIONIn ultrasonic testing the signal from a discontinuity in the test piece can show itself in one of three ways.A-scan presentationB-scan presentationC-scan presentationA-Scan PresentationThe discontinuity is represented on the screen of a cathodic ray tube as a peak (echo).The distance of the peak from zero on the time axis is proportional to the path covered by the beam before it meets the discontinuity.The peak amplitude is proportional to the acoustic pressure reflected by the discontinuity.

B-Scan PresentationThe discontinuity is displayed on the screen of a CRT as it could be seen on a cross-section of the test piece.The position of the discontinuity is shown in a Cartesian plane system.The presence of the discontinuity cause no signal to be detected on the line corresponding to the bottom surface of the discontinuity zone.The intensity of the image displayed on the screen is proportional to the amplitude of the pulse reflected by the discontinuity.

C-Scan PresentationThe discontinuity is displayed on the screen as if it were projected on a plane parallel to the surface being tested.This presentation makes it possible to estimate the extension (length and width) of the discontinuity.No data is given (or can be obtained) regarding the depth of the discontinuity in relation to the surface.This type of presentation is generally used when UT is carried out by means of automatic systems combined with a permanent recording system.

Area-Amplitude BlocksThe blocks are used to determined the relationship between flaw size and signal amplitude by comparing signal responses for the different sized holes.Area-Amplitude blocks are available in an eight block set.They have a constant 3 inch metal path distance with Flat Bottom Holes (FBH).Hole sizes are varied from 1/64 to 8/64 steps and numbered from 1 to 8.Area amplitude blocks are also available as square stocks of 1-15/16They are also known as Alcoa Series A blocksFBH231/64 to 8/64

Distance Amplitude BlocksNineteen block sets with FBHs of a single size and varying metal path distances are available.Sets have either a #3 (3/64) FBH, a #5 (5/64) FBH or a #8 (8/64) FBH.The metal path distances are 1/16, 1/8, , 3/8, , 5/8, , 7/8, 1, 1-1/4, 1-3/4, 2-1/4, 2-3/4, 3-1/4, 3-3/4, 4-1/4, 4-3/4, 5-1/4 and 5-3/4.They are also known as Alcoa Series B blocks These blocks are designated by a 5-digit code.E.g. 3-0075 stands for 3/64 dia. FBH with a metal distance of 75/100 = 0.75 = QuestionsThe transducers used in ultrasonic probes:Transform an electrical signal into a mechanical oneTransform a mechanical signal into an electrical oneCan perform both conversionsThe piezoelectric transducers transform an electrical signal, the electrostrictive transducers transform a mechanical signal

The commonly used frequencies in straight beam transducers are:1 25 MHz10 40 MHz15 60 MHz60 100 MHz

In angle beam transducers the refracted beam is composed of:Lambs wavesLongitudinal wavesShear wavesLongitudinal waves and Shear waves

The reject circuit is used to:Attenuate signalsDefine the full scale pointEliminate the spurious signalsDefine the maximum value of the acceptable signals

Going from 10 dB to 20 dB amplification, how much will increase the signal :About 2 timesAbout 3 timesAbout 10 timesAbout 20 times What can be evaluated with C-scan presentation:The depth of discontinuityThe length of discontinuityLength and width of discontinuityLength, width and depth of discontinuity