Magnetic particle inspection (MPI) is a nondestructive testing (NDT) technique used to detect surface and subsurface defects in ferroelectric materials such as iron, nickel and a few steel alloys. However, the effectiveness and accuracy of MPI in detection of subsurface defects is debatable. In most cases, MPI is used as an auxiliary technique in conjunction with ultrasonic inspection to enable accurate defect detection of the object under test. Considered by some as an outdated technology, the benefits and need for MPI has been debated vociferously by industry experts.
Technology Overview
As the name suggests, MPI employs the principles of magnetism to characterize and detect defects in ferromagnetic materials. In this NDT technique, the ferromagnetic component is magnetized using a strong magnetic field created by a magnet or special equipment. If there is a discontinuity in the component under test, the magnetic field introduced in the object is disrupted and a leakage field is produced at the point of the discontinuity. To characterize the defect, the surface needs to be coated with magnetic particles that are attracted to these leakage fields. Hence, at the point of flux leakage, there is an indication of discontinuity which is visible under proper lighting conditions. This is the general working principle of MPI.
To obtain accurate results, MPI needs to be performed using a standard procedure, involving the following steps:
To obtain accurate results, MPI needs to be performed using a standard procedure, involving the following steps:
1. Component pre-cleaning
2. Introduction of magnetic field
3. Application of magnetic media
4. Interpretation of magnetic particle indications
5. Demagnetization of the component
First, the component under test needs to be pre-cleaned thoroughly as any contaminants, such as oil, dirt or grease, impede the passage of magnetic particles to the leakage fields. Next, the component is magnetized using a strong magnetic field and the magnetic media is applied on the surface of the component. The magnetic media or the magnetic particles can either be dry or wet. The decision to use either dry particles or wet particles depends on the nature of the surface to be inspected. In the majority of cases, wet particles have several advantages over dry particles; however, in case of rough surfaces, dry particles are preferred as the wet particles are comparatively smaller and can settle in surface valleys. As a result, the wet particles lose mobility and become less effective than dry particles. After interpreting the magnetic particle indications, the component must be demagnetized, as the residual magnetic field in the component may interfere with subsequent manufacturing operations.
2. Introduction of magnetic field
3. Application of magnetic media
4. Interpretation of magnetic particle indications
5. Demagnetization of the component
First, the component under test needs to be pre-cleaned thoroughly as any contaminants, such as oil, dirt or grease, impede the passage of magnetic particles to the leakage fields. Next, the component is magnetized using a strong magnetic field and the magnetic media is applied on the surface of the component. The magnetic media or the magnetic particles can either be dry or wet. The decision to use either dry particles or wet particles depends on the nature of the surface to be inspected. In the majority of cases, wet particles have several advantages over dry particles; however, in case of rough surfaces, dry particles are preferred as the wet particles are comparatively smaller and can settle in surface valleys. As a result, the wet particles lose mobility and become less effective than dry particles. After interpreting the magnetic particle indications, the component must be demagnetized, as the residual magnetic field in the component may interfere with subsequent manufacturing operations.
Alternative Technologies
MPI is a conventional NDT technique that has become a standard for inspection of surface cracks in ferromagnetic materials. In comparison with other conventional NDT techniques for surface inspection, such as penetrant testing (PT), MPI is considered a fast and low cost method for inspection with the ability to also detect sub-surface defects. However, over the past decade, the NDT industry has invested significant resources to develop advanced NDT techniques that offer competition to the conventional techniques such as MPI and PT. Alternating current field measurement (ACFM), eddy current testing (ECT), and remote field testing (RFT) expose the limitations of the conventional techniques, such as low productivity due to time-consuming operations of surface cleaning, low reliability due to the human factor, high costs for material expendables, and impossibility to estimate the flaw depth. Also, the advanced NDT techniques do not employ the use of toxic chemicals, hence are environmentally friendly as well.Frost & Sullivan research indicates that ACFM in particular provides exceptional cost savings over MPI. This is backed by a case study performed by TSC Inspection Systems. The company compared one-man inspection of 100 separate 1m long welds, where it is assumed that there is an average of one significant defect (>1mm deep) per weld. Comparisons are made between ACFM and MPI (the latter used in conjunction with alternating current potential drop (ACPD) or ultrasonic testing (UT) to provide depth information on the defects) for both unpainted and painted welds. The depth information is needed to allow decisions on subsequent removal of cracks to be made. If no other technique is used in conjunction with MPI to give depth information, it is common to grind about 1mm at a time and re-inspect between grinds to check if the crack has disappeared. In this case, the extra time spent grinding is even longer than that spent depth sizing.
The example stated above concludes that the savings using ACFM range between 60 percent (unpainted surface) and 70 percent (painted surface). Additionally, ACFM has a higher probability of detection. Hence, in comparison with MPI, ACFM is a far superior technique for weld inspection. Although, the case study presented earlier gives a comparison between ACFM and MPI, parallel conclusions can be drawn with other alternative techniques such as ECT, and RFT, amongst others.
Market Overview
MPI along with PT are considered standard inspection techniques for surface inspection due to the comparative simplicity in operation and significant ease of inspecting irregular component shapes. When used in conjunction with other techniques, such as ultrasonic inspection, a component can be completely tested for surface, sub-surface and internal defects.Frost & Sullivan research indicates that MPI and PT combined accounts for 2.3 percent of the total NDT equipment market in 2011. This is negligible when compared with other technologies in the market such as ultrasonic, eddy current, visual and radiography. Although, to a certain extent, emerging nations such as China and India drive the growth for MPI, a low replacement rate and lack of technological advancement has resulted in end-users opting for newer NDT techniques. Also, as mentioned earlier, evolution of alternative techniques such as ACFM, ECT, and RFT, amongst others, is expected to decrease the demand for MPI over the next three to five years. As a result, this market is expected to grow at approximately 1.5 percent compound annual growth rate (CAGR) from 2011 to 2016. The competition in MPI market is highly concentrated with the top two competitors; Magnaflux Global NDT Solutions and Gould-Bass Company, Inc. accounting for approximately 75 percent of the market. Other market participants include Johnson & Allen Ltd, KARL DEUTSCH Prüf- und Messgerätebau GmbH + Co KG and Magwerks Corporation.
Conclusion
For over 50 years, MPI has been the most popular NDT technique for surface inspection. It is an extremely simple method with little scope for game-changing technological innovations. The use of MPI in conjunction with conventional ultrasonics has evolved in to a standard procedure for defect detection in ferromagnetic materials. However, with advances in alternative technologies such as ACFM, ECT, and RFT, along with greater acceptance from end-user industries, the demand for MPI is expected to decrease over the next three to five years. With that being said, MPI will not disappear completely from the market, as most standards for surface inspection stipulate the use of MPI or PT for defect detection. Also, in emerging economies such as India and China with abundant material, manpower resources and low capital investment, MPI is still expected to be the primary choice of NDT technique for surface inspection.Thus, the use of MPI is expected to diminish significantly in regions such as North America and Europe; however, demand from Asia-Pacific (APAC) is expected to continue to create growth opportunities for market participants, albeit at extremely low revenue growth rates. NDT
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