Calibration of Phased-Array Automated Ultrasonic Testing System

Calibration of Phased-Array Automated Ultrasonic Testing System

Abstract: This study describes the composition, testing method and calibration of the phased-array automated ultrasonic testing (AUT) system.

Keywords: automated, ultrasonic testing, zones, phased array TOFD, calibration

1.Phased-array automated ultrasonic testing (AUT) system

The phased-array automated ultrasonic testing (AUT) system is applied to the girth weld inspection of automated welding of long-distance pipelines. Compared with conventional manual ultrasound testing and X-ray testing, the testing speed, defect testing ability, defect quantitative accuracy, reduction of environmental pollution, and decrease of workload of phased-array AUT has obvious advantages and has wide applications. See Fig. 1 for its composition.?

2.Standard test block

The standard test block is for calibrating the testing system. It uses standard holes or grooves to simulate defects at the locations where defects are prone to occur in automated welding. Generally, φ 2 flat-bottom holes perpendicular to the fusion line are used on the fusion surface of the weld, as shown in Fig. 2, for A-scan calibration. In the middle of the weld, a reflector with the bottom surface of the φ 2 flat-bottomed hole at 45 degrees to the horizontal surface is set at the centerline of the weld (see Fig. 3), for B-scan calibration. In addition, a square groove or a triangular groove that matches the root fusion line is used as the root, occasionally a square groove (10-20mm long, 2mm wide, 1mm deep) is also set on the surface fusion line. These artificial defect reflectors can be used for calibrating parameters of the instrument, probes and crawlers, which ensures that the system works effectively. The drawings of common standard test blocks are shown in Fig. 4.??

3.Zones and Testing Methods

3.1 Zones

The Zones method is to divide the weld into several vertical zones according to the wall thickness, groove and fill count. The height of each zone is generally 1-3mm, and each zone is scanned by a set of individual excitation apertures. The angle of the main beam for testing is set according to the direction of the main defect. In automated welding, the defect is mainly incomplete fusion. The beams shall be kept perpendicular to the fusion line as possible. A-scan uses focused beams for scanning, and the focus size is generally 2mm or smaller. They can effectively inspect respective zones, and the overlap on the reflectors in the adjacent zones is minimal. Each zone is bounded by the center line of the weld and is divided into two symmetrical channels: UpStream and DownStream. Fig. 5 is a schematic diagram of the zones. Referring from the root, they are: Root, LCP, HotPass 1, HotPass 2 (which may expand to more zones subject to the thickness of HotPass), Fill 1, Fill 2 (which may expand to more zones subject to the thickness of Fill) and Cap.

3.2 Testing method

The system adopts the testing method mainly based on phased array and assisted by TOFD.

Linear array that accommodates 60 crystals is used by phased array probes. The crystals are distributed in lines the probe at intervals. Two array probes are placed on both sides of the weld center to detect from two directions, which can produce thousands of ultrasonic beam patterns. The scanning angle ranging from 35 to 75 degrees can meet various angles required for scanning from the Root to the Cap. See Fig. 6. For the specific description of the testing method, see No. 1~8 in Table 1.

In addition, B-scan of PA is used for testing inside the weld, which is called volumetric channel. For details, see No. 9~11 in Table 1.

As an independent ultrasonic testing method, TOFD is mainly for auxiliary characterization, measurement of defect height and porosity testing. For details, see No. 12 in Table 1.

The coupling channel is for monitoring the coupling of phased array probes. For details, see No. 13 in Table 1.

To sum up, the phased-array automated ultrasonic testing adopts the zone method, which divides the weld into several zones in the vertical direction, and the phased array detector controls the probe to carry out zone scanning, supplemented by B-scan and TOFD. The test results are displayed in the form of a dual-door strip chart, as shown in Fig. 7. The quality of the weld can be analyzed and judged by automatic or manual assessment.?

3.3 Testing Requirements

1)The signal peak value of PA zone is adjusted to 80%, and the display amplitude of the reflectors in the adjacent zones on both sides should be 6~24dB lower than that of the main reflector.

2)The lateral wave signal of the TOFD zone is adjusted to 40%-90%.

3)The gate of the fusion is determined by the reflector of the fusion. The gate start should be at least 3mm in front of the fusion line, and the gate end should exceed 1mm of the weld joint.

4)When the wall thickness ≥12mm, adjust the sensitivity of the centerline reflector of the weld joint in the Fill (Cap and HotPass) for the Volumetric, the gate start should be at least 3mm before the fusion line on the probe side, and the gate end should cover the groove fusion line on the opposite side, with the scanning sensitivity +8dB~14dB based on the reference sensitivity.

When the wall thickness <12mm, use the fusion reflector to adjust the sensitivity for the volumetric channel. The start ?of the gate should be at least 3mm before the fusion line on the probe side, and the gate end should cover the groove fusion line of the opposite side, with the scanning sensitivity +8dB~14dB based on the reference sensitivity.

5)TOFD: the general single-channel setting shall meet the requirements.

6)Time gate thresh: 20%, assessed gate thresh: 40%.

7)The scan shall record the entire length of the joint with a certain overlapping area.

8)Scanning speed Vc ≤Wc*PRF/3, where Vc is the scanning speed (mm/s); Wc is measured by the half-wave height method, with the narrowest beam width at the valid distance of the probe (mm); PRF is the pulse repetition frequency for exciting the probe (Hz).

9)Coupling channel: when the surface is dry without couplant, it prompts poor coupling; when the testing has good coupling, it prompts good coupling.

10)The error of the recorded encoder position of the reflector relative to the actual circumferential position is ±10mm.

4.Calibration method

4.1 Assembly of crawler and probe set

Assemble 4 sets of probes (2PA+2TOFD) to the truss, and install the truss and the main body of the crawler to the standard test block, ensuring the probe spacing is the same as the spacing setting in the laptop. See Fig. 8.

Check that each set of wheels is fitted on the guide rail. Try to test the crawler to confirm that there is no abnormal situation such as abnormal noise, metal chips or slippage during the movement of the crawler.

4.2 Input of test block parameters and focusing simulation

1)See Fig. 4 for the drawing of the standard test block for the calibration.

2)According to the number of zones in the test block, input in the laptop software. See Fig. 9; the laptop software will automatically input the position parameters of each group of reflectors in the zones according to SYT 4112-2017. The user can adjust the items according to the relevant specification parameters of the drawing in Fig. 4. See Fig. 10.

3)According to the simulated workpiece automatically generated by the Suporup laptop software, set the beams and SoundBeam focusing law.

4)During the beam setting process, the software will automatically match the T/R mode to "P/E (pitch and emit)" or "P/C (pitch and catch)" according to the position of each reflector in the test block.

5)As shown in Table 1, the software in HotPass uses the work mode "P/E" by default, because the secondary wave is sufficient to find the incomplete-fusion defect in the same side of HotPass. See Fig. 11 for the beam setting.?

6)If the fusion area adopts the work method "P/C", pay attention that there should be an intersection between the two sets of beams. The intersection is the position of the reflector, as shown in Fig. 12.?

7)During the beam setting process, when the beams in each zone pass through the reflector hole, adjust the focus position, and move the focus of the beam direction to the reflector hole to coincide with it. See Fig. 13.?

8)In addition to the fusion zone and HotPass using A-scan strip chart to display defects, the system has included Volumetric (B-scan) and TOFD scan channel to assist in the testing of porosity and volumetric defects. The setting of volumetric beams is the same as 5) of 4.2, and the TOFD channel can be calibrated through conventional contact. Please refer to relevant sections for details.?

4.3 Amplitude Calibration of each channel

1)Adjust the crawler so that the probe is facing the marker line of the reflector.

2)Select the channel in the upper right of the software interface, move the crawler back and forth, and observe the position of the peak. Take the channel in Fill 4 as an example. See Fig. 13.

3)Adjust "StartEle" on the right menu to make sure that the amplitude is the highest in the current state. If there is higher amplitude, keep the changed value of "StartEle".

4)Adjust "Aperture" on the right menu to make sure that the amplitude is the highest in the current state. If there is higher amplitude, keep the changed value of "StartEle".

5)Move the crawler back and forth to make sure that the amplitude is the highest in the current state.

6)Click “Auto Scan” on the remote control, input the distance of the next reflector, at the speed "20mm/s", click Start, and the crawler will automatically move to the position facing the next reflector.

7)Switch to the next channel in the software, repeat 3) to 5), and make sure that the echo amplitude of each channel is the highest.

4.4 UpDownCov calibration of each channel

1)When the MainAmp of each channel reaches the highest, start to adjust the UpDownCov. Take Fill 4 as an example, set the channel to "Fill 4”, move the probe to the peak position, keep this channel, and move up and down to "Fill 3" and "Fill 5" of the adjacent reflector. See Fig. 14.?

?2)If the UpDownCov of the adjacent areas exceed the testing requirement of "-6dB~ -24dB lower than the reflector of the current channel", return the probe to the position of "Fill 4" facing the reflector, and adjust "RefAngle” by +3°. Repeat step 1) and observe whether it is satisfied. If satisfied, increase AutoGain of MainAmp to 80%; If not, adjust "RefAngle” by -3°, and repeat step 1).

3)If the steps above still cannot meet the UpDownCov standard, adjust the angle error by ±5° and repeat step 1).

4)When the MainAmp reaches the peak at the reflector position, and complies with the UpDownCov of the adjacent area, go to auto calibration.

4.5 Automatic verification and calibration setting

1)Enter the Scan menu, click to start scanning, move the crawler, and observe the strip chart of each channel.

2)After generating the strip chart, select the reflector echo of each group, and measure the MainAmp sensitivity and UpDownCov to see if they comply with the requirements.

3)The software will automatically identify the channels not conforming to the standard. See Fig. 15.

4)Click "Calib", and the software will automatically reject the excessive amplitude and compensate the low amplitude.

5)Repeat steps 1) and 2) till MainAmp and UpDownCov values of each channel are conforming and tend to be stable, i.e. the calibration is finished. You may start the normal testing job now.?