Abstract

Contents

• 1. Introduction
• 2. The pulse-echo method - general remarks
• 3. Simple evaluation methods
• 4. Evaluation of diffracted and converted sound waves
• References

1. Introduction

Ultrasonic inspection is used to determine whether or not a test piece may be used according to its intended purpose ("fit for purpose"); that is, whether it is free of discontinuities or contains defects. The definition of "defects" cannot be determined solely from ultrasonic information : knowledge of the construction of the test piece, its intended purpose, its material and its fabrication process is necessary. All this information is important to decide whether an inspected part may be used or not.
Therefore, in the following text the term "defect evaluation" will not be used if we are referring to the interpretation of ultrasonic pulse echoes.

2. The pulse-echo method - general remarks It is essential in the evaluation of a reflector that one has scanned for it and detected it (1). The procedure can be divided into the following steps: A reflector is detected. A simple evaluation can be made of the pulse echo characteristics, for example, the transit time or the maximum echo amplitude. A rough classification of the type of reflector can be made with additional effort. In this case a combination of different characteristics from a number of echoes during a scan procedure (signal analysis) must be linked in order to determine the type of reflector. A precise classification of the reflector according to type, position and size can be attemped by means of ultrasonic imaging. However, besides being quite difficult, these methods are often constrained by the limits of physics.

Fig. 1 - Information of the pulse echo method

Using the pulse echo method, in principle, all information about the reflector can be derived only from the echo signal. Simple evaluation methods use only parts of the resulting information. According to fig. 1, the following will be available:

• the position of the transducer, when an echo occurs;
• the directional characteristics of the transducer;
• the transit time of the pulse echo;
• information based on the shape of the echo.

From the interaction between the sound waves and the reflector, three spheres of influence can be differentiated (1,2):

• The transducer influences the transmitted and received pulse. It has a directional characteristic;

• The reflector influences the shape and direction of propagation of the reflected pulse, and could even cause wave transformation;

• The material also influences the shape and amplitude of the echo pulse by sound absorption, anisotropy and scattering (and sometimes also the direction of propagation).

  The interaction between the sound wave and the reflector does not always take place in an ideal manner as shown in fig. 1 (above). In most cases a portion of the sound beam strikes the reflector and sometimes only part of the reflected waves can be received. It is also possible that mode conversion has taken place and the wave type received is different from the transmitted pulse. Therefore, when using simple evaluation methods, the many possibilities of interaction sometimes lead to an uncertain or even wrong result. However, the opposite can also be true : if the type and position of a possible reflector is known, then these simple evaluation methods can give just as good results as the more sophisticated analytical methods.

3. Simple evaluation methods

3.1. Evaluation of the transit time:
With crack growth in tensile test pieces, the reflector type and position is known. The crack depth can be determined from the measurement of the transit time (3,4) (see fig. 2). Stress cracks on parts having simple shapes can be determined in a similar way if the direction of crack propagation is known.

3.2. Reflector edge scanning:
If information about transducer position is added to the transit time data, then one obtains a scanning method with which the dimensions of large reflectors can be determined. This method is used when the size of the detected reflector is distinctly larger than the sound beam diameter (fig. 3).

Fig. 2 - Simple transit time method (End-on-crack)

Fig. 3 - Simple reflector scanning (amplitude drop by x dB)

From the transit time, the depth position of the reflector can be derived, the measurement of the transducer scanning track will give reflector expansion. In most cases the projection of the reflector area on the surface of the test piece is used (C-scan method). However, the echo amplitude must also be considered in order to determine the reflector edge. It is assumed that the reflector edge is positioned under the centre of the transducer when the echo amplitude has dropped below its maximum by a predetermined value of x dB (5,6,7), i.e. by 6 dB (the half value method). An amplitude decrease of 20 dB is also quite common (8).

3.3. Evaluation of the maximum echo amplitude:
If the reflector area, contrary to paragraph 3.2., is smaller than the sound beam diameter, then evaluation is accomplished using the maximum echo amplitude combined with appropriate transit time information. This is the most common evaluation method used today in manual testing.
The transit time determines the reflector position and the maximum echo amplitude determines a (fictitious) reflector size. The DGS-method uses the ideal circular reflector as an equivalent reflector. Independent of the actual reflector type and possible inclined position, the echo is evaluated as if it had come from a circular reflector of equivalent size which was hit perpendicularly (9).

In addition to the uncertainties which the use of an equivalent reflector imply, the rectified video signal is normally used rather than the RF-echo presentation (fig. 4). Properties of the electrical transmission line also influence the result (rectification, smoothing, filtering).


Fig. 4 - Simple evaluation of echo amplitude (DGS method)

3.4. Reflector scanning using automatic methods :
Information about probe position, time of flight and maximum echo amplitude can be automatically obtained by scanning the test object, storing the results and evaluating them with a computer (10,11).

COMSON is a system which carries out a reflector diagnosis by which the echo amplitude is evaluated over different sound paths when testing in the manual mode (12). Fig. 5 shows an example of this.

Fig 5: - Multiple-path scanning by the COMSON method. (44Kb)

Rapid beam steering (parallel to surface movement and swivelling) is carried out electronically using phased arrays (13, 14, 15).
The reflector size is mostly displayed as a plane area. Fig. 6 shows the principal method :

• Linear scanning by shifting the sound beam in a line or within a plane;
• Sector scanning by swivelling the sound beam within a certain angle range;
• Compound scanning by combining linear scanning with sector scanning.


Fig. 6 - Scanning a reflector by phased arrays

The same reflector can be detected a number of times from different locations.

If the location of a reflector changes within the sound beam, then characteristic changes occur with echo transit time and amplitude. The ALOK method (Amplitude -Time of Flight- Local Curve Method) makes use of this physical law (16, 17, 18), (refer to fig. 7).

Fig. 7 - Evaluation by the ALOK-method Evaluation: Defect border reconstruction or or pattern recognizition (classification) required corrections: - transmitter/receiver characteristics - spectrum

The amplitude value, A=f(s), and the time of flight, L=f(s), received via the probe shifts, are freed from any interference by using a computer. The influence from the receiver and transmitter probes is eliminated. A mathematical reconstruction is made of the reflector edge or characteristic values which one can process using a pattern recognition program. Reflector classification is then automatically carried out. This also applies to the COMSON method.

3.5. Multiple frequency method
There is another analysis possibility by which the ultrasonic pulse changes its spectral composition, because reflection behaviours of plane and inclined reflectors are strongly dependent on frequency.
An analysis of the echo signal, with dependence on the frequency, can also give reliable results in manual testing especially when the reflectors do not have ideal reflective characteristics, e.g. castings or welds (19, 20).

Fig. 8 shows reflection behaviours at different frequencies (according to (19)):

• At very high frequencies, only small areas of the fissured reflector return to the probe (shaded).
• At low frequencies, these areas become larger.
• At very low frequencies the complete area of the reflector is reflecting back to the probe (transition from reflection to scattering).

  

4. Evaluation of diffracted and converted sound waves

The considerations made up until now have assumed that a plane wave hits the reflector which then reflects back only this wave mode in an ideal sort of way (Fig. 9a).

Fig. 9a - Taking into account of diffraction phenomena

Fig. 9b - Taking into account of diffraction phenomena

In reality, with reflections from real reflectors, diffracted waves appear, mostly from the edge of the reflector. Depending on the wave's angle of incidence onto the reflector surface, other types of waves can be generated : longitudinal or transverse waves, even surface waves. This is illustrated in Fig. 9b.

Basically, information about the reflector can be obtained from all waves produced, whether it be from the times of flight or from the echo amplitudes. Using focused sound fields, diffracted waves can be greatly excited at the edge of the reflector (Fig. 10).

Fig. 10 - (Focal distance / diameter)

Fig. 11 - Diffraction at crack tips time-of-flight technique

By scanning the edge, the reflector size and angle can be determined from the time of flight and the amplitude (when the reflector is larger than the diameter of the sound beam)(21).
In spite of the term "superintensity", it is the sound pressure amplitude which is evaluated and not the intensity of the sound.
Even if it is only possible to evaluate the times of flight from the diffracted sound waves, an accurate measurement can be made of the reflector dimension. This is of special interest regarding natural cracks where there is no clear relationship between echo amplitude and crack extension. This type of evaluation, from the time of flight, is now known as TOFD (Time-Of-Flight Diffraction technique) (22, 23).
Figure 11 shows detection of the crack tips by determination of the diffracted wave's time of flight to various probes or probe positions for surface cracks and for internal cracks as well. An overview of possible ultrasonic methods for determination of the crack extension is explained in (24).

In order to separate diffracted sound pulses from the tips of smaller reflectors (regarding time of flight) short, broad band pulses must be used (25). The phase information from a RF pulse, which is diffracted at the tips of the reflector, can determine whether it is a connected reflector, a flat reflector, or a voluminous reflector (26). This determination is especially important in the field of weld testing. Figure 12 shows the decision tree using this method of evaluation.

Fig. 12 - Evaluation of diffracted waves (Radio frequency pulse)/Sign of first oscillation

It should be noted here that the COMSON method (12) uses rectified pulses.
If incoming waves are converted to surface waves at the reflector, as seen in Fig. 13a, then these will produce additional echoes (longitudinal and transverse waves) from the reflector boundaries, which will return to the probe later than direct echoes from the edge of the reflector. This is the reason why they are called "satellite echoes" (Fig. 13b).

With cracks, the surface wave propagates along the crack surface; with spherical or cylindrical reflectors it travels around them (27, 28)


Fig. 13a Fig. 13b - Mode conversion satellite pulses

From the type of satellite echo and its time of flight, the type and extension of a reflector can be determined, e.g. differentiation between pores and cracks (29-31). Converted waves are reflected at another angle than the original incoming waves. By application of crystals for two wave modes (longitudinal and transverse waves), which are arranged at various angles, the dimensions of reflectors need only be measured using one probe (containing two crystals). This is of advantage with reflectors which are not positioned perpendicularly to the striking wave and which, up until now, could only be detected using the tandem method (with two probes). Due to longitudinal-longitudinal-transverse mode conversion (Fig. 14), people speak of the LLT technique (32) or, according to the type of probe, of multiple-beam-multiple-mode probes (33)

Fig. 14 - Mode conversion with LLT-method

References

1. Serabian, St.: An assessment of the detection ability of the angle beam interrogation method. - Mater. Eval. 39 (1981), 1243 - 1249
2. Wüstenberg, H.; Erhard, A.: Development of ultrasonic techniques for sizing defects. In: Nichols, R.W.(Ed.): Nondestructive Examination Relation Structural Integrity. London, Appl. Sc. Publ., 1980, 59 - 83
3. Winter,D.C.: End-on-crack measurement. In: McAvoy, B.R.(Ed.): Ultrasonics Symposium Proceedings 1975. New York, Inst. Electr. Eng. 1975, 572 - 574
4. Hess,A., Thoma,Ch.: Moglichkeiten der Rißtiefenbestimmung mit Ultra- schall nach dem Rißspitzenverfahren. Materialprüfung 25 (1983), 1O, 340
5. Schlengermann,U., Frielinghaus,R.: Beitrag zur FehlergroBenbestimmung mit Ultraschall durch Fehlerabtastung mit relativer Schwelle. Materialprufung 15 (1973), 2, 50 - 56
6. Schlengermann,U.: Beitrag zur Fehlergroßenbestimmung durch Fehlerabtastung mit fester SchwelIe. Materialprüfung 16 (1974), 10, 319
7. Schlengermann, U.; Frielinghaus, R.: Remarks on the practice of determining the size of reflectors by scanning. In: ComFranc Etud. Ess. Non Destruct. (Ed.): Proceedings 8th WCNDT Cannes (1976), report 5 C 10
8. Jackson, B.: The use of edge-of-beam methods for the assessment of defect size. Can. Soc. Nondestructive Testing Journ. 5 (1983),1; 48,50
9. Krautkrämer, J.; Krautkrämer, H.: Werkstoffprüfung mit Ultraschall. Springer Verlag Berlin,1986, 101 -112.
10. Anderson, E.B.: The evaluation of defects with P-scan equipment and focused search units. In: Conf. Period. Inspect. Press. Components, London, Inst. Mech. Engin., 1982, 163-168.
11. Rensmeyer, M.E.; Grothues, H.L.: Automated data processing for ASME section Xl requirements using the SUTAR system. In: ASNTFall Conf. Proceed., Denver, Am. Soc. Nondestr. Test., 1978, 175-181.
12. Außerwoger, J.; Ganglbauer, O.; Wallner, F.: Computerunterstützte Fehlerdiagnose bei der Ultraschallprüfung von Schweißnahten. Berg-Huttenmann. Monatsh. 130 (1985) 11, 417-421.
13. Gebhardt, W.; Bonitz, F.; Woll, H.; Schmitz, V.: Fehlercharakterisierung und Fehlerrekonstruktion mittels elektronischem Sektorscan und Verbundscan. Saarbrucken, Fraunh. Ges. Inst. Zerstorungsfr. Pruf., 1979, Bericht 790520-TW.
14. Gebhardt, W.; Bonitz, F.; Woll, H.: Defect reconstruction and classification by phased arrays. Mater. Eval. 40 (1982)1, 90-95.
15. Wüstenberg, H.; Schulz, E.; Volker, J.; Sonnenberg, G.J.: Turbinenteile uberwachen mit Ultraschall-Prufkopfen. MM Maschinenmarkt 90 (1984) 55/56, 1308-1311.
16. Barbian, O.A.; Grohs, B.; Licht, R.: Signalanhebung durch Entstorung von Laufzeitmeßwerten aus Ultraschallprufungen von ferritischen und austenitischen Werkstoffen - ALOK 1. Materialpruf. 23 (1981 ) 11, 379-383.
17. Grohs, B.; Barbian, O.A.; Kappes, W.; Paul, H.: Fehlerbeschreibung nach Art, Lage und Dimension mit Hilfe von Laufzeitortskurven aus Ultraschallprufungen - ALOK 2. Materialprüf. 23 (1981) 12, 427-432.
18. Kappes, W.; Grohs, B.; Barbian, O.A.: Berechnung von Laufzeitortskurven fur die Tandemprufung mit Ultraschall und Fehlerlage-Rekonstruktion aus Tandem- Laufzeit-Ortskurven beim ALOK-Verfahren - ALOK 3. Materialprüf. 24 (1982) 5, 161-165.
19. Crostack, H.A.; Roye, W.: Verbesserung der Ultraschallprüfung von GuBteilen durch Anwendung der Mehrfrequenzentechnik. GieBereiforsch. 36 (1984) 3,115-120.
20. Crostack, H.A.; Roye, W.: Einsatz der Ultraschall-Mehrfrequenzentechnik zur verbesserten Fehlerbeschreibung bei der Schweißnahtprüfung. Düsseldorf, Deut. Verb. SchweiBtechn., 1986, DVS- Berichte 106, 37-41
21. Vadder, D. de; Azou, P.; Saglio, R.; Birac, A.M.: Determination of orientation and size of badly oriented quasi plane defects by means of focused probes. Proceed. 9th WCNDT, Melbourne (1979), report 4G-1.
22. Silk, M.G.: Defect sizing using ultrasonic diffraction. Brit. J. Nondestr. Test. 21 (1979) 1,12-15.
23. Silk, M.G.: A time approach to crack location and sizing in austenitic welds. Brit. J. Nondestr. Test. 22 (1980) 2, 55-61.
24. Schlengermann, U.: Zur Bestimmung von Rißtiefen durch Ultraschallverfahren - ein Überblick. DGZfP-Jahrestagung 1987, Berichtsband 10, 70-81, auch Krautkrämer SD 264.
25. Lam, F.K.; Tsang, W.M.: Flaw characterization based on diffraction of ultrasonic waves. Ultrasonics 23 (1985) 1,14-20.
26. Proegler, H.: Recent ultrasonic methods for the distinction between different types of defects in welds. Proceed. 9th WCNDT, Melbourne (1979), report 3B-3.
27. Charlesworth, J.P.; Temple, J.A.G.: Creeping waves in ultrasonic nondestructive testing. Proceed. Ultrasonics International (1981),390-395.
28. Budenkov, G.A.; Khakimova, L.I.: Measurement of the dimensions of spherical and cylindrical defects. Sov. J. Nondestr. Test.17 (1981) 7, 540-545.
29. Ogura, Y.; Ishii, Y.: A measurement of the height of internal defects using the mode-converted surface wave at the defect. Proceed. 9th WCNDT, Melbourne (1979), report 4G-5.
30. Sachse, W.; Golan, S.: The scattering of elastic pulses and the nondestructive evaluation of materials. ASME report ADM-29 (1978) 11-31.
31. Gruber, G.J.: Defect identification and sizing by the ultrasonic satellite pulse technique. J. Nondestr. Eval.1 (1980) 4, 263-276.
32. Gebhardt, W.; Walte, F.: Ultraschallprufung auf senkrecht orientierte Risse durch Ausnutzung der Wellenumwandlung (Tandem-ersatzprufung). Materialpruf. 30 (1988) 3, 73-77.
33. Gruber, G.J.; Hamlin, D.R.; Grothues, H.L.; Jackson, J.L.: Imaging of fatigue cracks in cladded pressure vessels with the SLIC-50. Nondestr. Test. Int.19 (1986) 3, 155-161.

---

Author:

U. Schlengermann
works many years in the development and application department
for Krautkrämer GmbH D-Hürth.
This Article is a 'reprint' and was published in the Echo 33,34,35.
"the echo" is published free of charge at various intervals. Produced by:
Krautkrämer GmbH Co.
Robert-Bosch-Str. 3
P. 0. Box 13 63 D-50354 Hürth
Phon: +49-2233-6010
Fax: +49-2233-601402
E-Mail: 100656.65@compuserve.com
Contact: Krautkraemer in NDT online exhibition

| UT-Front Page | |Top of this page|