CA2003672A1 - Bidirectional beamspread blocks - Google Patents

Abstract

ABSTRACT
Bidirectional Beamspread Blocks In non-destructive testing using ultrasonics as the test method, the dimensions of defects are established by measuring the move-ment of the ultrasonic transducer that is required to reduce the defect signal to a predetermined height. Various engineering standards and codes have established calibration blocks and tech-niques to compensate for sound beam dimensions in the vertical plane but until now have been without an acceptable means of determining sound beam dimensions in the horizontal plane. This invention is a pair of calibration blocks that provide a means of accurately determining both horizontal and vertical beamspread of ultrasonic tranducers.

This Abstract is submitted as part of the application for a patent on the Bidirectional Beamspread Blocks.

Description

2C~6~2 ~peciflcation for Bidirectlonal Beamspread ~lock~
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~hls lnventlon provldes a means of determlnlng the 30und beam shape and dimenslons in both vertical and horlzontal planes for ultrascnic transducers used ln nondestructlve testlng (NDT).
There exists in ultrasonic testing several styles of calibration blocks to determine characterstlcs of the sound beam generated by ultrasonic transducers. For example; the International Institute of Welding ~IIW) block i~ used to determine transducer exit point, test range and refracted angle. The Institute of Welding (IOW) block is used to determine vertical beamspread and depth resolution. Although the IOW block is suggested for determlning horizontal beamspread by the British Standards Institution ~BS
4331 Part 3), that same standard acknowledges the shortcomings of trying to use signals from the tips of holes. In the commonly used ASME ~American 80ciety of Mechanical Engineers) code Section V Article 4 Appendix B-70, provision is made for a technique to establish beam width ~horizontal beamspread) but until now no technique has proven acceptable. Attempts to evaluate beam dimen-slons in water by directlng the soundbeam at ball or wire targets have been developed but these techniques are carried out in water they are restricted to longitudinal wave mode. Also, since beamspread is a function of the wavelength of sound, the results obtained in water would be far dif ferent than the results in steel which has sound velocities typically two to four times that of water.

To overcome the shortcomings of the previous techniques and calibration blocks we desiqned the blocks shown in Fiure 1.
8ince the most uniform and reliable reflecting surface for beam evaluation is a small diameter hole at right angles to the beam, we designed a block that would allow the refracted sound beam to strike a hole at right an~es. Since most shear wave ultrasonic testing is done usinglfixed~angles of refract~ion; 45 , 60 , 70 and 35 degrees, we required four lncllned surfaces. To reduce the weight of this calibration piece we decided to make two blocks, one with the 45 and 60 degree scanning surfaces and the 35 and 70 degree scanning surfaces on the other. Other less common angles could be eva}uated provlded the inclined angles are machined for the specific angle in question.
The through hole provides a reflected slgnal on the ultrasonlc machlne's scope. This slgnal is maxlmised and the distance from the transducer to the hole ls read off the graticules on the 6cope which will have been calibrated for range ~distance).

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The horizontal position on the block is noted then the probe ig moved horizontally left untll the maximlsed signal ls reduced to 50~ or 10~ of its maximized height, depending on whether the 6 dB
or 20 dB beamspread is required. The soundpath distance as read off the scope and the horizontal displacement from the position whe.e the signal was maximized is notedO The signal is aqain max-imized at the same range and then the procedure is repeated wlth a horizontal movement to the right. The three readings, centre, left and right, are repeatd until 3 or more different ranges have been recorded. The results of these readings are recorded on a map illustrated in Figure 2.
Connecting the points for the centre of beam back to the origin will indicate if the beam ls skewed. Connecting the points for the left displacements back to the origin indicate the half angle of divergence for the left side of the beam as measured from the centreline. Similarly connecting the points of right displacement back to the origin indicate the half angle of divergence for the right side of the beam.
In addition to the through hole, 4 blind holes 25mm deep are ar-ranged as shown in Figure 1. These are used to determine vertical extent using the techniques in ASME Section V or ~S 4331 Part 3.
The through hole has been off-set from the middle of the block so the 4 blind holes do not generate interfering signals when evaluating horizontal beamspread. To more readily evaluate probe dlplacement in the vertical plane the blocks have been provided with metric graticules alonq the edges indicated in Figure 1.
The 5 holes at their different depths can also be used to deter-mlne ~eamspreads of Normal Beam (0 degree incidence) transducers.
The Bidirectional Beamspread Blocks can also be used to in-directly determine the dominant frequency the transducer is ring-ing at. By placing the transducer on the block so as to achieve a maximised signal from the through hole at the shortest possible range, the transducer is then moved along the block increasinq the sound path from the transducer to the hole maintaining a straight line with the central axis of the beam. Amplitude fluc- -tuatlons on the ultrasonic~machine's scope will be observed as the sound path increases. At some point the signal will reach a -maximum then drop off smoothly with increasing sound path. The --- ;distance at which the signal is maximum is the Near Zone. Since :
we would know the diameter of the transducer we are using, we could then substitute the Near Zone and diameter into the equa-tion N = D~f/4v where N = Near Zone D = transducer dlameter -v = velocity of sound 3.23mm/us for transverse waves ' -in steel f = frequency ' ' .

Claims (5)

1. Calibration blocks that permit accurate horizontal beamspread determination for ultrasonic transducers.

2. Calibration blocks defined in claim 1 that permit beam skew determination for ultrasonic transducers.

3. Calibration blocks defined in claim 1 that permit both verti-cal and horizontal beamspread determination for ultrasonic transducers.

4. Calibration blocks defined in claim 1 that permit determina-tion of the dominant frequency of an ultrasonic transducer.

5. Calibration blocks defined in claim 1 that permit determina-tion of the near zone of an ultrasonic transducer's near zone.