NDT.net - September 2000, Vol. 5 No. 09

## Nondestructive Material Testing with Ultrasonics - Introduction to the Basic Principles -

 TABLE OF CONTENTS Introduction 1. Why use ultrasonics for nondestructive material testing? 2. Ultrasonic testing tasks 3. Detection of discontinuities 4. Method of testing and instrument technology 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 7. Documentation 8. Diagnosis of indications (outlook)     Reference list

### 5. Locating discontinuities

5.1 Calibration of the instrument
The location of a discontinuity can be instantly determined using its echo if the instrument is correctly calibrated. Calibration means, linear display, from the zero point on the scale, of a certain distance range of the object to be tested. The zero point on the scale corresponds to the surface of the test object and the 10th scale graduation the maximum distance, e.g. 100 mm steel, 10 mm aluminum, 25 mm brass etc. When specifying the calibration range the naming of the material is also important because the displayed distance of the echo, sound path s, is always deduced from the time of flight t of the pulse and the sound velocity c according to the equation:

 s = sound path [mm] c = sound velocity [km/s] t = transit time [ms]
 Fig. 39 USK 7: Backwall echo sequence with a straight-beam probe

This relationship is not unimportant for the ultrasonic operator but it is not required for the sequence of calibration. The rule simply says: Use a work piece of the same material as the test object whose dimensions are known. By coupling the probe onto an object of known thickness t an echo sequence appears on the display, Fig. 39. The associated sound paths correspond logically to the paths being travelled in the test object, for example with a straight-beam probe it is the multiple of the test object's thickness t, therefore:

1st Echo = t,
2nd Echo = 2t,
3rd Echo = 3t, etc.

We must now adjust 2 of these echoes on the corresponding scale graduation to the required calibration range. The instrument is then calibrated, i.e. by reading off the scale position T the sound path s (distance) of the associated reflector can be determined (location of reflectors, wall thickness measurement).

5.1.1 Calibration with a straight-beam probe
The reference piece used for calibration is called the Calibration Block, or Standard Calibration Block, if the block used is standardized. The Standard Calibration Block 1, also simply referred to as V1 block (according to BS 2704 - A2), has a thickness of exactly 25 mm and is made of low-alloyed fine grained steel so that it can be used for nearly all types of calibration when similar steels are to be tested.

Example 1: Calibration range 100 mm steel (longitudinal waves)

 Fig. 40 Calibration range: 0-10mm
The 10 scale graduations on the horizontal display scale are to have a range of 0 to 100†mm steel, Fig. 40. One scale graduation therefore corresponds to 10 mm in the test object. We say: the scale factor k (display scale) is 10 mm per scale graduation. We couple the straight-beam probe to the V1 block (laying flatwise), Fig. 39. The backwall echo sequence now comes from the 25 mm steel path. The allocation of sound paths si to the corresponding scale positions Ti is carried out using the calibration table:

 Echo-No i Sound path si[mm] Scale factor k [mm/scale grad.] Skalen-position Ti [scale grad.] 1 25 10 2.5 2 50 10 5.0 3 75 10 7.5 4 100 10 10.0

The corresponding scale position Ti is calculated by using the formula:

 si = sound path of umpteenth echoes Ti = scale position of the umpteenth echo k= scale factor

 Fig. 41 USK 7: Calibration in the 100 mm range Fig. 42 USK 7 D: Consideration of the probe delay

The exact adjustment of echoes from the calibration block, as in Fig. 41, is made with analog ultrasonic flaw detectors using the controls pulse shift (or delay) as well as coarse and fine ranges. In doing this, the adjustments must be alternately carried out at these points until the echo flanks are at the correct scale positions. With modern digital instruments the calibration range of 100 mm and the sound velocity of 5920 m/s are firstly entered. After coupling the probe to the calibration block, the function delay or probe delay is changed until the echoes are correctly positioned, Fig. 42.

Example 2: Calibration range of 250 mm in aluminum

10 scale graduations correspond to 250 mm in aluminum: k = 25 mm/graduation. We couple the straight-beam probe to an aluminum test block which is 80 mm thick, i.e. a backwall echo sequence is produced from this thickness (t = 80 mm), Fig. 43.

The calibration table now looks like this:

 Echo-No i Sound path si[mm] Scale factor k [mm/scale grad.] Skalen-position Ti [scale grad.] 1 80 25 3.2 2 160 25 6.4 3 240 25 9.6

Exact reflector location is only possible after correct calibration of a test instrument. The ultrasonic operator moves the probe over the test object. In a normal case, i.e. when a discontinuity does not exist, only the initial pulse and the backwall echo are visible on the display. As soon as a discontinuity is within the area of the sound beam, an additional echo appears between the initial pulse and the backwall echo, Fig. 44, e.g. an echo at scale graduation 1.4. With calibration in the 250 mm range the distance to the reflector s is therefore 1.4 x 25 = 35 mm.

 Fig. 43 USK 7 D: Calibration of a 250 mm range with an 80 mm aluminum path Fig. 44 USK 7 D: Sound path measurement.

5.1.2 Calibration with a TR probe
For technical reasons, the calibration with a TR probe can only be made to a certain extent using a backwall echo sequence from a comparison object. Due to the slight angular beaming, Fig. 35, transverse waves occur with the TR probe which cause strong interference behind the 1st backwall echo so that the 2nd backwall echo is often unable to be identified. Therefore, a stepped calibration block is used for the adjustment of both echoes, alternately going between two steps (2 point calibration).

Example 3: Calibration range for 10 mm steel

Step block VW (steel: 1 - 8 mm). The 3 mm and 6 mm steps should be used for calibration. The step selection depends on the depth range of the expected reflectors. Here the echo from 3 mm must be adjusted to the 3rd scale graduation and the echo from 6 mm to scale graduation 6, Fig. 45a+b.

 a) Firstly, we couple the TR probe to the 3†mm step and use the delay control for adjusting the echo flank to the 3rd scale graduation. b) Now we couple the probe to the 6 mm step and bring the echo to the 6th scale graduation with the range control. c) Steps a) and b) are alternately repeated until both echo flanks are exactly on the 3rd and 6th scale graduations, Fig. 45a+b.
The main application of TR probes are wall thickness measurements, but they are also suitable for the detection of near-to-surface discontinuities, Fig. 46a+b.

 Fig. 45b Calibration echo at the 3rd graduation (top) Calibration echo at the 6th graduation (bottom) Fig. 45a The two positions (3 mm and 6 mm step) of the TR probe on the stepped calibration block VW Fig. 46a Probe DA 312 on a speciemen with a side drilled hole in a depth of 1 mm. Fig. 46 b Detection of the drill hole from Fig. 46 a

5.1.3 Calibration with an angle-beam probe
For calibration of the test instrument with an angle-beam probe the standard calibration block 1, Fig. 47a, and the calibration block V2 (according to BS 2704 - A4), Fig. 47b, are almost exclusively used because no backwall echo sequence is received due to the angular beaming from a plane-parallel calibration block.

 Fig. 47a WB 60-2E on Calibration Block 1 Fig. 47b MWB 45-4E on Calibration Block 2 Fig. 48 Different probe angels at V1 block Fig. 49a Sound path in the V1 block without angle reflection Fig. 49b Sound path in the V1 block with angle reflection.

The advantage with echoes from the circle segment of the calibration block is that the same sound path is always given independent of the probe angle, Fig. 48. When the angle-beam probe is exactly coupled in the center of the circle segment, a first echo is exactly received from 100 mm out of V1 block. According to the reflection law, the sound waves coming out of the arc are reflected away from the coupling surface to the back, this means away from the arc, Fig. 49a. A second echo out of the arc, needed for the calibration sequence, cannot therefore be produced. For this, there are two saw cuts made in the center of the quarter circle: in the edges, which these saw cuts form with the surfaces, the sound waves are reflected back within themselves due to double reflection (angle reflection effect) so that they go back to the arc, Fig. 49b.

Because the radius of the circle segment is exactly 100 mm we will regularly receive an echo sequence with distances of 100 mm, 200 mm, 300 mm etc. with which we are able to carry out calibration of the test instrument the same way as the straight-beam probe. Fig. 50 shows calibration of the 250 mm range.

 Fig. 50 Range: 250 mm with a WB 60-2 on V1 block Fig. 51a Path of a sound wave in a V2 block, radius 50 mm

For the miniature angle-beam probe one uses the considerably smaller and lighter Standard Calibration Block 2 (V2 block). This has, as opposed to the V1 block, two circle segments with a common center point, however it does not have saw cuts. The required echo sequence is produced here by the alternating reflection of the sound waves, Fig. 51a+b.

 Fig. 51b Path of a sound wave in a V2 block, radius 25 mm Fig. 52 Range: 100 mm calibrated on V2, radius 25 mm.

The corresponding echo sequence is produced according to whether the probe beams into the 25 mm radius or the 50 mm radius. No echoes appear with sound paths by which the sound pulses from the "wrong" direction meet at the center point because these pulses are absorbed by the front damping element of the probe. Fig. 52 shows calibration of the 100 mm range by scanning into the 25 mm radius of Standard Calibration Block V2.

5.1.4 Locating reflectors with an angle-beam probe
 Fig. 53 Scanning a reflector using an angle beam probe
The echo of a discontinuity on the instrument display does not now give us any direct information about its position in the material. The only available information for determination of the reflector position is the scale position and therefore the sound path s, this means the distance of the discontinuity from the index point (sound exit point) of the probe, Fig. 53.

The mathematics of the right-angled triangle helps us to evaluate the Surface Distance and the Depth of a reflector which are both important for the ultrasonic test, Fig. 54a. We therefore now have the possibility to instantly mark a detected flaw's position on the surface of the test object by measurement of the surface distance from the sound exit point and to give the depth. For practical reasons, the reduced surface distance is used because this is measured from the front edge of the probe. The difference between the surface distance and the reduced surface distance corresponds to the x-value of the probe, this is the distance of the sound exit point to the front edge of the probe, Fig. 54b.

 Fig. 54a The flaw triangle Fig. 54b Reduced surface distances and x-value

 Fig. 55 USN 50: A hole being scanned with the probe MWB 60-4E
With ultrasonic instruments having digital echo evaluation these calculations are naturally carried out by an integrated microprocessor and immediately displayed so that the operator does not need to make any more time-consuming calculations, Fig. 55. This is of great help with weld testing because with the calculation of the flaw depth an additional factor must be taken into account, namely: whether the sound pulses were reflected from the opposing wall. If this is the case then an apparent depth of the reflector is produced by using the depth formula which is greater than the thickness T of the test object. The ultrasonic operator must acertain whether a reflection comes from the opposite wall and then proceed with calculating the reflector depth, Fig. 56b.

 Fig. 56a The apparent depth Fig. 56b The real reflector depth after sound reflection

5.1.4 Locating reflectors with an angle-beam probe
The echo of a discontinuity on the instrument display does not now give us any direct information about its position in the material. The only available information for determination of the reflector position is the scale position and therefore the sound path s, this means the distance of the discontinuity from the index point (sound exit point) of the probe, Fig. 53. The mathematics of the right-angled triangle helps us to evaluate the Surface Distance and the Depth of a reflector which are both important for the ultrasonic test, Fig. 54a. We therefore now have the possibility to instantly mark a detected flaw's position on the surface of the test object by measurement of the surface distance from the sound exit point and to give the depth. For depth of the reflector is produced by using the depth formula which is greater than the thickness T of the test object. The ultrasonic operator must acertain whether a reflection comes from the opposite wall and then proceed with calculating the reflector depth, Fig. 56b.