Ultrasonic Testing of Materials

Acoustics, the science of sound, describes the phenomenon of mechanical vibrations and their propagation in solid, liquid or gaseous materials. Empty space knows no sound because it is the particles of matter themselves which vibrate, in contrast for instance to the oscillations of light or other electromagnetic waves where the electric and magnetic state of free space oscillates. In air a sound wave moves a discrete volume of air back and forth around its neutral position, whereas a light wave has no influence on its state of rest or motion.

Ultrasonic testing of materials utilises mechanical waves in contrast, for instance, to X-ray techniques which use electromagnetic waves. Any mechanical wave is composed of oscillations of discrete particles of material. The motion carried out by a small mass attached to a spring as shown in Fig. 1.1 if pulled down once and released, is called an oscillation. Left to itself, the mass oscillates about the equilibrium position. The nature of this oscillation is of particular importance inasmuch as it is sinusoidal, the path recorded as a function of time being a sine curve. It is obtained only if the driving force, in this case supplied by the spring, increases proportionately to the displacement. It is then also referred to as an elastic oscillation. Furthermore, one can imagine the body to consist of individual particles kept in position by elastic forces. Very much simplified, the model of an elastic body can be visualised as shown in Fig. 1.2, but three-dimensionally. Provided such a body is not stressed by compression or tension beyond its elastic limit, it behaves like this spring model. In it, the particles can perform elastic oscillations. How then does a wave arise from an oscillation?

Analysis of a wave in an infinitely extended substance is possible only theoretically because in practice every substance terminates somewhere, i.e. it has a boundary. There the propagation of the wave is disturbed. If the material concerned borders on an empty space, no wave can go beyond this boundary because the transmission of such a wave always requires the presence of particles of material. At such a free boundary the wave will therefore return in one form or another. At a smooth boundary one then speaks of reflection, and at a rough boundary of scattering. In this connection the roughness of the boundary should be measured in terms of the wavelength. If another material is beyond the boundary, and adheres to the first material so that forces can be transmitted, the wave can be propagated into it, although usually in a more or less changed direction, intensity and mode.

Geometrical optics use light rays which can be drawn as straight lines. Applying the simple laws of refraction and reflection at interfaces, it permits a very clear presentation of the effect of mirrors and lenses with curved surfaces for example. We have also used this convenient method when discussing reflection and refraction in Chapter 2. In this connection it should, however, not be overlooked that this method fails to take into account a very important property, both of light waves and ultrasonic waves, viz. the wave structure.

Ultrasonic waves are generated by a source, the so-called probe, and we require to know how the wave motion propagates into a material as the ultrasonic field.

The problems of material testing by ultrasonics can be illustrated qualitatively by an optical experiment.

So far, ideal materials have been assumed in which the sound pressure is reduced only by virtue of the spreading of the wave. A plane wave would thus show no reduction whatever of the sound pressure along its path, and a spherical wave, or the sound beam of a probe in its far-field, would merely decrease inversely with the distance from the source. Natural materials, however, all produce a more or less pronounced effect which further weakens the sound. This results from two basic causes, viz. scattering, and true absorption, which are both combined in the term attenuation (sometimes also called extinction).

So far we have discussed the propagation and behaviour of ultrasonic waves in various materials without pre-supposing anything regarding their generation except that they are excited in the material concerned by the contact face of a radiator which oscillates with the desired wave-form and frequency. For detection a microphone has been assumed which likewise has a contact face and which is capable of measuring the sound pressure of an incident wave. Both devices are referred to in materials testing as a probe, or transducer, and where applicable as a transmitting probe or a receiving probe. We shall now discuss its mode of operation, which is based almost exlusively on the piezoelectric effect. Other methods for generating ultrasonics are discussed in Chapter 8.

As well as the piezo-electric effect, other physical properties can be utilized for generating and receiving ultrasound. Although many of these produce weaker signals than are obtainable by the piezo-electric effect, they nevertheless offer a number of advantages which in special cases make their application in the testing of materials useful. In the case of many of these effects the energy is transmitted by electrical or magnetic fields which in principle make mechanical contact with the metallic test piece unnecessary. The conversion into, or from, acoustic energy takes place in the surface of the workpiece concerned. Compared with the piezo-electric oscillator, which is coupled to the workpiece, the surface of the work piece forms in the case of these “direct” methods a part of the acoustic transducer. The direct or dry methods thus require no coupling medium, and so avoid some of the difficulties analysed in the paragraph below.

Table 9.1 lists all current methods of ultrasonic testing of materials. They are categorized by reference to three basic criteria: namely the type of primary measured quantity, the form of radiated ultrasound used (continuous wave or pulses) and the effect of an anomaly within the material under test or on its surface. Based on the presentation in the Table, each method will be discussed to an extent depending on its practical importance.

Figure 10.1 shows the principle of the method in which an ultrasonic pulsed wave, usually in form of a damped oscillation, is generated by a probe and propagates into a specimen with the ultrasonic velocity corresponding to the material concerned. Part of the ultrasound will be reflected if it strikes an obstacle in the form of an inhomogeneity and, if this is not too large, the remainder will travel further to a boundary of the specimen and will be reflected back to a receiver, if the rear surface and the receiver are in favorable positions. The signal obtained from the receiver is displayed as a peak on a base line of a CR tube (Fig. 10.2). The horizontal sweep is proportional to the time, so that the transit times of the pulse to and from the reflector, and to and from the back wall, correspond respectively to the distances on the screen from the initial peak to the echo peaks corresponding to reflector and back wall. To obtain a standing image the pulses and the sweep of the CR tube are synchronised at the so-called pulse-repetition frequency.

In this Chapter all those methods will be discussed in which the only ultrasonic testing result used is transit time. Information about the amplitude of an echo, which was essential in Chapter 10, is not now considered since it can only be of interest if it should restrict the range of measurement. A variety of instruments have been developed which measure pulse-transit times to evaluate such quantities as wall thickness, residual wall thickness, sound velocity and physical strain with ever improving ease and precision.

This method is also called the intensity-measurement or through-transmission method and is explained in Fig.12.1. The shadow of an inhomgeneity, which is illuminated by an ultrasonic wave, reduces under certain conditions the intensity of the wave received by a second probe. The name through-transmission method arises obviously from the fact that two probes are often positioned face to face on opposite sides of the specimen but that may not always be the case. Figure 12.2 shows an alternative arrangement of the shadow method where the beam is reflected before being influenced by the defect, and equally is could also be reflected afterwards. In this situation in which a reflection takes place at a free boundary, the two probes can even be combined into one. Nevertheless if only the influence of the defect on the back-wall echo is observed, it is in principle an application of the shadow method, and in addition the shadowing may even have a double influence on both the outward and return sound paths.

After having located a defect it is of great importance to know something about its size, since this is the basis for a decision concerning its importance for the practical integrity of the specimen which contains it. Its shadow, as revealed by X-ray screening could already be very useful and this is the aim of all imaging methods. By the use of sound sensors, signals are obtained to transform the sound “picture” into a visual image.

This method of non-destructive testing will be treated only briefly because it has little similarity to traditional ultrasonic testing, and also because it has not achieved widespread applications in practice. Testing specifications for pressure vessels have, however, already been produced.

In any ultrasonic test the shape and roughness of the test surface is of decisive importance. On the one hand these factors often limit the sensitivity of the method applied, making it necessary to first prepare the surface; on the other they have an important influence on the wear of the probes used for continuous and routine tests if in direct contact with the specimen. The surface conditions, therefore, greatly influence the economics of testing.

Mode changing by reflection at boundaries of the test piece sometimes results in a basically quite simple testing technique becoming more complicated and interpretation of the screen picture more difficult. The principles of wave propagation discussed in Chapter 2 will therefore be applied to a few frequently occurring cases.

These waves are used only rarely for two principal reasons. As explained in Section 2.3 the longitudinal waves are always accompanied by oblique transverse waves at a smaller angle and these often generate interfering echoes from geometrical features of the specimen. In addition, if they fall on any reflecting surface at an angle other than 90° they lose energy by mode changing. The zigzag technique for example in plates is not useful and even the corner reflection is poor (Section 2.8, Fig. 2.25).

In order to correct internal faults of ultrasonic instruments, only the relevant service instructions should be consulted. In the case of external interference, however, certain remedies of general validity apply. If several adjacent pulse-echo instruments with not exactly identical repetition frequencies are used, the pulses are transmitted from one instrument to the other electrically and they travel across the image. This requires synchronisation of the repetition frequencies of the instruments concerned and instruments designed for use in testing installations are usually equipped with this synchronisation facility. If not installed, synchronisation is usually possible by slightly changing the wiring and for this purpose it is recommended to consult the manufacturer.

The detection of defects by observing their ultrasonic echoes is usually quite simple and rapid. More difficult and much more time-consuming is the classification of defects according to their type, shape and size. This is fundamentally due to the basic handicap of ultrasonic testing in having a poor lateral resolution because of the relatively long wavelength used. Even when using the most complex imaging methods of Section 13 a precise defect image cannot be expected. Also because these methods have to be restricted in practice to relatively simple-shaped specimens, other means have to be used in the wider applications to establish a little more about a defect than only its geometrical position.

Where the same testing problems for identical or very similar specimens occur frequently, it is recommended that the head of the testing department works out a test specification so that the tests can be carried out by a trained examiner. From service experience and information gained from destructive tests, the nature, size and position of flaws will be generally known, so that non-destructive tests can then concentrate on these. The specification should include: designation of the specimen with a sketch, instructions on whether and where the surface should be prepared, type of couplant, setting of instrument and choice of probe for each testing procedure, together with indication of scanning patterns for suspected flaw positions as well as reference to possible echoes which could be confused with flaw echoes. In the case of mass tests, the examiner can, as a rule, be relieved of the necessity of preparing individual reports since on the evidence of the flaw echoes he can quickly make his own decision whether a given specimen should be rejected or not. In the case of more costly test pieces a preliminary sorting out of doubtful specimens is advisable after which these can then be submitted to the more experienced head of the testing department for final decision.

In parallel with the gradual mastering by test operators of manual testing methods in the course of time, there came also the development of automated continuous testing installations [1226, 119, 1369] up to the specialised single-piece testing automats [1190]. Examples for the former type of installation are those for testing plate, rods, rails and tubes, which are built into transport roller ways (Fig.21.1). Example for the second kind are automats for testing steering joints, valves, rollers, balls, and other individual machined construction elements. Their design depends on the shape of the piece for manipulation, and for scanning by the probes (Fig.21.2). For piece and probe manipulation industrial robots are today used to an increasing degree.

In view of the high machining costs and the long replacement times, testing of large forgings is started as early as possible in the manufacturing process, in order to detect and exclude defects already present in the cast ingots, and which would be dangerous in service for the finished piece.

A separate treatment of this group of testing problems, which individually could be classified under other sections, is justified in view of the extent and the importance of ultrasonic testing in railway engineering. Most interesting in this respect is the detection of defects generated during service by dynamic stresses, especially considering the latest high-speed trains. For these trains acceptance testing is of increasing importance.

Test problems for plate are mainly concerned with production testing. However, there are a few applications arising from in-service stresses and working conditions. These will be dealt with first.

Manufacturing defects which occur in this group of semi-finished products can either be on the surface or internal (Fig. 25.1). Internal defects originate from ingot defects such as shrinkage cavities and inclusions which have been elongated during rolling and drawing or are rolling or drawing defects, such as piping, particularly in the case of non-ferrous metals, and cracks in the core, which in cross-section appear flat or star-shaped. Surface defects are usually the result of the drawing operation on defects such as radial cracks or laps which reach the surface at a shallow angle. Since all flaws extend in the longitudinal direction, this requires that the axis of the sound beam in a cross-sectional plane lies either normal or obliquely to the surface and for some applications can be surface waves running in a circumferential direction.

Manufacturing defects are once more the main inspection problem for pipes and tubes, and it also happens that in-service defects such as fatigue cracks and corrosion cracks require the same testing techniques as the manufacturing defects because they are usually in the same position and orientation. Other in-service inspection such as the detection of corrosion and the measurement of wall thickness, are discussed in Section 33.1.

In castings flaw detection is almost exclusively concerned with manufacturing defects and only rarely as in-service inspection. Suitable testing techniques and the subsequent evaluation of indications in castings is very different from the testing of forged and worked material so that the differences must not be forgotten or difficulties can occur. In-service inspection, as in the case of forgings, depends on the local stresses and the piece geometry so it is not necessary to treat it specially in this section.

The commonly occurring defects in welded joints are porosity, slag inclusions, lack of side-wall fusion, lack of inter-mn fusion, lack of root penetration, undercutting and longitudinal or transverse cracks.

At one time this inspection requirement in steel pressure vessels was very important, but today it is mainly of historical interest. The rivet itself can develop transverse cracks between head and shank and these can be detected by testing from the rivet head, as on bolts (Section 22.4). Figure 29.1 illustrates the formation of inci- pient cracks at the hole surfaces, which can penetrate through the wall and even extend from one rivet hole to the next. The testing technique is illustrated in Fig.29.2 and 29.3 and is similar to the probe-scanning movement for welds. The echoes from the top and bottom edges of the hole serve as reference echoes. Fig.29.1Partially and fully penetrating cracks in rivetted jointsFig.29.2Rivet-hole testing; reference echo from top and bottom edges of the rivet holeFig.29.3Testing rivet holes, swing-motion of the probe and indication on the CR screen of an incipient crack in a rivet hole

Because of the very high safety demands for nuclear power stations all components of the primary circuit, including the reactor pressure vessel and the circulation system, have to undergo several detailed utrasonic tests. Before the plant is put into service the so-called zero test or “fingerprint” inspection is made. Additionally in-service tests have to be made in Germany of the pressure vessel every 4 years along with 50% of the remaining components after the first 4 years and the other 50% at the following in-service inspection, and so on. Other countries have similar specifications, corresponding wholly or in part with the German requirements (see Chapter 34). In several detailed specifications (rules of the Reaktorsicherheits-Kommission RSK [1745], of the Kerntechnischer Ausschuss (KTA) [1732] and DIN-Standards [1719]) the areas of application of the different tests are specified including the relevant sections of the components involved, the timing and extent of testing, and the techniques and equipment to be usded. Ultrasonic testing is specified as the basic method in these requirements.

The Table of the sound velocities for different steels, published in the 3rd edition of this book, page 527, shows differences of less than 5% for different states of working and for the various alloying metals no systematic influence can be recognized. The processing conditions such as heat treatment, hardening, quenching and cold working have a larger influence than the alloying elements. In general both longitudinal and transverse velocities as well as the attenuation decrease as a result of such processes. The deviations from the value 5.93 km/s for most practical applications can be neglected, but not if precise wall-thickness measurements are to be made. Care must also be taken when using angle probes because a change of only 1% in the transverse wave velocity at a nominal beam angle of 70° causes a change of 1.5°. In critical cases, as for instance at the critical angles for the generation of Rayleigh or creeping waves, velocity changes must be taken into account, and in these cases the decrease of the velocity with temperature also plays a role [437, 1162, 450].

The main application of ultrasonic testing for ceramics and glass material is that used for electrical insulators. The unfired porcelain blanks, the so-called slugs, as long as they are saturated with water, are sufficiently transmissive at frequencies of 0.5 to 2 MHz to be tested in thickness of a few 100 mm for larger voids and insufficiently bonded joints between two or more slugs. In practice, however, tests are not made on the material in this state and dried blanks are no longer transmissive. Densely fired porcelain, as far as transmittance and acoustic velocity are concerned, reaches values close to those for steel. Using longitudinal waves of 5 MHz and higher, 1-m lengths or more can be penetrated ultrasonically. Other dense ceramics used for insulators show similar behaviour, for example steatite, whose acoustic velocity exceeds even that of steel (see below) and glass has also very low attenuation and high velocity.

This Chapter deals with the measurement of material properties and of elastic constants, as far as they are of interest for materials testing in general and where they can be carried out by using commercial testing instruments. This excludes therefore many purely scientific problems and measuring methods or permits only their brief mention. For further and more detailed studies the textbook of Tietz [41] is strongly recommended.

The growing importance of ultrasonic testing has persuaded associations for the application of non-destructive testing methods, of manufacturers and users, as well as standardizing organizations, to publish standards ranging from non-compulsory recommendations to specifications with legal status. In every country national standardizing commissions are investigating the specifications proposed by the various associations and societies. In the Federal Republic of Germany this is the DNA (Deutscher NormenausschuB) which publishes the DIN standards. On an international scale the ISO (International Organization for Standardization) is making efforts to reconcile and harmonize the standards of the various countries. For the same purpose on a european scale the CEN in Paris has been established (European Standards Committee). In addition, similar efforts are being made by international professional associations, e.g. in the field of welding, such as the IIW (International Institute of Welding).