Back to Basics

Introduction
There are four essential aspects involved in the proper performance of a wet method magnetic particle inspection (MT) system. The item being inspected must be suitably magnetized in regard to magnetic field strength and orientation. The bath solution must be in proper concentration and state of cleanliness. The light environment must be properly maintained to provide sufficient UV and visible light levels. Inspectors must be thoroughly schooled and capable in performing the functions of accepting good items and rejecting bad ones.

A shortcoming in any one of these four areas can destroy the integrity of the MT system. This paper focuses on the properties of the bath that can be monitored and controlled to ensure the success of the inspection operation. Relevant specifications and industry practices are identified and reviewed to highlight this aspect of the MT operation.

 

Satisfying the Auditor
Auditors may be seen as being the NDT police, because they serve a very important function. They may or may not have an MT background or even an NDT background, which may not necessarily be disadvantageous. An auditor should understand that MT is a process. Like any other process, the procedures involved should be clearly stated and documented. Auditors should look at the process objectively, from the written procedures to their implemented practices, and make a determination as to whether the process is effective. The difficulty in auditing occurs in the interpretation of the "gray areas" of procedures that are inherent in the qualitative nature of MT.

The National Aerospace and Defense Contractors Accreditation Program (NADCAP), a unit of the Performance Review Institute, has devised an effective approach to performing audits on MT systems. It consists of a series of questions on its Laboratory Process Controls Document AC7114/2 checklist for MT systems. This document addresses the operating standards for MT systems based on requirements spelled out in ASTM E-1444 Standard Practice for Magnetic Particle Examination.

---

A variety of criteria must be satisfied for an MT bath to be found useable.

---

Process Requirements
A variety of criteria must be satisfied for an MT bath to be found useable. Some of the bath properties pertain to the materials going into the initial bath. Particle, fluid, and water conditioner characteristics and requirements are described by documents written and maintained through the Society of Automotive Engineers (SAE). Physical characteristics for particles include size, color, uniformity, etc. Oil fluids are rated by of color, clarity, flashpoint, and viscosity, among others.

Especially with the advent and much broader use of water bath systems, there are more factors than ever before that must be at least recognized as areas of concern. Water conditioners have some of the same properties as oil, in addition to characteristics such as pH (acidity or alkalinity), surface coverage, foaming, chemical compatibility, etc. The quality of the water itself is an issue; it is dependent on the locality of the inspection facility and the nature of water treatment that has been applied.

Many of the critical properties of the bath cannot be readily quantified or objectively measured. For instance, not only the bath but also the nature of the anomaly, the light intensity, and contrast against the surface can affect the brightness of a fluorescent indication. The determination of the usability of the baths depends on a relative comparison to other materials. A comparison of this kind must be carefully performed so as to minimize the variables from one test to the next.

 

Potential Problems
A variety of circumstances can affect the status of the MT bath solution (whether the fluid in use is oil or water, for instance):

• Drag-out of magnetic particles, by mechanical and magnetic adherence to parts.
• Drag-out of liquid due to the film that adheres to the surface of the parts.
• Loss of liquid by evaporation.
• A gradual accumulation of contaminants.
• Miscellaneous objects and materials that fall into tanks.
• Dilution of the bath from wet test pieces, dripping overhead pipes and moisture condensation.
• Improper mixing and measuring of bath components.
• Excessive foaming that can entrap particles.
• Insufficient or excessive agitation of the bath.
• Leaks in the reservoir and recirculation system.

 

Settling Tube
The settling tube stands as probably the most recognized and universal tool for evaluating the MT bath. It allows the inspector to observe several critical characteristics about the bath, including:

• Volume of solids (both fluorescent and nonfluorescent) present in the bath.
• Consistency of the solid material in the bath.
• Settling characteristics of the solid material.
• Presence of contaminants (both solid and liquid).
• Degree of dye separation, or, degradation of fluorescent particles.

Clearly the majority of facilities performing MT in their facilities, especially those who answer to any form of formal customer audit, base their procedures on ASTM E 1444. Paragraph 20.6 contains several subsections that describe the bath testing that can be performed with the settling tube. Bath properties such as settling volume can be readily quantified by directly observing the tube after a specified period of time. Other properties, such as percentage of contaminants, fluorescent brightness, etc., are more qualitative than quantitative and require training and sound judgment to be properly evaluated.

There are specifications, such as BAC-5424, used by The Boeing Company, that require an inspection operation to store a small separate portion of freshly made bath each time that a new bath is made. This separate sample is retained as a control for the process bath. The observations of the bath tests are more meaningful when the used bath is compared to its initial state, because any changes are more evident.

 

Use of Known Anomalies
Having a test piece with a known anomaly is often considered to be the most critical and significant test of an MT system. The more difficult an anomaly is to find the more valuable the piece becomes. The ability to find the anomaly should indicate that all parts of the MT system are working effectively. The only difficulty with this type of test is the failure to detect the critical anomaly. Any of the functions of the MT system bath, magnetic field, illumination, or personnel, are subject to error. It is not possible from the mere failure of the system as a whole to determine which part of the system has failed.

Artificial anomalies, including shims like the Burmah Castrol strips, quantitative quality indicators, and others, may or may not be effective for testing the bath. Such devices not only depend on the system to be performing entirely properly, but also require that the shims themselves be securely mounted to the inspection surface. Any gap between the surface and the shim drastically reduces or eliminates the meaning of the shim reading.

The ketos ring is probably more meaningful as a bath verification device since thorough reviews, resulting in the specification AS 5282 Tool Steel Ring Standard for Magnetic Particle Inspection have identified and corrected inconsistencies that have clouded the usefulness of the ring in the past. Previously, baths were used to verify the rings which, in turn, were used to verify the baths. The use of a mechanism using magnetic probes to indicate the magnetic properties of a ring have eliminated this seeming conundrum and resulted in a much more uniform industry standard. Still, the procedure for magnetizing the ring and applying the bath needs to be uniformly administered for consistently meaningful observations.

 

Bath Isolatation Test Pieces
Several devices are available that isolate the evaluation of the bath from the magnetizing apparatus. The common characteristic of these devices is that there is no magnetizing or demagnetizing involved with the use of the test piece. Any variables involved with the strength or orientation of the magnetizing force are eliminated.

In one form or another a residual magnetic field is used to establish a consistent magnetic force over a known constructed surface anomaly. These devices are conveniently small and, given their residual magnetic field, designed to provide a consistent test of the materials being evaluated. The value of the observations using such pieces is in observing and comparing their relative results from one test to the next for one material over time, or for different materials side by side.

The first of these is an MTU test block. This small disk is - in its simplest description - a quench cracked permanent magnet. The smooth surface has been shattered by small cracks through a controlled heat treating process. Bath is poured over the surface and the block is observed under UV illumination (see Figure 1) .

Figure 1 - Particles are attracted to the cracks on the surface of this magnetized disk. (Courtesy of Karl Deutsch.)

 

Figure 2 - (a) The Fluxa-Test Block contains a permanent magnet inside two precision ground steel blocks; (b) The seam between the blocks is incremented. (Courtesy Karl Deutsch.)

Another device is called the Fluxa test block, in which a small permanent magnet is embedded inside small, precisely ground steel blocks which form an artificial crack at their contact surfaces. The crack is incremented, thus allowing a quantified record to be made of the sensitivity of a particular bath (see Figures 2a and 2b) .

Finally, magnetic stripe cards are available, which, instead of using a residual magnetic field, employ a characteristic of magnetic stripe encoding. As a magnetic stripe is encoded, the domains of the particles making up the stripe are reversed. Each reversal establishes a magnetic anomaly that can attract magnetic particles. While a stripe provides no measurable external magnetic field, particles are attracted to the encoded pattern and the stripe becomes, in effect, a very effective test piece. Such cards are reliable, inexpensive, and reusable. (See Figures 3 and 4.)

Figure 3 - The magnetic encoder reverses the direction of the domains of the tiny particles making up the magnetic stripe.

 

Figure 4 - The particle indications can be lifted off the magnetic stripe card with transparent tape and used for documentation purposes.

 

High Technology
There are applications of higher technology being effectively applied to MT bath control. At least one installation has supplanted human inspectors with a machine vision system. This same installation uses automated turbidity measurement to control the bath. This installation is an example of electronic equipment doing what it is told to do. Turbidity measurement can be very effective if, and only if, it operates in a controlled environment with some particular conditions.

Turbidity can be defined as an "expression of the optical property that causes light to be scattered and absorbed rather than transmitted in straight lines through a sample," (Hach, 1990). Suspended solids obstruct the transmission of light through a transparent fluid. While it is a measure of the relative clarity of a fluid, turbidity is technically a measure of the scattering effect such particles have on light.

Solid particles exposed to a directed beam of light absorb and reradiate the light in all directions. Relatively large particles (greater than the wavelength of light) scatter light mostly in the forward direction (see Figure 5). Light scattering intensifies as particle concentration increases. A "ratio" turbidimeter compares 90 degree scattered light to forward scattered light and generates and output signal to a display or monitoring device (see Figure 6).

The sample cell is constantly supplied with recirculating MT bath. Upper and lower control limits are established by the inspecting party monitoring both minimum and maximum strength solutions. The output signal of the turbidimeter can be used in an LED display observed by an operator, or to set off an alarm of some sort when control points are breached (see Figure 7).

The limitation of using a turbidimeter is that it cannot see what solid material is in suspension, but only how the solid materials in the bath affect the light beam transmission. This machine cannot distinguish between good particles and scale, contamination, or any other form of suspended solid matter. As a practical matter the system using this technology inspects a shot blasted casting that brings only a very minimal, if any, amount of solid or liquid contamination into the system. It also uses a relatively small reservoir so that the tank is effectively drained and recharged fairly frequently.

Figure 5 - Typical patterns of light scatter from particles.

 

Figure 6 - Schematic view of ratio turbidimeter components.

 

Figure 7 - Turbidimeter hardware.

 

Conclusion
The bath is only one component of the MT system. If the bath is in good condition the system may be working. If the bath is not in good condition the system is not working. Testing of magnetic particle inspection baths is a relative methodology and requires that the number of bath test variables is minimized. Responsibility for bath monitoring, evaluation, and control must be assigned and understood.

 

References and Specifications
AC7114/2 NADCAP Audit Criteria for Nondestructive Testing Magnetic Particle Survey, Performance Review Institute, 161 Thornhill Road, Warrendale Pennsylvania 15086-7527.

AMS 3040-3046: Magnetic Particles, available from Society of Automotive Engineers (SAE), 400 Commonwealth Drive, Warrendale, Pennsylvania 15096.

AS 4792: Water Conditioning Agents for Aqueous Magnetic Particle Inspection, available from Society of Automotive Engineers (SAE), 400 Commonwealth Drive, Warrendale, Pennsylvania 15096.

AS 5282: Tool Steel Ring Standard for Magnetic Particle Inspection, available from Society of Automotive Engineers (SAE), 400 Commonwealth Drive, Warrendale, Pennsylvania 15096.

AS 5371: Reference Standards Notched Shims for Magnetic Particle Inspection, available from Society of Automotive Engineers (SAE), 400 Commonwealth Drive, Warrendale, Pennsylvania 15096.

ASTM E-1444: Standard Practice for Magnetic Particle Examination, available from American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, Pennsylvania 19428-2959.

ASTM E-709: Standard Guide for Magnetic Particle Examination, available from American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, Pennsylvania 19428-2959.

Audit Criteria 114/2, NADCAP, Performance Review Institute, 161 Thornhill Road, Warrendale, Pennsylvania 15086-7527.

BAC 5424, The Boeing Company, Seattle, WA.

Hach, C.C., R.D. Vanous, and J.M. Heer, Understanding Turbidity Measurement, Technical Information Series Booklet, No. 11, Loveland, Colorado, Hach Company, 1990.

Technical Order 33B-1-1/NAVAIR 01-1A-16/TM 55-1500-335-23, Departments of the US Army and US Air Force.

* Circle Systems, Incorporated, 479 W. Lincoln, Hinckley, IL 60520-1228, (815) 286-3271; fax: (815) 286-3352; e-mail circlesys@worldnet.att.net; Web site http://www.circlesafe.com..

Copyright © 1999 by the American Society for Nondestructive Testing, Inc. All rights reserved.

IRRSP, NDT Handbook, The NDT Technician and www.asnt.org are trademarks of the American Society for Nondestructive Testing, Inc. ACCP, ASNT, Level III Study Guide, Materials Evaluation, Nondestructive Testing Handbook, Research in Nondestructive Evaluation and RNDE are registered trademarks of the American Society for Nondestructive Testing, Inc. ASNT exists to create a safer world by promoting the profession and technologies of nondestructive testing.