Ultrasonic Flaw Length Measurement

by Eddie Pompa

Does transducer diameter affect the accuracy of flaw length measurements because of near field more than beam spread?

Introduction

To determine if transducer diameter affects the accuracy of flaw length measurements because of near field more than beam spread, an experiment was performed. The overall length of the certified calibration notch was measured using transducers of different diameters with the same refracted angle and frequency for each to determine if the diameter affected the accuracy of the length measurement.

In measuring the notch length, scanning back and forth across the notch end was avoided. The aim was to approach the notch in one smooth movement until the signal dropped 6 dB. The calculated near field and half angle beam spread values for each diameter transducer are provided in Table 3 and visually illustrated in Figures 2, 3, and 4 which is the primary contributing factor for the notch length accuracy. The smaller-diameter probes produced a shorter near field than the larger-diameter probes. See Tables 1 and 2 for the notch length results and the delta for each.

The 0.25 in. probe was more challenging to manipulate compared to the larger probes as it slid along the edge of the T-square from time to time, making it harder to capture the end of the notch. With some practice, control of the probe improved enough to allow for consistent notch length measurements. An additional observation during this exercise was the immediate signal drop as the probe reached the end of the notch resulting from the small beam spot produced. This characteristic required a deliberate focus on probe movement to capture the end of the notch to avoid overshooting the notch length and having to scan back and forth across the notch end.

The most accurate and repeatable notch length measurements appear to be on the thicker block due to the longer sound path, thus removing the near field from interacting with the inside diameter (ID) or outside diameter (OD) notches.

Therefore, when possible and required, technicians should utilize a small-diameter probe to accurately record flaw length where the flaw is within the focal distance of the probe. The wide beam angle of the small-diameter probes can produce more noise where long sound paths are required.

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Table 1. 45° wedge angle data

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Table 2. 60° wedge angle data

Equipment

The instrument used was a standard 5 MHz pulse-echo unit with single-element transducers (0.250, The instrument used was a standard 5 MHz pulse-echo unit with single-element transducers (0.250, 0.375, and 0.500 in.). Lucite wedges were used (0.250, 0.375, and 0.500 in.) with 45° and 60° wedge angles. The calibration block was a distance and sensitivity calibration (DSC) ASME basic block (0.50 and 0.75 in.) made of aluminum. Gel (for the wedge) and water (for the contact surface) were used as couplant. A T-square, dial calipers, infrared thermometer, tape, and pencil completes the list of equipment used.

Technique: Contact Angle Beam

Each diameter transducer was calibrated prior to making length measurements using the DSC block. The probes were placed in the center of the notch length where the signal was adjusted to 80% full screen height (FSH) as the reference level. The length of each notch was made with no adjustment to the gain setting. The notch length was marked on the T-square surface so that final length measurements could be made with a pair of dial calipers.

Procedure

First, the temperature of the part and calibration block were measured to verify they were within 2 °F of each other prior to performing standardization and measurement.

The DSC and ASME block temperature range over the duration of the exams was 56–63 °F. To complete the procedure, the following steps were taken:

1. Calibrate on the DSC block.
2. Locate the ID notch and set the T-square in place to ensure a parallel surface during length measurements.
3. Adjust gain to achieve 80% FSH for the ID notch.
4. Slide the transducer along the notch length away from center toward one end of the notch until the signal amplitude drops by a factor of 6 dB.
5. Record this point on the T-square surface.
6. Slide the transducer along the notch length back toward the center of the notch and toward the inspector until signal amplitude drops by a factor of 6 dB.
7. Record this point on the T-square surface.
8. Repeat steps 3–7 for the OD notch.

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Table 3. Data recorded from experiment

The half angle beam spread has an increased diameter the further away from the transducer face, which can be seen in Figures 2 through 4, but it is hard to visualize the impact this has on final notch length measurements.

Another contributing factor toward repeatable notch length measurements was the 60° wedge angle, which added to the sound travel path distance increase and kept the near field away from the ID and OD notch.

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The beam spread illustration in Figure 5 shows the large and uncertain beam spread along the sound path, which has a 4:1 ratio applied.

All length measurements were made using the 6 dB drop method by approaching the end in one smooth direction as opposed to extending beyond the notch and coming back to it.

For each diameter probe, the signal peaked and set to 80% FSH from the ID notch as the reference point. No adjustments were made when moving to the OD notch measurement.

The 0.25 in. diameter probe was harder to control as the end of the notch was approached, perhaps as a result of the damping material catching the T-square and the small grip area. The signal did drop off more rapidly as the end of the notch approached.

The effects of beam spread and near field are illustrated in Figure 2 on a 0.75 in. thick calibration block using a UT software tool.

Conclusion

Beam spread affects the approach to the notch end/length but does not interfere with the signal quality as does the near field. Beam spread can be visualized as a concentration of sound over the diameter of the cone at a given distance as seen in the simulated beam spread illustrations in Figures 2, 3, and 4. The near field contains various wave intensities that produce nonuniform signal responses, which makes it difficult to resolve and gauge indications at expected signal amplitudes. Because of these inconsistent waveforms, the notch length measurements within the near field are less repeatable than measurements made in the far field. The measurement data from the experiment revealed no consistent trend based on how the different diameter transducer affects the accuracy of the flaw length measurement. Therefore, to generate the most accurate and repeatable results, it is best to make measurements within the wave form where signal quality is at its best.

References

• AWS, 2020, D1.1, Structural Welding Code – Steel, American Welding Society, Miami, Florida.
• Drury, J.C., 1992, Ultrasonic Flaw Detection for Technicians, OIS Power & Nuclear Division, Wales.
• ASNT, 2015, Ultrasonic Testing: Classroom Training Book, second edition, American Society for Nondestructive Testing, Columbus, Ohio.

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Eddie Pompa is an NDE Level 3 Quality Assurance Specialist, ndtheroes@gmail.com.