Proper Use of Quantitative Quality Indicators (QQI) for Field Direction and Strength

by Eddie Pompa

The purpose of this blog post is to provide inspectors with a better understanding of the purpose, preparation, and use of quantitative quality indicators (QQIs) for determining the required magnetic field strength for magnetic particle inspection. QQIs are magnetic particle test pieces (shims) created with artificial flaws (notches) that are used to verify magnetic field direction and relative strength.

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QQI History

The QQI was developed by Kermit Skeie in the late 1980s. Skeie started with a Japanese concept that was thicker and modified it to create a more versatile reference standard. He worked together with Pat Jenkins and D.J. Hagemaier to publish papers and develop procedures for the proper use of these QQIs during the 1990s. This work led to the creation and inclusion of industry standards SAE AS 5371-98: Reference Standards Notched Shims for Magnetic Particle Inspection and the 2001 edition of ASTM E1444-01 Standard Practice for Magnetic Particle Examination.

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QQI Design

The design of the QQI requires the notch depth be a set percentage of the shim thickness regardless of how thick the shim is. Each shim type has a reference number to describe it and its dimensions. A shorter number is etched on the shim for quick reference.

The breakdown for each type of shim and its associated identifying number is provided in Table 1.

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Table 1

The initial concept and design for QQIs was adopted into industry standard SAE AS 5371-1998 to ensure uniformity in the quality and to establish consistent sensitivity. The portion of the circle where flux lines intersect perpendicularly allows particles to gather and become visible once the appropriate amperage level is achieved.

This reference standard was originally added to ASTM E1444-01, Annex A1.5 as a “nonmandatory” requirement but in 2005 it was updated to a “mandatory” requirement that remains in effect.

The current version of ASTM E709 continues to include QQIs as a nonmandatory requirement for use in general magnetic particle testing (MT) applications. The use of a QQI may come into play where procedures are required to be proven and demonstrated. In these cases, the QQI is an adequate representative reference standard that can be used similarly to those used in other NDT methods.

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QQI Function

The creation of the notched shim was crucial to establishing sensitivity requirements in MT. Its use as a reference standard is similar to the use of the IQI in radiographic testing with hole penetrometers and wire gauges. When a sufficient length of the circle notch of the shim appears during magnetization, the inspector has achieved the minimum amperage needed to detect the critical flaw size.

The objective in any NDT method is to properly detect the smallest rejectable flaw size with a high level of confidence and repeatability. With this in mind, procedures are written to include the use of the QQI to establish the proper technique that can be recorded and referenced for future use on similar part shapes and sizes.

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QQI Step-by-Step Use

The steps for using QQIs vary depending on the part itself and the required QQI per specific procedure. The following are basic steps that apply to QQI preparation, application, observation, demonstration, and storage both prior to and after use.

Care must be taken when removing the coating from the QQI to avoid damaging the notch profile. To remove the coating from the QQI follow these steps.

1. Place the QQI on a smooth flat surface such as a piece of glass.
2. Soak the surface with acetone for at least 1 min.
3. Gently rub the QQI with a fingertip to loosen the coating.
4. Repeat on both sides of the QQI.
5. Once the coating has been removed, wipe the QQI with isopropyl alcohol to remove any remaining residue, using the same technique as with the acetone application.
6. Dry the QQI and apply it to the part surface.

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QQI Application

1. Identify the high stress areas of the part being inspected.
2. Determine the number of QQIs needed.
3. Remove the “photoresist” protective coating from the QQIs using acetone followed by isopropyl alcohol. (Coating removal is not directly communicated in any of the standards mentioned in this blog post.)
4. Using a solvent, clean the surface of the part where the QQIs will be placed.
5. Secure each QQI (notches face down to the part) using super glue or transparent tape along all four edges to prevent fluid or particles from getting into the notches.
6. Apply amperage in small increments until the appropriate sensitivity is achieved.
7. When the appropriate amount of particle accumulates along the circle notch, the technique is considered adequate and shall be recorded. The tangential notch will appear in full to represent the field direction. Record this information in a report, a photograph, or both for recordkeeping.
8. Remove the QQIs with care to avoid distorting them.
9. Store the undamaged QQIs in a solution such as oil that will prevent corrosion.

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Summary

The QQI notches simulate a surface break on the part while providing a realistic flaw length to set the proper technique for detecting the required flaws. The 30% notch depth also equates to 30 gauss (G) which is the minimum required field strength for adequate detection of magnetic particle indications.

The application of the QQI has become a staple in the oil and gas industry as it pertains to API standards that require the NDT procedure be qualified which is where the QQI has made its way in, but not correctly as it relates to proper removal of the protective coating. Improper coating removal leads to QQI damage and inaccurate test results which has led to poor confidence in its use. This paper fills the gap in this regard which should improve test results and confidence in its use across all sectors.

The objective in this blog post is to fill in gaps that exist in our industry standards so that they may be added to MT procedures.

The use of a QQI when performing circular, longitudinal, or multidirectional magnetization with a bench or power unit is essential and provides evidence of the technique’s ability to detect the minimum critical flaw size as specified by your acceptance criteria.

The evolution of a quantitative reference standard for MT is a much-needed tool that removes variations and uncertainty in MT.

Proper use of the QQI requires cleanliness of both the part with a solvent solution and QQI coating removal prior to applying the shim to the surface.

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References

• ASTM, 2001, ASTM E1444 Standard Practice for Magnetic Particle Examination, ASTM International, West Conshohocken, PA
• ASTM, 2021, ASTM E709 Standard Guide for Magnetic Particle Testing, ASTM International, West Conshohocken, PA
• Hagemaier, D.J., 1991, “Quality Indicators for Magnetic Particle Inspection,” Materials Evaluation, March 1991
• Hagemaier, D.J., 1990, ATA NDT Forum, Quebec, Canada, September 1990, McDonnell Douglas, Douglas Aircraft Company
• Magnaflux Product Data Sheet
• SAE AS5371-1998, Reference Standards Notched Shims for Magnetic Particle Inspection, SAE International, Warrendale, PA

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Author

Eddie Pompa is an ASNT Face of NDT for 2023. He is ASNT NDT Level III in MT, PT, UT, and RT; ACCP Professional Level III in MT, PT, and UT; and NAS 410 Level 3 in ET, MT, PT, RT, and UT. He teaches eddy current testing MT, PT, RT, and UT classes at Lone Star College in Houston, Texas.