This is the fifth 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'

Chapter5.
PROBE CONSTRUCTION

COMPRESSION WAVE PROBES

Standard compression wave probes can be for contact scanning or for immersion testing. The contact scanning probes are either single crystal or twin crystal (dual) in construction. The construction of a typical single crystal contact probe is shown in the diagram in figure 1.

Fig. .1

The thickness of the crystal determines the operating frequency as we described in an earlier article and the faces of the crystal are coated in silver to make electrical contact.

The damping slug is cast onto the rear of the crystal and bonds to it as the epoxy sets. The amount of damping used determines the pulse length. A fine wire is soldered to the back of the crystal, using a solder that melts at low temperature, before adding the damping slug.

The wear face is glued to the front face of the crystal to protect it during contact scanning. The thickness of the wear face is important. It is made to be one quarter of the wavelength at the test frequency for the velocity of sound in the wear face material. This thickness gives maximum transmission of sound out of the probe into the test sample. Some wear faces are made from shim steel, others from a hardwearing ceramic material. The steel wear faces can be used to earth the front face of the crystal to the probe housing and are less fragile if you drop the probe, but are inclined to stretch and disbond from the crystal with use. If a non-conductive wear face is used, an alternative earthing method must be used.

The wear face, crystal and damping slug assembly are then fitted into the housing, the other end of the centre wire is soldered to the centre terminal of the connector and the cap and connector fitted to the housing. Figure 2 is a photograph of a typical single crystal compression wave probe.

Fig. .2

Twin crystal, or ‘dual’ probes are used to eliminate the ‘dead zone’ occupied by the transmission pulse with a single crystal probe. In this type of probe one crystal acts as a transmitter, the other as a receiver and the amplifier is isolated from the transmitting crystal. The two crystals are mounted on acrylic or polystyrene wedges these components are illustrated in figure 3. An acoustic barrier, usually made of cork, is fitted between the wedges and crystals toprevent cross talk between the transmitter and receiver. Figure 4 shows a typical twin crystal probe.

Fig. 3

Fig. 4

Immersion probes are similar in construction to that shown in figure 1 except that it is not necessary to fit a wear plate and so the silvered face of the crystal is usually visible. Probes can be focussed and this is achieved by fitting a plastic or epoxy lens to the front of the crystal, or by making a curved sectioned crystal. Figure 5 shows a 20MHz immersion probe with a small diameter spherically focussed crystal.

Figure 5

The lens or curvature can also be cylindrical as illustrated in figure 6. The cylindrical version is often referred to as a ‘Paintbrush probe’ because it allows a wide scan.

Fig. 6

Focussing can also be achieved using a technique called ‘Phased Array’, although not with conventional ultrasonic sets. The phased array probe contains a number of small crystals and the pulsing circuit is designed to be able to apply a pulse to all crystals simultaneously to produce a conventional zero degree compression wave, or to pulse each crystal separately with a small time interval between each. In the diagram shown in figure 7, the outer elements are triggered first and a progressive delay is used to pulse inner elements, the centre crystals being the last to be triggered. The result is that the ultrasonic wavefront reinforces in the curved way shown in the diagram to focus at a region determined by the delay intervals. By changing these intervals, the focal length can be changed. The principles of constructive and destructive reinforcement will be dealt with later in a later article.

Fig. 7

Single crystal ‘Delay line’ probes are sometimes used in contact scanning to reduce the ‘Dead Zone’ below the beam entry surface occupied by the transmission pulse and probe noise. The delay line is usually Perspex or a similar material and provides a stand off just like the water path in immersion testing. The length of the delay line must be sufficient to allow one or more backwall echoes in the specimen depending on the application. Figure 8 is an example of a delay line probe.

Fig. 8

SHEAR WAVE PROBES

Since shear waves cannot travel through liquids or gases, angled beam probes use compression waves in the incidence wedge in contact probes. The incident angle will be an angle between the first and second critical angles (described in the third article in this series) so that we only have the mode converted shear wave in the test material. Figure 9 is a sketch of the typical arrangement.

Fig. 9

We not only get a mode converted, refracted shear wave in the test piece, but we also have a reflected compression wave in the wedge. If this internal reflection manages to get back to the crystal face as it bounces around the wedge, we would have a standing echo that would be confusing. Several methods of avoiding this problem have been used over the years. The earliest probes used a long Perspex shaped ‘Cusp’ so that the reflection would be absorbed before it could return to the crystal. The Cusp made a rather unwieldy probe and the next design used ‘V’ shaped grooves in the front and top surfaces of the incident wedge to scatter the internal reflections. Some had a plastic material moulded onto these grooves to further damp the reflection. In the latest, most compact, designs the wedge is surrounded by a material that has a good acoustic match to Perspex, but a much higher absorption of sound. The internal reflections are transmitted easily into this layer and then absorbed. Figure 10 shows examples of the three designs and illustrates their relative size.

Fig. 10

Figure 11 is a photograph of a sectioned shear wave probe, showing the crystal, incidence wedge and the blocking medium for the internal reflections.

Fig. 11

Phased Array transducers, such as the one already discussed (figure 7), are also used to generate angled shear waves in the test piece. These transducers have the advantage that the phase delay between the crystal elements can be varied to give different angles of refraction. The delays can be swept through a range of values to give a shear wave beam that sweeps through a desired range of shear wave angles rather as a Radar scanner sweeps the skies.

In the fourth article, we said that EMAT probes could generate compression or shear waves, but that shear waves were often used because they can be directed perpendicular to the test surface (that is a 0 0 probe). That has advantages in resolution, because the wavelength for a shear wave is about half the wavelength for a compression wave and because the velocity of the shear wave is about half that of the compression wave, we are able to measure thinner sections than we can with conventional 0 0 probes of the same frequency. The EMAT probe shown in figure 12 is a radially polarised shear wave probe operating broadband between 1-10MHz, with a centre frequency of about 5MHz.

Courtesy of Ultrasonics Group,

Dept of Physics, University of Warwick

Fig. 12

‘Q’ FACTOR AND BANDWIDTH

Up to this point we may have gained the impression that our transducer produces a pure note at the calculated frequency, but this is not true. In fact the sound wave produced contains a band of frequencies related to the thickness of the crystal, its diameter or length and width plus the effects of the damping medium. In addition the electrical characteristics of the transducer and associated circuits affects the overall spectrum of frequencies. We refer to this spectrum as the ‘Bandwidth’ of the probe. In a well-designed probe, the centre of this band should be the desired probe frequency and the lower and upper limits are usually defined as the frequencies at which the amplitude is reduced by a given factor. Some people use 30% (-3dB) and others 50% (-6dB) as the factor we will use 50% in the following examples. Figure 13 illustrates the bandwidth of a 5MHz probe in which the –6dB bandwidth is equal to the centre frequency, in other words, from 2.5MHz to 7.5MHz.

Figure 14 shows the bandwidth for another 5MHz probe, but this time the bandwidth is only from 3.75 to 6.25MHz.

The probe shown in figure 13 can be described as having a broad bandwidth whereas the probe in figure 14 has a narrower bandwidth. In practice, short pulse probes have a broad bandwidth and long pulse probes are narrow bandwidth. For a given crystal size, material and frequency damping not only reduces pulse length, but also reduces pulse amplitude, so the narrower bandwidth probes will have longer pulses but more amplitude in the pulse therefore giving deeper penetration.

Another way of expressing bandwidth that is also common in other branches of electronics is the ‘Quality Factor’ or ‘Q’ of the probe and is defined by the formula: -

Where f 0 = the centre frequency

f 1 = the upper –6dB frequency

f 2 = the lower –6dB frequency

Example

Calculate the Q factor for the probe illustrated in figure 13.

Example 9

Calculate the Q factor for the probe illustrated in figure 14.

Undamped crystals can have a Q factor as high as 20,000 but for ultrasonic flaw detection the Q factor is normally in the range 1 to 10.

References: -

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

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