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What is the enduring appeal of magnetic particle inspection, a technique with a century-long history?
Release Date:
2023/07/01
Every day, numerous facilities around the world employ nondestructive testing (NDT) techniques to verify product quality. The most commonly used NDT methods include magnetic particle inspection, liquid penetrant testing, ultrasonic testing, eddy current testing, and radiographic testing. Among these, magnetic particle inspection—a widely applied technique—leverages magnetic properties to effectively detect surface and near-surface defects in ferromagnetic materials, thereby ensuring that products meet quality standards before they are put into service.
Magnetic particle testing can be applied across a wide range of industries and fields, from roller coasters and nuclear power plants to aerospace—and even to the inspection of underwater pipelines and structures. In short, as long as the material being inspected is ferromagnetic, magnetic particle testing is generally a viable option.
In the aerospace industry, magnetic particle inspection is frequently employed to detect defects in components that are in service, particularly in critical parts that play a vital role in flight safety. As an increasing number of aerospace organizations worldwide place greater emphasis on leveraging science, technology, and advanced inspection methods to maximize safety performance, the value of magnetic particle inspection continues to grow.
1. Magnetic Particle Testing Process
Magnetic particle inspection (commonly abbreviated as MP, MPI, or MT) has long been regarded as a highly reliable nondestructive testing technique, particularly for detecting surface and near-surface defects in materials, where it offers distinct advantages. For instance, the inspection equipment is portable and readily amenable to automation; magnetic indications (magnetic marks) are directly visualized on the test material; the method provides highly reliable and sensitive detection of discontinuities at or near the material surface; compared with liquid penetrant testing, it requires minimal surface preparation; and, relative to other NDT techniques, the associated equipment and consumables are comparatively inexpensive.
In essence, the principle of magnetic particle inspection involves magnetizing the test component with an appropriate magnetizing field and then applying ferromagnetic particles—either in dry form or as a suspension—to the surface of the area to be inspected. Following magnetization, any discontinuities in the material cause distortion of the magnetic field, resulting in partial magnetic flux leakage and the formation of a leakage field at the component’s surface. This leakage field attracts the magnetic particles, forming visible indications—magnetic particle accumulations (magnetic marks)—which, under suitable lighting conditions, reveal the location and shape of defects. Subsequently, based on the relevant material acceptance criteria, these defects are analyzed and compared with respect to their size, shape, and distribution.
All components subjected to magnetic particle inspection shall be inspected in at least two different directions. Upon completion of multi-angle inspection, as with liquid penetrant testing, qualified and non‑qualified samples must be stored separately and clearly labeled. In some cases, a demagnetizer may be used to eliminate or reduce the magnetism of the parts to meet customer requirements or industry regulations; the surface is then cleaned with a degreasing agent and lightly coated with anti‑rust oil to prevent corrosion, after which the parts are repackaged.
2. Application Fields of Magnetic Particle Testing
In the manufacturing sector, numerous inspection organizations have incorporated magnetic particle testing as a critical component of their nondestructive testing services, supporting one-stop, integrated component inspections as well as large-scale inspections.
With the rapid advancement of technology, nondestructive testing (NDT) has evolved to not only identify the scope of repairs required for products or equipment but also to verify the integrity of components after repair. Consequently, in many companies, steel components undergo at least one NDT inspection—often multiple inspections—to comprehensively validate product performance.
The aerospace industry requires the effective detection of defects in aircraft parts, assemblies, and individual fixed-wing and rotary-wing components, so that such defects can be promptly repaired before the relevant products are returned to service. In most cases, defects in aircraft components typically manifest as cracks, which can render the component airworthy.
How important is magnetic particle inspection technology?
For example, a company that provides more than 10,000 fixed-wing and rotary-wing maintenance services—approved by the U.S. Federal Aviation Administration—to customers in over 60 countries can, through magnetic particle inspection alone, deliver components sufficient for approximately 100 aircraft worldwide each day. These inspection assignments may involve parts ranging in size from several inches in length down to small components such as bolts and fasteners.
In any industry, magnetic particle inspection shall be performed by qualified personnel holding Level I, Level II, or Level III certification. In our industry, magnetic particle inspectors undergo specialized training and certification in accordance with NAS 410 and the relevant requirements of equipment manufacturers, thereby ensuring that they possess the requisite expertise and knowledge to conduct effective inspections of components within critical equipment.
3. Advantages and Limitations of Magnetic Particle Testing
Like all nondestructive testing techniques, magnetic particle inspection (MPI) has both advantages and limitations. In general, the decision to employ or forego MPI depends on the nature of the product and the specific inspection requirements, which are typically determined by the inspection agency or by the company’s in-house Level III MPI technicians and engineering team.
On the positive side, MPI is a relatively simple process. Parts require only minimal preparation, such as cleaning and degreasing, and the inspection setup demands very little. In contrast, many other NDT instruments necessitate extensive pre-use inspections by operators in multiple ways. In addition to surface defects, MPI can also detect subsurface flaws, typically down to a depth of several inches.
Although magnetic particle inspection appears nearly perfect, it is limited to ferromagnetic materials. Its accuracy also depends, to some extent, on the operator’s level of expertise, including their ability to verify that the magnetic field strength is adequate and to perform subsequent subjective visual inspections.
4. Summary
Magnetic particle testing has a history of nearly a century since its inception. Owing to its unique advantages in the evaluation of ferromagnetic materials, this technique has firmly established its position in the field of nondestructive testing, making it one of the most popular and widely used NDT methods worldwide. Moreover, its popularity appears to be steadily increasing, particularly as industries across the board increasingly rely on inspection technologies to maximize safety. Under these circumstances, magnetic particle testing will continue to be favored by inspectors of steel products.
Author: Clifford Murphy
Translator: Vince
Translated from: qualitymag
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