|NDT.net - September 2000, Vol. 5 No. 09|
- Introduction to the Basic Principles -
|TABLE OF CONTENTS|
1. Why use ultrasonics for nondestructive material testing?
2. Ultrasonic testing tasks
3. Detection of discontinuities
4. Method of testing and instrument technology
4.1 The ultrasonic flaw detector
4.2 Near resolution
4.3 The probe
4.4 Refraction and mode conversion
4.5 Characteristics of angle-beam probes
4.6 The TR probe
5. Locating discontinuities
5.1 Calibration of the instrument
5.1.1 Calibration with a straight-beam probe
5.1.2 Calibration with a TR probe
5.1.3 Calibration with an angel-beam probe
5.1.4 Locating reflectors with an angle-beam probe
6. Evaluation of discontinuities
6.1 Scanning method
6.2 Evaluation of small discontinuities: The DGS method
6.3 Sound attenuation
6.4 The reference block method
6.4.1 Comparison of echo amplitudes
6.4.2 Distance amplitude curve
8. Diagnosis of indications (outlook)
Corresponding Author Contact:
c = Sound velocity [km/s]|
f = Frequency [MHz]
l = Wave lenght [mm]
This means that ultrasonic waves must be used in a frequency range between about 0.5 MHz and 25 MHz and that the resulting wave length is in mm. With lower frequencies, the interaction effect of the waves with internal flaws would be so small that detection becomes questionable. Both test methods, radiography and ultrasonic testing, are the most frequently used methods of testing different test pieces for internal flaws, partly covering the application range and partly extending it. This means that today many volume tests are possible with the more economical and non-risk ultrasonic test method, on the other hand special test problems are solved, the same as before, using radiography. In cases where the highest safety requirements are demanded (e.g. nuclear power plants, aerospace industry) both methods are used.
Instead of using the word "reflector", the ultrasonic operator very often uses the term "discontinuity". This is defined as being an "irregularity in the test object which is suspected as being a flaw". In reality, only after location, evaluation and diagnosis has been made, can it be determined whether or not there is a flaw which effects the purpose of the test object. The term "discontinuity" is therefore always used as long as it is not certain whether it concerns a flaw which means a non-permissible irregularity.
|Fig. 1a Straight-beam probe (section)|
Fig. 1b Angle-beam probe (section)
The operator then scans the test object, i.e. he moves the probe evenly to and fro across the surface. In doing this, he observes an instrument display for any signals caused by reflections from internal discontinuities, Fig. 2.
|Fig. 2a Plane flaw - straight-beam probe|
Fig. 2b Plane flaw - angle-beam probe
Fig. 3 Sound field
Every probe has a certain directivity, i.e. the ultrasonic waves only cover a certain section of the test object. The area effective for the ultrasonic test is called the "sound beam" which is characteristic for the applied probe and material in which sound waves propagate. A sound beam can be roughly divided into a convergent (focusing) area, the near-field, and a divergent (spreading) part, the far field, Fig. 3. The length N of the near-field (near-field length) and the divergence angle is dependent on the diameter of the element, its frequency and the sound velocity of the material to be tested. The center beam is termed the acoustic axis.
The shape of the sound beam plays an important part in the selection of a probe for solving a test problem. It is often sufficient to draw the acoustic axis in order to show what the solution to a test task looks like. A volumetric discontinuity (hollow space, foreign material) reflects the sound waves in different directions, Figs. 4a + 4b.
Fig. 4a Volumetric discontinuity - straight-beam probe
Fig. 4b Volumetric discontinuity - angle-beam probe
The portion of sound wave which comes back to the probe after being reflected by the discontinuity is mainly dependent on the direction of the sound wave; i.e. it does not matter whether scanning is made with a straight-beam probe or an angle-beam probe or whether it is carried out from different surfaces on the test object, Fig. 5. If the received portion of the reflected sound wave from the probe is sufficient then the detection of the existing volumetric discontinuity is not critical, this means that the operator is able to detect it by scanning from different directions. A plane (two-dimensional) discontinuity (e.g. material separation, crack) reflects the ultrasonic waves mostly in a certain direction, Fig. 6.
Fig. 5 Volumetric flaw - detection form different directions
Fig. 6 Reflection on angled plane discontinuity
If the reflected portion of the sound wave is not received by the probe then it is unlikely that the discontinuity will be detected. The possibilities of detection only increase when the plane discontinuity is hit vertically by the sound beam. This applies to discontinuities which are isolated within the test object.
Fig. 7 Apparent deformation of the sound beam on a side wall
Fig. 8a Crack detection with 45° scanning
|Fig. 8b Angle reflection effect|
|Fig. 9 Plane, vertical reflector near the surface|
Fig. 10a Angle reflection effect
Fig. 10b Tandem testing: center zone
Fig. 10c Tamden testing: lower zone F
With this type of testing, the Tandem Technique, one probe is used as a transmitter, and the other probe is used as the receiver. Both probes are moved over the surface of the test object and are spaced apart at a fixed distance. Scanning is made for vertically positioned discontinuities at different depths of the test object, depending on the probe spacing, Figs. 10a, 10b and 10c.
Although, with angle scanning in thin test objects, there is a possibility that plane discontinuities cannot be vertically hit, Fig. 11 a, the detection sensitivity is much better, especially by suitable selection of the scanning angle and the test frequency so that the user favours the single probe test as opposed to the more complicated tandem method. This is normally the case when testing welds up to a thickness of about 30 mm.
Of course the possibility of detecting discontinuities which are not vertically hit is reduced. However, this deficiency is often compensated by an additional test with another angle of incidence, Fig. 11 b, or by using a probe with a lower frequency, Fig. 11 c. A typical procedure can be found in the corresponding specifications (test instructions) for weld testing.
Fig. 11a 70° scanning: unfavourable angle
Fig. 11c 70° scanning with 2 MHz; detection by large divergence of the sound beam
Fig. 11b 45° scanning: favourable angle
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