This is the fourth chapter of an eleven part article on Ultrasonics by John Drury, the Author of Ultrasonic Flaw Detection for Technicians. This article was first published in INSIGHT magazine throughout 2004/5. The chapters can be downloaded in PDF for you to build into a complete series.

To access the other chapters please use the navigation at the bottom of this page.

J.C. Drury ' BACK TO BASICS - ULTRASONICS'

Chapter4.
TRANSDUCERS FOR GENERATING AND DETECTING SOUND WAVES

There is an amusing story about a nautical gentleman at the time when wooden ships were being superseded by iron ones. This sailor thought up a new way to determine the depth of water under the hull to replace the old lead weight on a rope method. He decided that it should be possible, with a large hammer and a stopwatch, to bang on the iron bottom of the ship with the hammer and time the return echo from the seabed with the stopwatch. The measured time could be used to calculate depth using the speed of sound in water. Fired with enthusiasm, he gathered together a number of marine dignitaries in the bilges of his ship, passed a large sledgehammer to a muscular boatswain, took out his stopwatch and ordered the ‘swain to wallop the floor. This he did with such vigour that the hull boomed for ten minutes and the assembled observers were deafened for a month! Nobody heard an echo.

There are parallels with ultrasonic flaw detection in the story; we need our sound pulses to be ‘loud’ enough to penetrate to the depth of the anticipated flaw and we need the duration of the pulse to be short so that it does not mask any returning echoes. We also need the sound frequency of the pulse to produce a wavelength short enough to detect the smallest reflector that must be detected to ensure safety. In this chapter we will discuss various ways of generating and detecting suitable pulses and some of the limitations we face in terms of penetration and flaw sensitivity.

ULTRASONIC TRANSDUCERS

A transducer is a device that will change one form of energy into another. Ultrasonic transducers change electrical energy into mechanical energy (sound waves) or vice versa. There are several methods used to generate and detect ultrasonic pulses in modern flaw detection and the most common of these makes use of the Piezo Electric effect found in certain materials. Other methods, such as the Electro Magnetic Acoustic Transducer (EMAT) and Laser technology will also be described.

PIEZO ELECTRIC TRANSDUCERS

In 1880 the Curie brothers discovered that slices cut in a particular way from certain crystal materials would generate an electrical potential across the faces of the slice when distorted by a mechanical force. They called this phenomenon ‘Piezoelectricity’ from the Greek words for ‘Pressure’ and ‘Electricity’. A year or so later Lippman reported that the reverse was true; that a voltage applied across the slice would produce a mechanical distortion. Quartz was the prime example of a piezoelectric material, but Rochelle salts and Tourmaline crystals also displayed the same effect.

For the first thirty years of ultrasonic flaw detection from Sokolov in 1929, until the end of the nineteen fifties, quartz was the most common transducer material. Appropriate slices were cut from a single crystal. Later new polycrystalline materials were developed that had lower electrical impedance (resistance to high frequencies) and gave better ultrasonic performance, as much as 60 to 70 percent more efficient than quartz. These materials have to be ‘Polarised’. During polarization the individual crystals align themselves in the same direction so that their combined effect is coherent. The polarisation process involves heating the discs in an oil bath to a critical temperature called the ‘Curie Temperature’, applying a strong electrostatic field across the disc and then allowing the temperature to cool slowly. Figure 1 illustrates the polarising process.

Fig. 1

The Curie temperature differs for each of the common materials used in ultrasonics, so that the oil bath will need to be heated to a suitable temperature for the material in use. For Barium Titanate the Curie temperature is around 120 oC whereas for various grades of Lead Zirconate Titanate (PZT) the temperature is from 190 o to 350 oC and for Lead Metaniobate (PMN) it is about 400 oC. If the material is subsequently heated to a temperature near to the Curie temperature, the disc will ‘depolarise’ and lose its piezoelectric properties. It follows that care needs to be taken to avoid depolarisation when testing hot materials and this will sometimes influence the choice of transducer material.

MODE OF VIBRATION

Whether the transducer disc is made from a naturally occurring piezoelectric crystal, or one of the polarised polycrystalline materials, we usually refer to the disc as ‘the crystal’ when talking about probe construction. The crystal ‘disc’ or ‘plate’ may be round or rectangular and for some applications may be curved plates or concave discs to focus the sound. The way in which the plate vibrates when stimulated by an electrical pulse depends upon the ‘cut’, in the case of quartz, or the direction of polarisation in the case of polycrystalline materials. Figure 2 represents a typical quartz crystal showing the three axes defined by crystallographers, and two plates cut from a crystal, one an X-cut plate and the other a Y-cut plate.

Fig. 2

An X-cut plate is taken from the quartz crystal so that the X-axis is perpendicular to the plate and the Y-cut plate has the Y-axis perpendicular to the plate. If a voltage is applied across the faces of these plates, an X-cut crystal will distort in the thickness mode whereas a Y-cut crystal will distort in shear mode. Figure 3 illustrates the changes in shape when an alternating voltage is applied to an X-cut crystal and Figure 4 shows the shape changes for a Y-cut crystal. The same two modes of vibration can be obtained using the polycrystalline materials by polarising across the faces of the plate (equivalent to X-cut), or parallel to the faces of the plate (equivalent to Y-cut)

Fig. 3	Fig. 4

The X-cut crystal is the one most commonly used in ultrasonic flaw detection, it can generate and detect compression waves, and can therefore transmit sound through the liquid couplant we use. Since shear waves cannot exist in liquids or gases, the only way in which a Y-cut crystal could be used to generate shear waves in a metal object would be to use a solid couplant or high viscosity liquid such as honey; in other words we would need to almost glue the crystal in position. This is done in a few very special applications.

METHOD OF PULSING AND FREQUENCY

When we generate a short pulse of sound with our ‘crystal’, we don’t ‘drive’ the crystal with an alternating voltage of suitable frequency; instead, we ‘pluck’ the crystal with a short sharp electrical shock and allow the crystal to ‘ring’ at its natural resonant frequency. This is rather like ‘plucking’ a guitar string that also vibrates at its natural frequency. In the case of the piezoelectric plate, the crystal stretches as the voltage is applied and only produces sound when the voltage is rapidly cut off. To increase the amplitude (loudness) of the ultrasound we increase the peak voltage (pulse energy) applied to the crystal. The frequency of our ultrasonic transducer is determined by the thickness of the crystal. As the crystal is made thinner, so the resonant frequency increases. Quartz crystals are split in the appropriate and then lapped to the correct thickness for the required frequency. The polycrystalline materials are made as slurry that is moulded and compacted under pressure and then sliced and lapped to the required thickness before polarising.

The required thickness for a given frequency can be calculated from the frequency-thickness constant for the crystal material to be used. Since this depends on the velocity of a compression wave in that material it can be seen that the thickness for a given frequency will not be the same for PZT and quartz, for example. The frequency-thickness constant is defined mathematically as: -

Where f = the desired frequency

t = the crystal thickness

V = the compression wave velocity in the crystal material

Example

Calculate the required thickness of a PZT crystal to produce a resonant frequency of 5MHz given that the compression wave velocity for PZT is 3000M/s.

t = 0.3mm

CONTROL OF PULSE LENGTH

In ultrasonic flaw detection we measure the time taken for each echo to arrive at the receiver after entering the scanning surface of the object. If we know the velocity of sound in the material we can determine the distance travelled by the sound wave. Suppose that a crack has grown from a bolthole in the object as in figure 5; some of the sound will reflect from the top of the bolthole, and a little while later, some will reflect from the crack. The arrival of the two echoes at the receiver will be separated by a short interval of time (T 2 – T 1). If the ringing time of the crystal (pulse length) is longer than this interval of time, then we may not be able to distinguish the crack from the top of the bolthole – we may miss the crack. We say that we have not ‘resolved’ the two echoes or that the resolution is poor. In order to improve resolution we need to ensure that the pulse length is as short as possible.

Fig. 5

In ultrasonics we shorten the pulse duration by applying a weight to the back of the crystal known as the ‘damping’ or ‘backing’ slug. The damping slug is often made of a mixture of tungsten powder in an epoxy resin. The amount of damping applied to the crystal will govern the resolution of the probe. A short pulse probe will have only one or two cycles whereas a longer pulse probe may have from three to five cycles. An undamped crystal may have twelve or more cycles in the pulse. For a given number of cycles in a pulse, the duration or space occupied by the pulse will depend on the wavelength, which in turn depends on the probe frequency and the velocity of sound in the material being inspected. Thus: -

Pulse length = Number of cycles in the pulse multiplied by the wavelength.

It is obvious that one way to improve resolution would be to increase the test frequency, however, since the penetration of sound into the object decreases as the frequency increases, this is not always possible. Choosing a suitable test frequency is often a compromise between resolution penetration and flaw sensitivity and sometimes ultrasonics will not be able to detect a particular discontinuity at the critical depth. While resolution is an important consideration in many applications, it is not always the case and sometimes a longer pulse is preferable. For example, in the examination of a long shaft such as a railway axle, the screen on the flaw detector may only be 75mm wide and the display may represent the length of the shaft, say 2.5m long. A short pulse of 2 cycles will occupy such a small part of the screen that it is too faint to see and it would be better to use a longer, more visible pulse

PIEZO-COMPOSITE TRANSDUCERS

In a more recent development of the piezoelectric transducer, the active plate in the test probe is made by slicing piezoelectric crystals into small squares and assembling them into a matrix separated with an epoxy or a rubber compound as shown in figure 6. The main advantages of this type of construction are firstly, lower acoustic impedance allowing better acoustic matching. This is an advantage when testing castings and stainless steel. Secondly, resolution – they tend to provide short pulses without additional damping, allowing the probes to have a low profile.

POLYVINYLIDENE FLUORIDE (PVDF) TRANSDUCERS

PVDF was also found to display Piezo electric characteristics and has been used in ultrasonic flaw detection. These thin plastic films have advantages and limitations compared with conventional crystals. On the plus side, they can be easily shaped to focus sound, they produce very short pulses and they give good transmission into water because the acoustic impedance is similar to water. Against these advantages, the films are fragile and cannot be used in contact scanning, and the power output is relatively low compared with ceramic crystals. The main application is in high resolution immersion testing.

ELECTROMAGNETIC ACOUSTIC (EMAT) TRANSDUCERS

EMAT transducers provide a non-contact alternative to piezoelectric transducers. Sound waves are generated in the surface of a conductive test object by an electrical pulse applied to a flat coil that is positioned between a strong magnet and the test piece. The interaction between the magnetic field generated in the coil by the electrical pulse and the fixed magnetic field of the magnet causes a rapid ‘shock’ deformation at the surface of the test piece and an ultrasonic wave travels through the metal object. The EMAT probe needs to be close to the test surface, but does not need to touch it. Returning echoes arriving at the scanning surface cause the surface to vibrate in the magnetic field. This generates eddy currents in the test surface and the coil detects the eddy currents.

EMAT probes can be used with an air gap when testing hot surfaces and on surfaces coated with non-conducting material such as rubber, paint and fibreglass because the sound wave does not have to travel through the gap material. The probes can be configured to generate horizontally polarised shear waves directly into the test object. This is an advantage when testing austenitic welds, castings and other materials with dendritic grain structure because the shear wave does not mode convert when it meets a reflecting surface that is parallel to the direction of polarisation. Because shear waves travel at roughly half the velocity of compression waves and have shorter wavelengths, it is possible to obtain better near surface resolution and this can be an advantage when testing thin materials.

However, there are some disadvantages with EMAT probes, they are relatively large and inefficient compared with conventional probes and they cannot be used on non-conducting test objects unless a conducting coating is applied.

LASER TRANSDUCERS

Another non-contact method of generating ultrasound uses laser technology. A short burst of a laser beam on the surface of the test object causes a thermal shock with rapid local expansion of the surface. The sudden distortion of the surface causes an ultrasonic pulse to travel through the test object. The returning echo distorts the test surface and this distortion can be detected by a separate laser interferometer without a couplant, or can be detected with a conventional piezoelectric crystal and couplant. The gap between the transducer and test surface can be greater than is possible with EMAT probes and can be as much as 250mm (10 inches). Typical applications include the inspection of composite materials in the aircraft industry.

References: -

‘Ultrasonic Flaw Detection for Technicians’ - Third Edition, June 2004 by J. C. Drury

‘Ultrasonic Flaw Detection in Metals’ – Banks Oldfield & Rawding – ILIFFE 1962