Phased Array Ultrasonics For Detection Of Cracks Engineering Essay

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A linear phased array transducer usually consists of 64 elements that can be independently driven. By incorporating suitable time delays, it is possible to steer the beam to any required angle and/or focus it to any particular depth. The potential advantages of the phased array ultrasonics include improved defect detection and imaging, easier interpretation of the defect signals displayed in an image format, faster inspections and identification of cracks and metal loss due to corrosion and discrimination between metal loss and crack-like defects. This paper reports the tests carried out using phased array linear scan on EDM notches of two types :rectangular and triangular and of different sizes. This work also explores the optimum number of active elements required for a particular type and size of the defect. All experiments were carried out using a contact linear phased array probe made of 64 elements on pipes with simulated defects. The thickness of the pipe used was 11mm and that of the elbow was 12mm respectively. Notches of three different sizes, 75%, 50% and 25% of the thicknesses were made on the test samples. Inspections were carried out for different settings of the phased array probe, each setting designed to optimize the detection and imaging of the defect. These experiments also show that phased array examinations can provide extended performances in terms of versatility compared to the conventional probes.

Keywords: phased array ultrasonic, beam steering, pipe inspection, defect imaging

1. INTRODUCTION:

Detection of cracks in pressure vessels and pipes typically require testing to guarantee structural integrity. Ultrasonic testing has become a reliable yet easy method to detect cracks in piping components. The latest innovation in the field of ultrasonics is phased array system and its evolution. Phased arrays differ from conventional ultrasonic testing in that beams can be focused, steered and scanned. With such flexibility being offered in detection, it becomes inevitable that optimum parameters be investigated and determined for specific testing conditions and defects. This paper gives emphasis on selection of optimum parameters for detection and sizing of surface breaking cracks.

1.1 INTRODUCTION TO PHASED ARRAY:

The Phased Array concept is based on the use of transducers made up of individual elements that can each be independently driven. To generate a beam, the various probe elements are pulsed at slightly different times. By precisely controlling the delays between the probe elements, beams of various angles, focal distance, and focal spot size can be produced. The illustration shows how a beam can be focused at an angle and a given distance by firing the left elements slightly ahead of the corresponding elements on the right. It is possible to change the angle, focal distance, or focal spot size, simply by changing the timing to the various elements. Beam focusing and simultaneous beam steering-focusing is shown in figures 1(a) and (b) respectively.

Figure 1. (a) Beam focusing in a phased (b) steered-focused beam in a phased

array system array system

1.2 BASIC COMPONENTS OF A PHASED ARRAY SYSTEM

The main components required for a basic scanning system with phased array instruments are presented in Figure 2(a) below.

Figure 2. (a) Basic components of a phased (b) Experimental setup of Phased Array

array system Omniscan MX with a 5 MHz probe

1.3 AMPLITUDE DROP METHOD:

The amplitude drop technique is an easy and fast technique for sizing large and planar defects.  It is based on the principle that when half of the ultrasonic energy is not reflected by a defect, the echo is 6 dB less than in the case when the entire beam is reflected.  It is then assumed that when half of the beam is returned, the transducer centerline is directly over the edge of the defect. This method also suffers from some disadvantages. Since, the sizing of the defect is based on the amount of reflected ultrasonic energy, planar defects with straight edges only can be accurately sized and hence in most practical cases it only gives an approximation of the defect size. For sizing flat bottom and side drilled holes, usually 2 dB, 3 dB and 6 dB amplitude drop methods are used. Figure 2 (c) shows 1 dB, 2 dB, 3 dB and 6 dB amplitude drop methods to size a 2mm deep side drilled hole. The basic procedure involved in this method is first locating the flaw and maximizing the echo and noting the signal amplitude. Now the probe is translated along the scan axis till a specified amplitude (example 6 dB) drop is observed. The length of the drop along the scan axis is an indication of the size of the defect.

Figure 2(c): various amplitude drop methods to size a 2 mm deep side drilled hole [1]

2. EXPERIMENTS:

The experimental setup used in this study consists of the R/D Tech Omniscan MX ultrasonic Phased Array system. The data presented in this paper were acquired using 5MHz, 64 elements (46 mm X 15 mm area) probe for 36 dB gain setting. The inspection was carried out for various settings of the probe parameters such as varying the number of active elements and varying the angle of incidence. The simulated defects include three EDM of different sizes and depths on an aluminium pipe sample and three EDM notches on a SS elbow sample.

Figure 3(a): 11 mm-thick Aluminium Pipe 3(b): 12 mm-thick elbow sample

sample with three simulated defects with three simulated defects

11 mm pipe sample :

A 13 mm aluminium flat specimen was taken. The sides of the specimen were brought to a correct rectangular shape using a shaping machine. It was then given in an end mill cutter and faced to 11.0mm. The specimen was then taken to a center-less rolling machine and was rolled to the requisite diameter. The specimen was then taken to an EDM and slots of various depths were cut in it, all of 10 mm length and 0.6 mm width. The specimen was then cleaned with kerosene to remove all adhering dirt and was tested using the phased array testing machine. In Al sample all the three notches are rectangular longitudinal EDM notches of depths 3mm, 6mm, and 9 mm. The sample is shown in figure 3(a). The details of EDM notches with respect to vertical of 11 thick Aluminium pipe sample is given in Table 1.

12 mm SS elbow sample:

A 12 mm thick SS elbow specimen of 818 mm diameter was taken. Since it is not possible to do gas cutting on a SS specimen, it was taken to plasma cutting machine and a piece of a suitable dimension was cut out of it. The ends were then given in a milling machine and it was brought to a straight dimensions. The specimen was not given any surface treatment, as it was planned to simulate realistic surface conditions. The specimen was then taken to an EDM and longitudinal slots of various depths were cut in it, all of 10 mm length and 0.6 mm width. The specimen was then cleaned with kerosene to remove all adhering dirt and was tested using the phased array testing machine. In SS sample the 3 mm deep notch is rectangular and the 6 mm and 9 mm notches are triangular EDM notches. The sample is shown in figure 3(b). The details of EDM notches with respect to vertical of the 12 thick SS elbow sample are given in Table 2.

10 mm MS Pipe sample:

A 11 mm thick MS pipe specimen of 165 mm diameter was taken. The outside surface was then turned to gat a uniform outer diameter of 10 mm. three circumferential EDM notches of 10 mm length and 0.4 mm width and of varying depths were made on the specimen. The sample is shown in figure 4(a). The details of EDM notches with respect to vertical of 10 thick MS pipe sample are given in Table 3.

3. INSPECTION OF CURVED SECTIONS

Inspections of curved sections are quite difficult as the number of mode converted signals reaching the receiver-transducer increases with increase in the curvature of the specimen. Moreover, in the case of surface breaking cracks the defect echo tends to merge with the front-wall and the back-wall echo. The problem of inspection of thin sections can be tackled by using higher frequency (>5 MHz) transducers, which increases the resolution of the B-scan image. This may be acceptable since the accompanying higher attenuation is less of a problem in thin samples. Using the phased array, it is possible to increase the angular resolution and vary the gain dynamically to clearly separate the defect echo from the back-wall.

Table 1. The details of longitudinal EDM notches with respect to vertical of 11 thick Aluminium pipe sample

Defect Number

Type

Length(mm)

Width(mm)

Actual Depth (mm)

D1

Surface breaking

[rectangular]

10

0.6

9

D2

Surface breaking

[rectangular]

10

0.6

6

D3

Surface breaking

[rectangular]

10

0.6

3

Table 2. The details of longitudinal EDM notches with respect to vertical of 12 thick SS Elbow sample

Defect Number

Type

Length(mm)

Width(mm)

Actual Depth (mm)

D4

Surface breaking

[triangular]

10

0.6

9

D5

Surface breaking

[triangular]

6.67

0.6

6

D6

Surface breaking

[rectangular]

10

0.6

3

Table 3. The details of circumferential EDM notches with respect to vertical of 10 mm thick MS pipe sample

Defect Number

Type

Length(mm)

Width(mm)

Actual Depth (mm)

D7

Surface breaking

[rectangular]

10

0.4

7

D8

Surface breaking

[rectangular]

10

0.4

5

D9

Surface breaking

[rectangular]

10

0.4

3

4. RESULTS AND DISCUSSIONS:

Figure 4(b) below shows the typical B-scan image of the crack obtained using the phased array probe functioning in the linear scan mode.

Figure 4(a): 10 mm-thick MS Pipe sample Figure 4(b): B-scan of a 6 mm deep EDM notch

with three circumferential notches obtained using a 64 element phased array probe

The defects were imaged using 45° shear wave and were sized using the 3 dB (≈ 70.7% drop) and 6 dB (≈ 50.11% drop) amplitude drop methods. The settings were made such that the defects were completely accommodated within the gating window. The percentage errors associated with both the cases were also determined. The defect images of the 11 mm aluminium pipe are shown in figure 4 and the defect images of the 12 mm SS elbow are shown in figure 5. The actual depths of the defects and the depths obtained using phased array for the 11 mm thick aluminium pipe sample and the 12 mm thick elbow sample are given in tables 5 and 6 respectively. It was observed that the 3 dB amplitude drop method gave more accurate results in sizing the defects when compared to the 6 dB drop amplitude drop method.

(a): 9 mm deep notch (b): 6 mm deep notch (c): 3 mm deep notch

Figure 5 : B-scan images of three defects in 11mm Aluminium sample using 45° shear wave

Table 4: Comparison of the notch depths obtained by amplitude drop method with the actual notch depth of 11 mm thick Aluminium pipe sample using 45° shear wave

Defect Number

Actual

Depth

(mm)

Defect depth by

3dB amplitude

drop method(mm)

Percentage

error

Defect depth by

6dB amplitude

drop method(mm)

Percentage

error

D1

9

8.9

1.11

9.7

-7.78

D2

6

5.7

5.00

6.7

-11.67

D3

3

2.9

3.33

3.2

-6.67

(a): 9 mm deep notch (b): 6 mm deep notch (c): 3 mm deep notch

Figure 6: B-scan images of three defects in 12mm thick SS sample using 45° shear wave

Table 5: Comparison of the notch depths obtained by amplitude drop method with the actual notch depth of 12 mm thick SS pipe sample using 45° shear wave

Defect Number

Actual

Depth

(mm)

Defect depth by

3dB amplitude

drop method(mm)

Percentage

error

Defect depth by

6dB amplitude

drop method(mm)

Percentage

error

D4

9

8.8

2.22

9.4

-4.44

D5

6

5.7

5.00

6.6

-10.00

D6

3

2.8

6.67

4.2

-40.00

In order to determine the optimum angle for sizing defects in an angle beam inspection, the defects in the MS pipe specimen were imaged by phasing the elements of the probe to generate a shear wave for five different angles: 30, 35, 40, 45 and 50 degrees in the linear scan mode and were sized using the 3 dB and 6 dB amplitude drop methods. The variation of the defect size obtained with the angle generated within the specimen using 3 and 6 dB drop methods is given in figure 7 below. It was seen that the 6 dB drop method is more accurate in sizing the circumferential notches as compared to the 3 dB drop method in case of the MS pipe specimen. It was also observed that, operating between the angles 40° - 45° would give relatively more accurate defect sizes. It was also observed in case of all the three specimens that as the size of the defect increases, the accuracy with which it can be sized also increases.

Figure 7: variation of the defect size obtained with the angle generated within the specimen using 3 and 6 dB drop methods

In order to find the effect of the number of active elements necessary to size a defect accurately, three notches of the MS pipe specimen were sized using 6 dB drop method for 45° shear wave in linear scan mode for five combinations of number of active elements. The variation of the defect size for various numbers of active elements is given in Table 6. The graph showing the variation of the defect size obtained with respect to the increase in the number of active elements of the probe. It is observed that the accuracy in sizing defects increases with the increase in the number of active elements.

Table 6: Variation of the defect size with respect to the increase in the number of active elements of the phased array probe using 6 dB drop method for 45° shear wave in linear scan mode for circumferential notches in the 10 mm thick MS pipe specimen

No. of

Elements

Defect size

(mm)

Defect depth by

3dB amplitude

drop method(mm)

Defect size

(mm)

Defect depth by

3dB amplitude

drop method(mm)

Defect size

(mm)

Defect depth by

3dB amplitude

drop method(mm)

32

3

4.8

5

4.6

7

6.1

40

3

4.7

5

4.6

7

6.2

48

3

4.5

5

4.7

7

6.2

56

3

4.2

5

4.8

7

7.3

64

3

4.0

5

5.2

7

7.4

Figure 7: variation of the defect size obtained for various numbers of active elements of the phased array probe using 6 dB drop method

6. CONCLUSIONS:

The study describes the experimental results of the detection of cracks in pipes and elbows using phased array ultrasonic technique for two pipe and one elbow specimens. The following are the conclusions:

The 3 dB drop method was found to be more accurate than 6 dB drop in sizing the defect using 45 deg longitudinal waves for the SS elbow sample and for the Aluminium pipe sample with longitudinal notches.

The 6 dB drop method was found to be more accurate than 3 dB drop method in sizing the defect using 45 deg longitudinal waves for the MS pipe sample with circumferential notches.

As the size of the defect increases, the accuracy of sizing of the defect increases.

For angle beam inspections, it was observed that operating between 40° - 45° is ideal for sizing defects accurately.

The accuracy of the defect sizing increases with the increase in number of the active elements of the phased array probe.

As the size of the notch decreases, the amplitude drop method tends to oversize the defect. Consequently, as the size of the notch increases, the amplitude drop method tends to undersize the defect.

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