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• Lab & Field Testing of AUT Systems for Steel Highway Bridges

Lab & Field Testing of AUT Systems for Steel Highway Bridges

4. EQUIPMENT QUALIFICATION AND CALIBRATION

According to section 6.15 of the AASHTO/AWS D1.5M/D1.5: 2002 Bridge Welding Code,⁽¹⁾ every ultrasonic unit shall be calibrated prior to inspection. Therefore, the P-scan system was calibrated by a qualified P-scan operator using approved standard reference blocks. Because the calibration procedures for the P-scan system and the traditional UT systems are alike, the calibration procedures for ultrasonic inspection outlined in section 6.21 of the AASHTO/AWS D1.5M/D1.5: 2002 Bridge Welding Code(1) were applied to the P-scan equipment. Note that IIW or any other standard reference block suggested by the code may be used for equipment qualification and calibration. Figures 15 through 17 show the IIW type I, IIW type Resolution Calibration (RC), and IIW type Distance/Sensitivity Calibration (DSC) reference blocks used during calibration.

EQUIPMENT QUALIFICATION PROCEDURES

The equipment qualification procedures outlined in section 6.15 of the AASHTO/AWS D1.5M/D1.5: 2002 Bridge Welding Code(1) were closely followed for the qualification of the P scan system. The equipment qualification procedures are summarized as follows:

• Pulse-Echo Type Equipment Qualification: Section 6.15.1⁽¹⁾ states that the instrument shall be a pulse-echo type capable of using transducers with frequency ranges of 1 to 6 MHz. The instrument shall be capable of displaying an A-scan rectified trace. In general, the pulse-echo instrument is an ultrasonic instrument that generates ultrasonic pulses at a regular interval and measures the time taken for the pulses to pass through the test object and return to the transceiver. The P-scan system (shown in figure 2) is a pulse-echo system that uses transducers for transmission and reception of ultrasound at frequency ranges of 1 to15 MHz. The P-scan is capable of displaying an A-scan rectified trace.
• Horizontal Linearity Qualification: Section 6.15.2⁽¹⁾ states that the horizontal linearity of the test instrument shall be qualified over the full sound-path distance being used during testing. For this qualification, a straight-beam search unit creating longitudinal waves is used. Since the velocity of a longitudinal wave is almost double that of a shear wave, a 250-mm (10-inch) screen calibration for shear waves would require a 500-mm (20-inch) screen calibration for longitudinal waves. The procedures for horizontal linearity qualification are outlined as follows:
  • Couple a straight-beam transducer on the surface of the IIW type I reference block at the position shown in figure 18 to obtain five backwall reflections.
  • Adjust the first and fifth backwall reflections to their proper locations by use of the distance calibration and zero delay adjustments.
  • Each indication shall be adjusted to the reference level with the gain or attenuation control for horizontal location examination.
  • Each intermediate trace deflection location shall be correct within ±2 percent of the screen width. This qualification is not applicable to the P-scan system because the position of the echo is indicated by the peak amplitude rather than the leading edge where the position of the peak remains the same for all values of echo height.
• Signal Stability Check After Warmup: Section 6.15.3⁽¹⁾ states that the instrument shall be internally stabilized after warmup so that no variation in response greater 1 dB occurs with a nominal supply voltage change of 15 percent for a battery-operated instrument within the charge operating life. The P-scan system satisfies this qualification. The P-scan system houses two rechargeable batteries (one battery is used at a time to operate the system). When a battery's capacity falls below 10 percent, the system will automatically switch to the second battery, allowing the discharged battery to be exchanged ("hot swapped"). If both batteries fall below a 10 percent capacity, the system will give a warning and will shut off automatically.
• Gain Control, Attenuation, and Decibel Accuracy Qualification: Section 6.15.4⁽¹⁾ states that the instrument shall have a calibrated gain control adjustable in discrete 1 or 2 dB steps over a range of 60 dB. The accuracy of the gain control setting shall be within ±1 dB. This qualification is not applicable to the P-scan system because the P-scan has a logarithmic amplifier. There is no manual gain control in the P-scan system.
• Dynamic Range Accuracy Check: Section 6.15.5⁽¹⁾ states that the dynamic range of the instrument's display shall be such that a difference of 1 dB of amplitude can be easily detected on the display. The P-scan's scope window can be set up to have a resolution such that a difference of 1 dB of amplitude can be easily detected.

Figure 15. Diagram. IIW reference block used for the P-scan calibration: Type I.

Figure 16. Diagram. IIW reference block used for the P-scan calibration: Type RC.

Figure 17. Diagram. IIW reference block used for the P-scan calibration: Type DSC.

Figure 18. Photo. Horizontal linearity check using a straight-beam transducer and IIW type I reference block.

CALIBRATION PROCEDURES

The calibration procedures in sections 6.18 and 6.21 of the AASHTO/AWS D1.5M/D1.5: 2002 Bridge Welding Code⁽¹⁾ were closely followed to calibrate the AUT system for angle beam transducers. The calibration procedures were completed as follows:

• Sound Entry Point (Index Point) Check: The sound entry point is the point at which sound exits the wedge or shoe that the transducer is mounted on and enters the test object. The manufacturer always marks the sound entry point on the wedge. However, its position may shift because of wear in the lower surface of the wedge. Section 6.21.2.1(1) states that the following procedures shall be used to check the sound entry point of the wedge to be used in testing:
  • Couple the angle transducer on the IIW type I reference block at the position shown in figure 19.
  • Maximize the echo signal from the 100-mm (4-inch) radius circular reflector in the reference block (figure 20).
  • Mark the point on the wedge that aligns with the radius line of the circular reflector on the calibration block.

Figure 19. Photo. Sound entry point check: Photograph shows the transducer position on the IIW reference block.

Figure 20. Screen capture. Sound entry point check: A-scan screen displays the echo reflected from the 100-millimeter (4-inch) radius circular reflector on the IIW type I reference block.

• Sound-Path Angle (Beam Angle) Check: An angle transducer is characterized by the angle of refraction of the sound wave in the object being tested. Wedges are manufactured to carry longitudinal sound waves to the interface of the object being tested. As a result of mode conversion at the interface, a shear wave is produced in the object being tested. The angle of refraction is generally marked on the wedge. However, the angle may change because of wear on the lower surface of the wedge, so calibration is required to ensure that the correct angle is being implemented during inspection. Section 6.21.2.2⁽¹⁾ states that the following procedures shall be followed to check the sound-path angle:
  • Couple the selected angle transducer on the IIW type I reference block at the position shown in figure 21, where the angle of the wedge is the same angle marked on the IIW reference block.
  • Maximize the echo signal from the 50-mm (2-inch) diameter hollow circular reflector in the IIW reference block by moving the transducer back and forth over the line indicating the wedge angle on the reference block (figure 22).
  • Compare the sound entry point on the wedge with the angle mark on the IIW calibration block. If the sound entry point is within ±2 degrees of the angle mark, the transducer is acceptable.
• Resolution Check: The resolution is a measure of an instrument's ability to resolve echo signals reflected from neighboring objects. The resolution check for an angle transducer may be accomplished by the use of the IIW type RC reference block. The instrument shall be able to identify the three 1.5-mm- (0.06-inch-) diameter side-drilled holes. Section 6.21.2.5⁽¹⁾ states that the following procedures shall be followed to check resolution:
  • Couple the selected angle transducer on the IIW type RC reference block at the position shown in figure 23. Note that the angle of the wedge corresponds to the angle markings on the reference block.
  • Resolve the echo signal to distinguish between the peak associated with each of the three 1.5-mm- (0.06-inch-) diameter holes (figure 24).
• Distance Calibration Check: Section 6.21.2.3⁽¹⁾ states that the following procedures shall be followed to check distance calibration:
  • Couple the angle transducer on the IIW type I reference block at the position shown in figure 25, where the transducer's sound entry point aligns with the radius line of the 100-mm (4-inch) radius circular reflector.
  • Adjust the instrument to attain indications at 100 mm (4 inches) and 225 mm (9 inches) (figure 26).

Figure 21. Photo. Sound-path angle check: Photograph shows that the selected angle transducer is positioned on the IIW type I reference block over the line indicative of the transducer angle.

Figure 22. Screen capture. Sound-path angle check: A-scan screen displays the echo reflected from the 50-millimeter (2-inch) diameter hollow-disk reflector in the reference block.

Figure 23. Photo. Resolution check: Photograph shows that the selected angle transducer is positioned on the IIW type RC reference block over the line indicative of the transducer angle.

Figure 24. Screen capture. Resolution check: A-scan screen displays the three distinguishable signals reflected from the three holes.

Figure 25. Photo. Distance calibration check: Photograph shows that the angle transducer is positioned on the IIW type I reference block so that the transducer's sound entry point aligns with the radius line of the 100-millimeter (4-inch) reflector.

Figure 26. Screen capture. Distance calibration check: A-scan screen displays the signals reflected back and forth from the 100-millimeter (4-inch) radius circular reflector and 25.4-millimeter (1-inch) radius groove reflector.

• Amplitude, Sensitivity, or Reference-Level Calibration: Section 6.21.2.4⁽¹⁾ states that the following procedures shall be followed for sensitivity calibration:
  • Couple the angle transducer on the IIW type I reference block at the position shown in figure 27.
  • Maximize the signal from the 1.5-mm (0.06-inch) side-drilled hole in the IIW type I reference block by moving the transducer back and forth (figure 28).
  • Adjust the instrument to attain a horizontal reference line (i.e., an 80 percent FS level).

Figure 27. Photo. Amplitude, sensitivity, or reference-level calibration: Photograph shows that the angle transducer is positioned on the IIW type I reference block.

Figure 28. Screen capture. Amplitude, sensitivity, or reference-level calibration: A-scan screen displays the signal reflected from the 1.5-mm (0.06-inch) sidewall hole that is maximized to attain a horizontal reference-line (i.e., green line) height indication.