The essential "tool" for the ultrasonic operator is the probe, Figs. 1a + 1b. The piezoelectric element, excited by an extremely short electrical discharge, transmits an ultrasonic pulse. The same element on the other hand generates an electrical signal when it receives an ultrasonic signal thus causing it to oscillate. The probe is coupled to the surface of the test object with a liquid or coupling paste so that the sound waves from the probe are able to be transmitted into the test object.
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.
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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
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.
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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
With plane discontinuities which are open to the surface of the test object, e.g. a crack running vertically from the surface into the test object, a vertical scan of the crack does not always produce the required success. In this case wave overlapping occurs (interferences) due to sound wave reflection on the side wall of the test object which seems as if the sound wave bends away from the corresponding side wall, Fig. 7. In such cases, the probability of crack detection is very good if the angle reflection effect is used, Fig. 8a. At the 90° edge, between the crack and the surface of the test object, the sound waves are reflected back within themselves due to a double reflection, Fig. 8b. Use of the angle reflection effect is often even possible when a plane discontinuity, which is vertical to the surface, does not extend to the surface and under the condition that the sound wave reflections at the discontinuity and the surface are received by the probe, Fig. 9.
Fig. 8a Crack detection with 45° scanning
Fig. 8b Angle reflection effect
Fig. 9 Plane, vertical reflector near the surface
Flaw detection is the process of identifying and sizing sub-surface defects in materials. One of the most common techniques to identify defects is ultrasonic inspection where sound waves, propagated through the material, are used to identify such anomalies. The high frequency sound behaves predictably when interacting with surfaces and internal defects.
Flaw detection can be applied in almost any industry from composites and metals used in aerospace, to petrochemical oil and gas pipelines and storage tanks, to power generation including nuclear power. The most common anomalies detected include cracks, voids and porosity in metals, ceramics and plastics in addition to delaminations and disbonds in composites.
Advantages of ultrasonic testing include:
- Access is only required from one side for pulse-echo mode
- The depth of penetration is superior to other methods
- Highly accurate flaw sizing and shape
- Minimal part preparation is required
- Results are in real-time
Modern portable flaw detectors interpret the distinctive sound echoes given off by the anomalies. Imaging flaw detectors provide color and manual or automated scanning ability to generate comprehensible, full-field, C-scan images of the material, reducing inspection time dramatically.
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1. Why use ultrasonics for nondestructive material testing?
The essential "tool" for the ultrasonic operator is the probe, Figs. 1a + 1b. The piezoelectric element, excited by an extremely short electrical discharge, transmits an ultrasonic pulse. The same element on the other hand generates an electrical signal when it receives an ultrasonic signal thus causing it to oscillate. The probe is coupled to the surface of the test object with a liquid or coupling paste so that the sound waves from the probe are able to be transmitted into the test object.
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
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
With plane discontinuities which are open to the surface of the test object, e.g. a crack running vertically from the surface into the test object, a vertical scan of the crack does not always produce the required success. In this case wave overlapping occurs (interferences) due to sound wave reflection on the side wall of the test object which seems as if the sound wave bends away from the corresponding side wall, Fig. 7. In such cases, the probability of crack detection is very good if the angle reflection effect is used, Fig. 8a. At the 90° edge, between the crack and the surface of the test object, the sound waves are reflected back within themselves due to a double reflection, Fig. 8b. Use of the angle reflection effect is often even possible when a plane discontinuity, which is vertical to the surface, does not extend to the surface and under the condition that the sound wave reflections at the discontinuity and the surface are received by the probe, Fig. 9.
Fig. 8a Crack detection with 45° scanning
Fig. 8b Angle reflection effect
Fig. 9 Plane, vertical reflector near the surface
Flaw detection is the process of identifying and sizing sub-surface defects in materials. One of the most common techniques to identify defects is ultrasonic inspection where sound waves, propagated through the material, are used to identify such anomalies. The high frequency sound behaves predictably when interacting with surfaces and internal defects.
Flaw detection can be applied in almost any industry from composites and metals used in aerospace, to petrochemical oil and gas pipelines and storage tanks, to power generation including nuclear power. The most common anomalies detected include cracks, voids and porosity in metals, ceramics and plastics in addition to delaminations and disbonds in composites.
Advantages of ultrasonic testing include:
- Access is only required from one side for pulse-echo mode
- The depth of penetration is superior to other methods
- Highly accurate flaw sizing and shape
- Minimal part preparation is required
- Results are in real-time
Modern portable flaw detectors interpret the distinctive sound echoes given off by the anomalies. Imaging flaw detectors provide color and manual or automated scanning ability to generate comprehensible, full-field, C-scan images of the material, reducing inspection time dramatically.
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