Measurements on extent and quantity of ultrasonic angle beam probe index point variability with inspected material
Author : My?h?nen Heikki - Huber Testing Oy, Finland
Co-author:Ruha Matti - Huber Testing Oy, Finland
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Introduction
The true angle beam probe index point should not change in reality when testing different materials. Although temperature changes and perspex wear can alter the sound beam in the probe the form and location of the sound beam inside the probe perspex is practically constant. However, it is often necessary to use a different index point for weld inspection in aluminium than in steel for accurate determination of defect locations. Yet another observation concerns the measurement of index point with the calibration block 2 according to EN 27963. With for instance austenitic calibration blocks the index point is not the same when taking the measurement from the 25 and 50 mm arc selected as the first reflection surface. In this article we present several different measurements with probe angle, frequencies and material as variables. Our main concern was to measure the index point variability, but in addition some other interesting results did come up as well.
Equipment used
For the tests we used five different probes: 2 MHz 45° and 60°, 4 MHz 45°, 60° and 70° all of which were Krautkr?mer type MWB probes. Test blocks were EN 27963 calibration blocks of steel, aluminium and austenitic steel. In addition blocks of steel, aluminium and austenitic steel with three side drilled holes with different depths for the probe angle and index point measurement according to EN 12668-3 were used. A Krautkr?mer USN 52 was used as the ultrasonic device.
All measurements were done in laboratory conditions as precise as possible but according to practice possible also at field work. No precision measurement tools were used. Knowingly we admit that there surely is error in some quantity in the measurements. Some of the following results, however, show such large deviations that they cannot be explained by mere inaccuracy due to manual measurements.
Probe delay measurement
The first measurement was the probe delay (probe zero) in time units. The three different EN 27963 calibration blocks were used to calibrate the ultrasonic device for the material. The 25 mm arc was used as the first reflection. After an accurate calibration the probe delay was recorded. Table 1 shows the results of this measurement.
Probe Fe Al SS
MWB 45-2 5,819 5,539 5,951
MWB 60-2 6,553 7,264 6,566
MWB 45-4 4,756 4,855 4,852
MWB 60-4 6,156 6,365 6,151
MWB 70-4 7,108 7,625 6,916
Table 1. Results of probe delay measurement in time [ms].
Surprisingly the results for a single probe are not equal although there should be no change in the time consumed in the probe perspex. Because the difference is hard to comprehend in terms of time, Table 2 below depicts the probe delays in terms of distance in perspex. The results have been calculated using perspex sound velocity 2730 m/s.
Probe Fe Al SS Max. difference
MWB 45-2 15,9 15,1 16,2 1,1
MWB 60-2 17,9 19,8 17,9 1,9
MWB 45-4 13,0 13,3 13,2 0,3
MWB 60-4 16,8 17,4 16,8 0,6
MWB 70-4 19,4 20,8 18,9 1,9
Table 2: Probe delay in perspex distance [mm] using 2730 m/s as sound velocity.
The result of Table 2 show that it is possible to measure almost 2 mm differences in the probe delay length by just changing the material of the calibration block. The largest differences are always measured with aluminium with respect to one of the other materials. Probe delays for austenitic steel and carbon steel are very close each other but calibration for aluminium results usually in a longer delay time.
Comparison between index point measurement from 25 and 50 mm arcs
In this measurement the index point measurement using the 25 mm and 50 mm arcs of calibration block 2 as the first reflector are compared. The results are shown in Table 3. With carbon steel calibration block there is no difference in the index point, but with austenitic steel the index point is further back the probe always when the measurement is made aiming at the 50 mm arc. This tendency is visible also with aluminium but with MWB 60-2 and MWB 70-4 the index point is the same with both measurements. The most interesting probe was the 4 MHz 70° with which the results are most peculiar. With aluminium the index point is stable but with austenitic stainless steel the difference is as long as 3 mm.
Although both aluminium and austenitic steel results show behaviour in a similar way there is no clearly consistent pattern involved. It is a known fact that austenitic steel is anisotropic with different sound velocities in different directions through the crystal structure. The crystal structure of aluminium is also face centred cubic. Aluminium and austenitic steel cannot be normalised in the same way as carbon steel. Hence, the material structure due to manufacturing may bear a substantial impact on how the sound beam interacts within the calibration block. The inconsistency of the aluminium block results points to this reasoning. Another fact is that both aluminium and austenitic steel have an oxide layer on their surfaces. The oxide layer of aluminium is strong and grows with time. This may also be a major factor affecting the virtual probe index point. The oxide layer should still be almost similar to both directions at the centre of the 25 mm and 50 mm arcs.
Fe Al SS
Probe 25 mm 50 mm 25 mm 50 mm 25 mm 50 mm
MWB 45-2 13 13 12 14 12 13
MWB 60-2 13 13 13 13 13 14
MWB 45-4 13 13 13 14 12 14
MWB 60-4 13 13 12 13 13 14
MWB 70-4 12 12 10 10 11 14
Table 3: Index point measurement with EN 27963 calibration block 2 using 25 mm and 50 mm arcs as the first reflector
Probe angle measurement with EN 27963 calibration block 2
The next measurement involves visual determination of the probe angle using calibration block 2 with the index point measured from the 25 mm arc. Table 4 shows the results compared to ones calculated with Snell's law and measured sound velocities.
Fe (3239 m/s) Al (3087 m/s) SS (3132 m/s)
Probe Nominal Measured Calc. Meas. Calc. Meas.
MWB 45-4 45 46 42,4 43 43,1 44
MWB 60-4 60 60 55,6 56 56,9 57,5
MWB 70-4 70 71 63,4 66 65,3 67
Table 4: Probe angle measurement with EN 27963 calibration block 2.
The measured results comply accurately enough with the calculated ones when the measured angle in carbon steel is taken into account. If the sound velocity of the tested material is known only the probe index point and delay are left as parameters which require accurate calibration blocks of different materials. Surprisingly these should be the parameters that are reasonably constant.
Probe angle and index point determination according to EN 12668-3
The EN 12668-3 approach to determination of probe angle and index point seems very accurate at first glance. The use of cylindrical side drilled holes at different depths and linear regression for the results should give us a good estimate of the true index point. Use of only straight sound paths with no reflections should also improve the accuracy. The only inaccurate measurement in this approach is the surface distance.
Our measurements for 45° and 60° probes were done with blocks which had three ?3 mm SDHs at depths 40, 60 and 80 mm. The same blocks were used also for the 70° probe but with depths 20, 40 and 60 mm, where the 20 mm depth is acquired flipping the block over. The longitudinal sound velocities in two different directions were measured and are shown in table 5. Transverse sound velocities in the direction of EN 12668-3 measurements were approximated measuring the full skip surface distance with 45° tandem arrangement. Index points were measured with carbon steel calibration block 2. Using the measured angle for the full skip and assuming sound velocity 3230 for carbon steel the sound velocities could be approximated with Snell's law. The results of this measurement are shown in table 6. Note that the transverse sound velocity of the aluminium test block is higher than values usually reported for aluminium.
Fe Al SS
v1 v2 v1 v2 v1 v2
5930 5939 6330 6384 5869 5728
Table 5: Test block longitudinal sound velocities. Velocity v1 is measured in the depth (100 mm) direction and v2 through the width (40 mm) of the blocks.
Fe Al SS
Angle vtr Angle vtr Angle vtr
45,8 3230 45,3 3207 39,8 3106
Table 6: Approximation of test block transverse sound velocity.
The results of measurements were calculated using three point linear regression and are shown in table 7 and figure 1. The abscissa in figures 1a-e shows the depth. Thus index points are read as the negative value of curves at depth zero. Probe angle is the angle between abscissa and curve.
Fig 1: Probe angle and index point determination for carbon steel (Fe), aluminium (Al) and austenitic steel (SS) according to EN 12668-3. a) MWB45-2, b) MWB45-4, c) MWB60-2, d) MWB60-4 and e) MWB70-4.
Fe Al SS
Probe Angle Index Angle Index Angle Index
MWB 45-2 46,9 15,6 45,7 13,3 39,8 9,7
MWB 60-2 60,1 12,5 59,0 16,6 58,9 19,4
MWB 45-4 47,3 17,5 44,3 13,4 41,1 10,3
MWB 60-4 59,9 13,6 60,2 21,2 58,7 19,2
MWB 70-4 71,5 20,0 68,4 22,2 66,8 14,9
Table 7: Probe angle and index point determination according to EN 12668-3.
The measured probe angles are roughly in accordance with approximated sound velocities. However, with 45° probes change of material alters the probe angle much more than with 60° probes. This observation is not consistent with Snell's law. Again the 70° degree probe measurements show angle change more in proportion to Snell's law but the angle in aluminium seems to be slightly too low.
The measurements for 45° probes in austenitic steel show a tendency to reduce the angle at longer distances. This can be seen easily following for instance the austenitic steel (SS) curve for MWB45-2 and the accompanied dots in figure 1a. The dot corresponding to the measurement at depth 80 mm is low when compared to the other points, which means that the surface distance is shorter and the angle smaller. This deviation may be due to beam refraction caused by austenitic structure and material texture. Another reason may be attenuation, which will alter the beam characteristics by low pass filtering the pulse frequency. Attenuation cuts down signal power more in the high frequency region of the sound beam near the centre line. This can flatten the power distribution and enable peak echo to be found with smaller angles. The filtering effect concentrates to shorter distances. The difference in angles for 60° probes should still be larger although all points are measured at longer distances. Table 7 compares the probe angle measurements with calibration block 2 and EN 12668-3.
Probe angle [°]
Calibration block 2 EN 12668-3
Probe Fe Al SS Range Fe Al SS Range
MWB45-4 46 43 44 43-46 47 44 41 41-47
MWB60-4 60 56 58 56-60 60 60 59 59-60
MWB70-4 71 66 67 66-71 72 68 67 67-72
Table 7: Measured probe angles in steel, aluminium and austenitic steel with calibration block 2 and EN 12668-3 method using three point measurement. Sound velocities between aluminium blocks are not the same.
The variability of probe index point with tested material is visible in all measurements. The deviations are, however, such large that it is obvious that three points for the EN 12668-3 measurement is not enough when the index point is determined. The measured index points are in some cases over 20 mm which is at least with a 70° probe quite out of possible range because the MWB probe contact surface is only 24 mm long. This gives a good reason to question the accuracy of the method. At least for 60° and 70° probes the depths of the SDHs used for the measurement should not be too large. With growing sound path the measurement becomes more and more inaccurate due to wider echo dynamics. Use of large number of points for the measurement would however make this method very tedious and time consuming to be suitable for field work. Table 8 compares the index point measurements with calibration block 2 and EN 12668-3.
Index point [mm]
Calibration block 2 EN 12668-3
Probe Fe Al SS Range Fe Al SS Range
MWB45-2 13 12-14 12-13 12-14 16 13 10 10-16
MWB45-4 13 13-14 12-14 12-14 18 13 10 10-18
MWB60-2 13 13 13-14 13-14 13 17 19 13-19
MWB60-4 13 12-13 13-14 12-14 14 21 19 14-21
MWB70-4 12 10 11-14 10-14 20 22 15 15-22
Table 8: Measured index points in steel, aluminium and austenitic steel with calibration block 2 and EN 12668-3 method using three point measurement..
Conclusions
Probe delay, angle and index point of a variety of commonly used transverse angle beam probes were measured using three different materials which were carbon steel, aluminium and austenitic steel.
The probe delay measurement with EN 27963 calibration block 2 show deviations when the inspected material was changed. This was unexpected because in theory the time consumed in the probe perspex is constant in constant temperature. The sound beam form and direction within the probe does not change if the inspected material is changed. This deviation must be due to sound beam interaction within the tested material. The change of material must create additive effects to the sound beam other than mere change of angle and beam spread for these kind of results to be possible. There was no consistent pattern involved in the results other than the fact that the largest deviations were always measured between aluminium and carbon steel or austenitic steel.
Probe angles were measured with calibration block 2 and the method described in EN 12668-3. The probe angles measured with calibration block 2 were in good compliance with Snell's law and sound velocities measured from these blocks. Measurements with EN 12668-3 resulted in slightly larger angles with the exception of 45° in austenitic steel. The sound velocities of the two aluminium blocks were significantly different, which explains the larger angles in EN 12668-3 measurement. The use of only three measurement points for this approach is clearly too few. Measurement error of 1 mm in every point may lead easily to a significant error in angle measurement.
Probe index points were also measured with calibration block 2 and the method described in EN 12668-3. The index points with carbon steel calibration blocks were the same regardless of the arc aimed at. With aluminium and austenitic steel the index points are not the same. They differ from the index point in carbon steel and are dependable on the arc aimed at. Compared to carbon steel the probe index may be shorter or longer depending on the arc aimed at. The index point was in most cases further back the probe when measurement was done aiming at the 50 mm arc. Austenitic steel measurements were consistent in this way but the results with aluminium calibration block were not. In some cases with the aluminium block the index point was the same regardless of the arc. Use of three points in the EN 12668-3 measurements for probe index point was clearly not enough. Even for carbon steel the results were totally different from the calibration block 2 results. Index points of 20 mm or more were measured for the 70° probe. This is most certainly a false result because the contact surface is only 24 mm long.
In our opinion it is advisable to check angle probe calibration including the index point at least for materials different from carbon steel with the actual object inspected whenever possible. This is usually not easy when there is lack of proper reflectors to use for the check. If known reflectors at two or more different depths are known then probe angle and index point can be estimated using the approach defined in EN 12668-3. One must, however, bear in mind that the estimate error is highly dependable on the number of different depths used for the check. Only two different depths are needed, but then the accuracy shall not be very good. Use of back wall reflections in order to acquire more measurements for the check may also lead to a distorted result due to sound
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