BASICS ULTRASONIC TESTING METHOD. -Dr.Oruganti Prabhakar Proprietor-OP-TECH

Transcription

1 BASICS ULTRASONIC TESTING METHOD -Dr.Oruganti Prabhakar Proprietor-OP-TECH INTRODUCTION Ultrasonic Testing (UT) is done by first generating high frequency acoustic waves at the outer surface of the component and transmitting these waves through a coupling medium into the component. The flaw indications are indirect unlike Radiographic Testing Method where the silhouette of the discontinuity can be seen on the radiograph. The UT of flat and parallel-sided components for large isolated discontinuities is quite straightforward. However, caution must be exercised in applying UT for complex shapes and for detecting smaller discontinuities. Operator skill would determine the correct interpretation of the results. It is better to restrict UT to materials having an attenuation coefficient less than 0.1 db/mm to reduce the trust deficit of the management on UT. widely used modes for UT are longitudinal, transverse, surface and plate waves. The directions of particle vibration and wave propagation direction will determine the vibration mode. If they are parallel then it is longitudinal and if they are perpendicular to each other then it is shear or transverse waves. In conventional UT we deal mostly with nondispersive type of US waves where the phase velocity is independent of frequency and specimen thickness. BASIC PRINCIPLES 1. PULSE-ECHO TECHNIQUE Two principles define this UT technique: Ultrasonic (US) waves have a fixed phase velocity in a given medium (non-dispersive type) for a given mode. US waves get reflected at materials interface where the acoustic impedances do not match resulting in an echo. ACOUSTIC WAVES Fig 2: Pulse-Echo technique principle Fig 1: Types of UT waves There are many ultrasonic vibration modes that travel through materials. The In the example given in Figure 2, a US pulse generated by the probe travels in 100 mm steel specimen and returns to the probe in about 32 microseconds (µsecs). In the same specimen if a flaw is present at 40 mm an echo will be received at 14 µsec. It is this time difference that we discern to locate the flaw. 1

2 Fig 3: Block diagram of an ultrasonic Pulse Echo equipment In pulse-echo technique we are looking for means to measure times in the microseconds range. So we use a CRT to measure the time of travel of the pulse. This is also known as the Time of flight) (TOF). We synchronize the electron beam travel in the CRT with the US pulse travel in the specimen. Figure 3 shows a schematic block diagram of US pulse echo equipment. Modern equipment are different, however, for training we still use CRT to explain the UT fundamentals. Some typical flaw indications are shown in Figure 4. What one should notice is an echo from the discontinuity should be received by the probe. Otherwise we cannot draw any conclusion. So planar defects like fatigue crack, hot tear, laminations etc. that reflect the US waves nicely back to the probe can be detected easily by UT. Shrinks, pipes etc. are poor reflectors and hence detection is more difficult. Fig 4: Typical flaw indicattions 2. ACOUSTIC IMPEDANCE: It is the product of the density and the phase velocity of a material. The degree of mismatch between two materials will determine the amount of pressure transferred from one material to the when in contact. Fig 5: Reflection and transmission 3. THROUGH TRANSMISSION TECHNIQUE 2

3 Sin α Sin γ L1 t 2 Velocity = Velocity L1 t2 where subscript L= longitudinal, t=transverse, 1= medium 1 and 2 =medium 2. Fig 6: Through transmission technique In through transmission technique US energy is transmitted through the zone of interest. The changes in the transmitted energy are taken as a measure of the quality of the zone of interest. Figure 6 shows the schematic of the method. This technique is employed in the resistance welding of plates and sheets. One uses a continuous train of US waves unlike a pulse in the pulse- technique. OBLIQUE INCIDENCE Fig 7: Mode conversions Mode conversion takes place as shown in figure 7 when a sound beam strikes an interface between two media at an angle other than 90 o. The angular relationships between the various modes can be determined by Snell s law given below. We generate longitudinal waves in the probe but scan the component using shear waves generated through mode conversion. By clever manipulation we totally eliminate the presence of longitudinal waves in the component. This law is used extensively in UT for testing and probe construction. One should remember while Snell s law indicates angular relationships between various modes, it does not tell us about the pressure distribution amongst these modes. PRESSURE Attenuation The gradual loss of intensity of US waves travelling through a specimen is termed as Attenuation. Grain size distribution, micro inclusions and other scatterers influence attenuation. If the scatterer is smaller than the wavelength then the scatterer is practically of no consequence. By employing a lower frequency or a higher wavelength one may reduce the scattering problem. However, lowering the frequency will decrease the sensitivity to detect discontinuities and will also reduce lateral resolution. SENSITIVITY It is ability to detect the smallest flaw. It is influence by the wavelength or the probe frequency employed in UT. Higher the frequency higher the sensivity of the UT method. The smallest size of the flaw one can detect is half of the wavelength used. 3

4 This is for spherical type of flaws. In the case of crack like discontinuities the crack thickness can be as low as one tenth of the wavelength or even smaller. RESOLUTION Resolution is the ability to separate two signals coming from two defects lying close to each other. Both axial and lateral resolutions are relevant in UT. 6 db difference may be used to differentiate two closely lying signal. Axial resolution may be altered by changing the damping of the crustal and the pulse voltage applied to the crystal. Lateral resolution is manipulated by changing the beam spread and beam focussing. IMMERSION TECHNIQUE In this technique water is used as the couplant. The part to be tested in immersed in a water tank during testing. In immersion technique it is possible to use different search units sizes and shapes. The search units are usually focussed probes. The water path and the focussing distance inside the steel specimen are of major concerns in this method. One tends to use scanners in immersion technique. The principle one employs in immersion testing and in any time delay technique in general, is that the second reflection from the water/ specimen interface should come after the first reflection from the backwall of the test specimen. This will ensure easier analysis of the signal patterns. Please see figure 8 for a general arrangement used in immersion testing. Fig 8: Immersion Technique PROBE TYPES The following types of probes are used in UT. 1. Normal Probe 2. Angle Probe 3. T/R Probe 4. Immersion Probe 5. Squirters 6. Bubblers 7. EMAT 8. Laser induced US waves 9. Air coupling Probe Each type of probe is used depending on the application. The most commonly used probes are the normal, angle and immersion probes. CALIBRATION/REFERENCE BLOCKS Calibration blocks are used to calibrate the time and the gain of the UT equipment. Two of the commonly used calibration blocks are shown in Figure 9. 4

5 1. Meter 2. Strip Chart 3. Cathode ray tube (CRT) Fig 9: V 1, V 2 reference blocks Reference blocks are made with a wide variety of reflectors such as notches, flat bottom holes (FBH) and side drilled holes (SDH). FLAW SIZE DETERMINATION Fracture control and fracture mechanics have placed a great emphasis on the flaw size determination. UT is of great help to determine the flaw size. One employs three methods to assess the flaw size: 1. 6 db drop method: This is used when the flaw size is bigger than the probe diameter. 2. Distance- Gain-Size diagrams (DGS), Distance-Amplitude- Correction Curve (DAC), Flat Bottom Holes (FBH) or Side drilled Holes (SDH): This is used when the flaw size is smallerr than the probe diameter. 3. Time of Flight: In this method one employs the time of flight to calculate the flaw size instead of the gain used. Cathode ray tube (CRT) is usually employed in UT. The CRT has two sets of plates, one pair X-Pair and the other- Y- Pair. Two electrical signals can be fed to these two pairs of plates. The type of scanning is determined by the electrical signals we choose to feed into CRT. A-Scan: X-Pair- Time of flight of the US pulse. Y-Pair- Amplification of the US signal, Gain. B-Scan: X-Pair- Probe location on the component in one (say x) direction. Y-Pair- Time of flight of the US pulse. So you find that the B-Scan has no information on the gain employed. So this is useful in Time of Flight Diffraction (TOFD), a type of UT which utilizes the diffraction effects of US waves. One may also call B-scan as parallel stacking of A- Scan signals. C-Scan: X-Pair- Probe location on the component in x- direction. Y-Pair- Probe location on the component in y- direction. In the C-Scan the test data like time of flight, gain or phase are displayed as the pixel intensity or depth. SCANNING METHODS The information obtained in UT can be displayed by three methods: 5

6 TESTING OF CASTINGS Plain carbon steel or alloy steel castings weighing up to 50 tons are cast in India for many industries including for cement and sugar mills. Heat treatment is an essential feature of these castings. Adequacy of the heat treatment can be in many situations monitored by UT. Ultimately the toughness of the castings is what the engineer focussing on. UT would reveal many factors like inclusions, hot-tears, blow holes, porosity, cracks, hydrogen blisters, hydrogen induced cracking etc. that affect toughness. We should not forget that some critical information on embrittlement including hydrogen or tramp element induced embrittlement, segregation or presence of harmful phases like sigma or Chi phase cannot be obtained by conventional UT on the shop floor. Minimum section thickness of the castings, surface roughness, the orientation of the discontinuities and microstructure are some of the factors that limit the use of UT in the casting industry. Curved surfaces such as the bore of an engine cylinder block would not permit adequate or easy coupling of the probe and so precautions should be taken. Cast iron castings pose a special problem due to the presence of free graphite that damps the US waves. Grades higher than 17 or Spheroidal graphite cast irons may be tested with UT. UT is more useful to assess the structure of cast iron castings than for flaw detection. WELD TESTING Fig 10: Weld Testing Welds are tested using angle probes as shown in Figure 10. By its very nature weld cannot be perfect. Welds have defects. Cracks or nicks, residual stresses and variations in microstructure constitute the major issues of concern. Many of the weld defects can be detected by UT and it is the main test method in many situations to accept welds. Hydrogen continues to be a major problem in welds. In its atomic state it can cause embrittlement or coalesce to form blisters. However, in the molecular form coupled with residual stresses can cause delayed cracking. UT is an invaluable tool to detect HIC (hydrogen induced cracks). Mostly angle probes are employed. Once a flaw is detected during scanning, the beam path from the flaw detector and the physical distance measured between the probe and the weld centre line are used to obtain information on the location, size and the nature of flaw. A UT operator has to undergo good practical training to do weld testing reliably. Codes like AWS and ASME are used extensively in the industries to carry out UT. 6

7 The main principle is that the UT beam should be very nearly at 90 o to the flaw to obtain good reflection. So in the case of thick plates one may virtually slice the plates into three or more zones and use different probes for different zones. This is practised in Automated Ultrasonic Testing (AUT). Just for one type of defect like Lack of side wall fusion (LOF) in thick welds, we may use time of flight diffraction technique which is a slight variation of UT. In TOFD we use the diffracted beam and not the reflected beam. programs are necessary if the reliability and usefulness of this method are to be further improved in our country. ADVANCES IN UT Major advances have taken place in UT. Most of these are propelled by improvements in signal and image analysis techniques. Phased array ultrasonic testing (PAUT), Guided wave testing and Time-of-flight diffraction technique are the important modern UT methods. Investments in equipment and manpower training are huge. In India there is a general belief amongst the technologists that more recent the technique the better it is. Remember the days when acoustic emission technique was introduced in early 70 s and when all the Indian NDT personnel rushed to embrace this technique as a cure all. However, at least one major advanced country has reservations about these modern UT techniques. They feel that conventional UT can do the job adequately and it is mostly the hype that is surrounding these modern techniques. Obviously truth should be lying somewhere between the two extreme views. GENERAL COMMENT For a successful application of UT in a jobbing environment, time and effort are needed. Reasonable remuneration for UT operators and tightening the certification 7