Table of Content
- Liquid Penetrant Testing
- Liquid Penetrant Testing Procedure
- Magnetic Particle Testing
- Magnetic Particle Inspection Procedure
- Liquid Penetrant Testing vs Magnetic Particle Testing
When carrying out non-destructive testing, it is important to make sure that you use the most appropriate testing method to identify any flaws or defects. Two of the options that you can use when looking for surface defects are Liquid Penetrant Testing vs magnetic particle testing.
It is a good idea to find out more about them before you choose which would be best for the materials or structure that you want to test. In this article we unpack Liquid Penetrant Testing and Magnetic Particle Inspection, explaining each test, before looking at their similarities and differences.
What is Liquid Penetrant Testing?
Liquid Penetrant Testing (sometimes also known as Dye Penetrant Inspection) is used to detect surface-breaking defects. The penetrant testing definition states that a dye is used to highlight surface cracks and then it is measured under different light sources. Normally, it is used on non-ferromagnetic materials, (these are materials that are non-magnetic and contain no iron).
Liquid Penetrant Testing Procedure
1. The test requires proper preparation of the surface, including cleaning it of contaminants or coatings that may hinder the inspection.
2. The penetrant is applied evenly over the inspected area, either through spraying, brushing, or immersion, depending on the size and shape of the component.
3. Dwell time refers to the duration the penetrant remains on the surface before removal. It's a critical parameter as it determines the effectiveness of the test.
4. Excess penetrant is removed from the surface using a solvent or emulsifier after dwell time to prevent false indications during inspection.
5. After removing excess penetrant, a developer is applied to the surface, acting as a blotter to remove the penetrant from defects and spread it on the surface.
6. The surface is visually inspected under suitable lighting conditions, revealing indications like cracks, pores, or leaks as coloured or fluorescent marks against the background.
7. After inspection, the surface should be cleaned thoroughly to remove any remaining traces of penetrant and developer.
8. The results of the inspection are evaluated based on the type, size, and location of indications observed.
Read More, Procedure for Liquid Penetrant Testing
What is Magnetic Particle Testing?
Magnetic Particle Testing, also called Magnetic Penetrant Testing is used to detect surface defects, particularly cracks, in ferromagnetic or magnetic materials. MPI Test is a widely employed NDT method used to assess the integrity of welds and various metallic components. This technique is particularly valuable in industries such as welding and manufacturing.
During an MPI Test for welding, the material or weldment under examination is magnetized, typically by inducing a magnetic field. This magnetic field allows iron or steel particles (dry powder), often suspended in a liquid solution, to be applied to the surface.
The magnetic particles are drawn to areas with Magnetic Flux Leakage, such as cracks, weld discontinuities, or other defects, revealing these flaws. The inspection can be conducted at different magnetic particle inspection temperatures, with variations in the test procedure to accommodate specific environmental conditions and material properties. By effectively detecting surface and near-surface defects, MPI plays a pivotal role in ensuring the reliability and safety of welded structures and critical components across various industries.
MPI- Magnetic Particle Inspection Procedure
1. First, the surface needs to be cleaned and then a white contrast paint can be applied.
2. The NDT MPI Testing component is magnetized, either locally or overall.
3. A magnetic particle powder or ink is applied to the surface.
4. If there is a surface-breaking flaw the magnetic field will have become distorted, causing local magnetic flux leakage around the flaw. The fine iron particles (either dry or in a liquid) will build up in the areas of flux leakage. This will be clear to see, even for narrow cracks. It is indicated as a line of iron powder particles on the surface which can be seen against the contrasting white paint background.
5. It is common to apply magnetization more than once in different directions to fully inspect the material.
6. If required, the component can be de-magnetized, and post-cleaning can be applied.
The method can be used on all metals that can be strongly magnetized and are often used during the manufacturing and testing of welds. Magnetic Particle Testing can be used for storage tanks, pipework, shafts, castings, structural steel, heat exchangers, and boilers.
Liquid Penetrant Testing vs Magnetic Particle Testing
Liquid Penetrant testing and Magnetic Particle testing belong to NDT methods that supplement visual inspection, revealing defects such as fine cracks or micro-porosity that would be invisible or difficult to detect with the naked eye. Neither liquid, nor MPI testing techniques can give a permanent record of the inspection, but photographs of the area are useful – reference markers can be helpful for accurate recording of the size and site of indications, particularly if repairs are needed. As the name suggests, Magnetic Particle testing requires the material to be magnetized (and therefore made of metal), whereas LPT can be used on a wider variety of materials, such as metals, plastics, and ceramics. Surface preparation is less critical in Magnetic Particle testing than in Liquid Penetrant Testing, but surfaces should be generally smooth for both.
There are pros and cons to both methods and generally, you should make your choice by considering the various factors involved, especially the type of material that you want to test, as Magnetic Particle Testing cannot be used on non-magnetic materials. Both tests, though, allow for cost-effective and time-efficient testing, meaning that reports can be turned around quickly.
Major Factors to Consider Between Liquid Penetrant Testing and Magnetic Particle Inspection
1. Detection Capabilities
Liquid Penetrant Testing (LPT):
- Suitable for detecting surface-breaking defects.
- Effective in identifying cracks, pores, and leaks on non-ferromagnetic materials.
- Limited to defects that penetrate the surface of the material.
Magnetic Particle Inspection (MPI):
- Primarily used for detecting surface and near-surface defects.
- Particularly effective in identifying cracks and weld discontinuities in ferromagnetic materials.
- Capable of detecting flaws below the surface layer, providing a more comprehensive inspection.
2. Material Compatibility
Liquid Penetrant Testing (LPT):
- Suitable for a wide range of non-porous materials, including metals, plastics, ceramics, and composites.
- Ideal for components that are non-magnetic and contain no iron.
Magnetic Particle Inspection (MPI):
- Specifically designed for ferromagnetic or magnetic materials.
- Limited to materials that can be magnetised, such as iron, steel, and certain alloys.
3. Procedure Complexity
Liquid Penetrant Testing (LPT):
- Relatively simple and straightforward procedure.
- It requires surface cleaning, application of penetrant, dwell time, removal of excess penetrant, application of developer, visual inspection, and cleaning.
- Can be performed using basic tools and materials.
Magnetic Particle Inspection (MPI):
- It involves magnetization of the component, application of magnetic particles, detection of flaws, and demagnetization.
- Requires specialised equipment such as a magnetising yoke or electromagnetic equipment.
- May involve additional steps depending on the specific requirements of the inspection, such as surface preparation and post-inspection cleaning.
While both liquid penetrant testing and magnetic particle inspection are effective methods for detecting surface defects, they differ in their detection capabilities, material compatibility, and procedural complexity. Understanding these differences is essential for selecting the most appropriate technique for a given inspection scenario. It is important to consider factors such as the material being inspected and the size of defects that need to be detected when choosing between liquid penetrant testing and magnetic particle inspection. Additionally, both methods require trained technicians to ensure accurate results.
Ultimately, the decision between Magnetic Particle Testing and Liquid Penetrant Testing will depend on the specific needs of your project and the material being tested. It is important to consider factors such as sensitivity, ease of use, and compatibility with the material in order to determine which method is most suitable for your application.
Reference
1. NDT Group
2. Dynamic Globe
3. The National Board of Boiler and Pressure Vessel Inspectors
4. IndiaMart
5. Research Gate
6. Fujifilm NDT
7. Bergeng
