5. CHAPTER CALIBRATION OF THE TESTING SYSTEM 1.PURPOSE OF CALIBRATION
5.2. STANDARD TEST BLOCKS
5.2.1. I.I.W. Calibration Block (V1 Block)
The most versatile calibration block is the block made from medium carbon ferritic and normalized steel described by the International Institute of Welding (I.I.W) and proposed by the International Standard Organization (I.S.O.). This block, called the I.I.W. V1 block, is as shown in Figure 5.1.
FIG. 5.1. Test block I.I.W. (V1) for calibration of an equipment with normal and angle probes.
This block is generally used for:
(i) The calibration of the time base using 25 mm, 100 mm and 200 mm thickness with the normal beam probes and 100 mm quadrant for angle beam probes;
(ii) The determination of probe index using 100 mm quadrant;
(iii) The determination of the probe angle using plastic wedge and degrees stamped on the side of the block. The angle beam transducer is subject to wear in normal use. This wear can change the probe index and the probe angle;
(iv) The checking of performance characteristics of the ultrasonic flaw detector such as:
Time base linearity;
Screen height linearity;
Amplitude control linearity;
Resolving power;
Penetrative power;
Horizontal linearity;
Dead zone check;
Pulse length.
(v) The setting of sensitivity;
(vi) The calibration of time base and sensitivity setting for DGS method;
(vii) The comparison of various materials due to their different acoustic velocities.
5.2.2. The Miniature Angle-Beam Block (V2 Block)
This is a more compact form of the V1 block suitable for site use, although somewhat less versatile in its function. It is also described by International Institute of Welding (I.I.W). The latest version of this block is shown in (Figure 5.2). It is particularly suitable for short near field lengths and the time base calibration of small diameter normal and angle probes.
This calibration block is made of steel with the dimensions in millimetres as shown in Figure 5.2. The tolerances are 0.1 mm except on the length of the engraved scale where they are 0.5 mm. This calibration block contains two quadrants having radii of 25 mm and 50 mm. Point
‘A’ is the common focal point for both quadrants. The uses of the block are as follows:
(i) Checking the characteristic of angle beam probe;
(ii) Calibration of time base of ultrasonic instruments;
(iii) Probe index point;
(iv) Beam angle;
(v) Metal distance calibration.
FIG. 5.2. Miniature Angle-Beam Block (V2).
5.2.3. Thickness Blocks
Thickness Blocks Stepped or tapered test blocks are used to calibrate ultrasonic equipment for thickness measurement. These blocks are carefully ground from material similar to that being inspected, and the exact thickness at various positions is marked on the block.
FIG. 5.3. Step wedge calibration block.
5.2.4. ASME reference block
The block is used to construct a distance-amplitude-correction (DC) curve on the CRT screen by noting the changes in echo amplitude from the hole with change in scanning distance (multiple skip). This block is made from the same material as that of the test specimen and contains side drilled holes. Thickness of the block and the diameter of the side drilled holes depend on the thickness of the test specimen. A typical ASME reference block called the basic calibration block (BCB) used for the inspection of welds is shown in Figure 5.4.
Weld Thickness, t, in.(mm)
Calibration Block Thickness, T,
in. (mm)
Hole Diameter, in. (mm)
Notch Dimensions.
in. (mm)
Up to 1(25) ¾ (19) or t 3/32 (2.5)
Notch depth = 2% T Notch width = ¼ (6) max.
Notch length = 1 (25) min.
Over 1 (25) through 2 (50) (18) or t 1/8 (3) Over 2 (50) through 4 (100) 3 (75), or t 3/16 (5)
Over 4 (100) t ± 1 (25) [Note (1)]
FIG. 5.4. ASME Basic Calibration Block (BCB).
5.2.5. ASME basic calibration block for piping
The basic calibration block configuration and reflectors shall be as shown in Figure 5.5 and in accordance with the following general notes. Thickness, T, shall be within 25% of the nominal thickness of the component to be examined. The block size and reflector locations shall be adequate to perform calibration for the beam angles used.
FIG. 5.5. Calibration block for pipe.
*Notches shall be located not closer than for 1 in. (25 mm), whichever is greater, to any block edge or to other notches.
GENERAL NOTES:
(a) The minimum calibration block length (L) shall be 8 in. (200 mm) or 8T, whichever is greater;
(b) For OD 4 in. (100 mm) or less, the minimum arc length shall be 270 deg. For OD greater than 4 in.
(100 mm), the minimum arc length shall be 8 in. (200 mm) or 3T, whichever is greater;
(c) Notch depths shall be 8% T minimum to 11% T maximum. When cladding is present, notch depths on the cladding side of the block shall be increased by the cladding thickness, CT (i.e., 8% T+ CT minimum to 11% T+ CT maximum). Notch widths shall be 1/4 in. (6 mm) maximum. Notch lengths shall be 1 in.
(25 mm) minimum;
(d) Maximum notch width is not critical. Notch may be made with EDM or with end mill upto ¼ in. (6mm) in diameter;
(e) Notch lengths shall be sufficient to provide for calibration with a minimum 3 to 1 signal to noise ratio.
5.2.5.1.Block Curvature
(i) Materials with Diameters 20 in. (500 mm) and Less;
For examinations in materials where the examination surface diameter is equal to or less than 20 in. (500 mm), a curved block shall be used. A single curved basic calibration block may be used for examinations in the range of curvature from 0.9 to 1.5 times the basic calibration block diameter. For example, an 8 in (200 mm) diameter block may be used to calibrate for examinations on surfaces in the range of curvature from 7.2 in. to 12 in. (180 mm to 300 mm) in diameter. The curvature range from 0.94 in. to 20 in. (24 mm to 500 mm) in diameter requires 6 curved blocks as shown in (Figure.5.6) for any thickness range.
FIG. 5.6. Ratio Limits for curved surface.
(ii) Materials with Diameters Greater Than 20 in. (500 mm);
For examinations in materials where the examination surface diameter is greater than 20 in.
(500 mm), a block of essentially the same curvature, or alternatively, a flat basic calibration block, may be used. An adjustment of receiver gain may be required when flat calibration blocks are used.
To determine the required increase in gain, the ratio of the material radius, R, to the critical radius of the transducer, Rc, must be evaluated as follows.
(a) When the ratio of R /Rc, the radius of curvature of the material R divided by the critical radius of the transducer Rc from Table 5-I and Figure.5.7 is equal to or greater than 1.0, no gain correction is required;
TABLE 5.1. TRANSDUCER FACTOR F1 FOR VARIOUS ULTRASONIC TRANSDUCER DIAMETER AND FREQUENCY.
Frequency MHz
Transducer Diameters, in.
0.25 0.5 0.75 1.0 1.125
F1, in.
1.0 2.58 10.3 23.2 41.3 52.3
2.25 5.81 23.2 52.2 92.9 118
5.0 12.9 51.2 116 207 262
10.0 25.8 103 232 413 523
FIG. 5.7. Critical radius RC for transducer/couplant combination.
Curve Couplant Transducer Wear face
A Motor oil or water Aluminum Oxide or Boron
Carbide
B Motor oil or water Quartz
Glycerine or syn. ester Aluminum Oxide or Boron Carbide
C Glycerine or syn. ester Quartz
D Motor oil or water Plastic
E Glycerine or syn. ester Plastic
(b) When the ratio of R/Rc is less than 1.0, the gain correction must be obtained from (Figure.5.8);
FIG. 5.8. Correction factor (gain) for various ultrasonic examination parameters.
This gain increase calibrates the examination on the curved surface after establishing calibration sensitivity on a flat calibration block.
Example:
Material with a 10 in. (250 mm) radius (R) will be examined with a 1 in. (25 mm) diameter 2.25 MHz boron carbide faced search unit using glycerine as a couplant.
(i) Determine the appropriate transducer factor, F1 from Table 5-I; F1 =92.9;
(ii) Determine the Rc from FIG.5.6 Rc= 100 in.(2 500 mm);
(iii) Calculate the R/Rc ratio; 10 in./100 in. = 0.1(250 mm/2 500 mm = 0.1).
Using (Figure.5.8) obtain the gain increase required; 12 dB 5.2.6. Area-amplitude blocks
Area-amplitude blocks provide artificial flaws of different sizes at the same depth. Eight blocks made from the same 50 mm (2 in.) dia round stock, each 95 mm (3-3/ 4 in.) high, constitute a set of area-amplitude blocks. The block material must have the same acoustic properties as the test piece material. Each block has a 20 mm (3/4 in.) deep flat-bottom hole drilled in the center of the bottom surface (Figure.5.9); the hole diameters vary from 0.4 to 3.2 mm (1/64 to 8/64 in.). The blocks are numbered to correspond with the diameter of the holes; that is, block No. 1 has a 0.4 mm (1/64 in.) diam hole, No. 2 has a 0.8 mm (2/64 in.) diam hole, and so on, up to No. 8, which has a 3.2 mm (8/64 in.) diam hole. Similar area-amplitude blocks made from 49 mm (1-15/ 16 in.) square stock are sometimes known as Alcoa Series-A blocks
12
FIG. 5.9. Area-amplitude block. (Dimensions given in inches).
The amplitude of the echo from a flat bottom hole in the far field of a normal beam probe is proportional to the area of the bottom of the hole. Therefore these blocks can be used to check linearity of a pulse echo inspection system and to relate echo amplitude to the area (or, in other words, the size of a flaw). Because a flat bottom hole is an ideal reflector and most natural flaws are less than ideal in reflective properties, an area-amplitude block defines a lower limit for the size of a flaw that yields a given height of indication on the CRT screen. For instance if the height of indication from a flaw in a test piece is six scale units and this is also the height of the indication from a 5/64" (1.98 mm) diameter flat bottom hole at the same depth as the flaw, it is not possible to determine accurately how much larger than the reference hole the flaw actually is. But the flaw is at least as large as the diameter of flat bottom hole.
5.2.7. Distance-amplitude blocks
Distance-amplitude blocks provide artificial flaws of a given size at various depths (metal distance). From ultrasonic wave theory it is known that the decrease in echo amplitude from a flat bottom hole using a circular probe is inversely proportional to the square of the distance to the hole bottom. Distance-amplitude blocks (also known as Alcoa series-B or Hitt blocks) can be used to check actual variations of amplitude with distance for normal beam inspection in a given material.
They also serve as reference for setting or standardizing the sensitivity of the inspection system so that readable indications will be displayed on the CRT screen for flaws of a given size and larger, but the screen will not be flooded with indications of smaller discontinuities that are of no interest. On instruments so equipped, these blocks are used for distance-amplitude correction so that a flaw of a given size will produce an indication on the CRT screen that is of a predetermined height regardless of distance from the entry surface.
There are 19 blocks in an Alcoa series-B set. All are 2" (50 mm) diameter blocks of the same material as that being inspected, and all have a 3/4" ( 19.6 mm) deep flat bottom hole drilled in the centre of the bottom surface (Figure.5.10). The hole diameter is the same in all the blocks of a set. Sets can be made with hole diameters of 3/64" (1.19 mm), 5/64" (1.98 mm) and 8/64"
(3.17 mm). The blocks vary in length to provide metal distances of 1/16" (1.59 mm), 1/8"
through 1" (25.4 mm) in 1/8" (3.17 mm) increments and 1-1/4" (31.7 mm) through 5-3/4" (146 mm) in 1/2" (12.7 mm) increments (see Table 5-I).
FIG. 5.10. Distance-amplitude block.
Each Alcoa series-B block is identified by a code number consisting of a digit, a dash and four more digits. The first digit is the diameter of the hole in one sixty fourths of an inch. The four other digits are the metal distance from the top surface to the hole bottom in one hundredth of an inch. For instance, as illustrated in the Table 5.2 a block marked 3-0075 has a 3/64" (1.19 mm) diameter hole and a 3/4" (19.6 mm) metal distance.
5.2.8. ASTM blocks
ASTM blocks can be combined into various sets of area-amplitude and distance-amplitude blocks. The ASTM basic set of distance-amplitude blocks consists of ten 2" (50 mm) diameter blocks, each with a 3/4" (19 mm) deep flat bottom hole drilled in the centre of the bottom surface. One block has a 3/64" (1.19 mm) diameter hole at a 3" (76.2 mm) metal distance. Seven blocks have 5/64" (1.98 mm) diameter holes at metal distances of 1/8" (3.17 mm), 1/4" (6.35 mm), 1/2" (12.7 mm), 3/4" (19 mm), 1-1/2" (38.1 mm), 3" (76.2 mm) and 6" (152.4 mm). The remaining blocks have 8/64" (3.17 mm) diameter holes at 3" (76.2 mm) and 6" (152.4 mm) metal distances. The ASTM basic set of area-amplitude blocks consists of three blocks with a 3" (76.2 mm) metal distance and holes with diameter of 3/64" (1.19 mm), 5/64" (1.98 mm), and 8/64" (3.17 mm). The remaining amplitude blocks with the 3/64" (1.19 mm) diameter holes form the set of distance-amplitude blocks.
In addition to the basic set, ASTM lists five more area-amplitude standard reference blocks and 80 more distance-amplitude blocks. Each ASTM block is identified by a code number, using the same system as that used for the Alcoa series-B set.
TABLE 5.2. DISTANCE-AMPLITUDE SET
Block
identification Metal distance Overall length
number Inches mm Inches mm
3-0006 0.062 (1/16) 1.57 0.875 22.2
3-0012 0.125 (1/8) 3.2 0.875 22.2
3-0025 0.250 (2/8) 6.4 1.000 25.4
3-0037 0.375 (3/8) 9.5 1.125 28.6
3-0050 0.500 (4/8) 12.7 1.250 31.8
3-0062 0.625 (5/8) 15.9 1.375 34.9
3-0075 0.750 (6/8) 19.1 1.500 36.1
3-0087 0.875 (7/8) 22.2 1.625 41.3
3-0100 1.000 25.4 1.750 44.5
3-0125 1.250 (1-1/4) 31.8 2.000 50.8
3-0175 1.750 (1-3/4) 44.5 2.500 63.5
3-0225 2.250 (2-1/4) 57.2 3.000 76.2
3-02753 2.750 (2-3/4) 69.9 3.500 88.9
3-0325 3.250 (3-1/4) 82.6 4.000 101.6
3-0375 3.750 (3-3/4) 95.3 4.500 114.3
3-0425 4.250 (4-1/4) 108.0 5.000 127.0
3-0475 4.750 (4-3/4) 120.7 5.500 139.7
3-0525 5.250 (5-1/4) 133.4 6.000 152.4
3-0575 5.750 (5-3/4) 146.1 6.500 165.1