Non-destructive testing (NDT) is a critical component of many industries. It allows for the detection of flaws or defects in materials or components without the need for damaging them. NDT is an incredibly useful tool that can help to ensure safety and reliability in a wide range of industries. In this article, we will explore the two main types of non-destructive testing: dye penetrant testing and magnetic particle testing.
What is Non-Destructive Testing?
Non-destructive testing (NDT) is a method of examining components and materials for flaws or defects without damaging them. NDT is used in a variety of industries, including aerospace, automotive, and construction. NDT is often used when it is not feasible or possible to examine components or materials using conventional methods.
The technicians performing non-destructive testing must be qualified to the requirements of ASNT SNT TC-1A Level II in order to ensure accuracy and reliability.
Dye Penetrant Testing
Dye penetrant testing is a method of non-destructive testing that uses a liquid of intense color to flow into cracks and cavities on the test surface. After a few minutes, the excess penetrant is wiped away, and a fine coating of white developer powder is applied to the surface to improve contrast and draw out the penetrant left in the cracks and cavities.
The equipment used for dye penetrant testing is relatively simple and consists of three aerosol cans. The first aerosol can contains the penetrant, the second aerosol can contains a solvent cleaner, and the third aerosol can contains a non-aqueous wet developer.
Advantages of Dye Penetrant Testing:
• Simple to use
• Can be used on ferrous, non-ferrousd metals, plastics and glass
• No power supply required
• Very sensitive and cost-effective
• Can inspect any geometry or shape of material
Disadvantages of Dye Penetrant Testing:
• Can only be used for defects that are open to the surface
• Pre-cleaning of the test surface is essential
Magnetic Particle Testing
Magnetic particle testing (MPT) is a non-destructive testing (NDT) method used in laboratories to detect surface and sub-surface discontinuities in ferromagnetic materials. This method relies on the contrast between magnetic particles caught in the magnetic flux leakage around discontinuities and the background color of the test surface. It is a simple and cost-effective way to detect defects.
What is Magnetic Particle Testing?
Magnetic particle testing is a non-destructive testing method that uses magnetic particles to detect the surface and sub-surface discontinuities in the magnetized test surface. This method relies on the difference between magnetic particles caught in the magnetic flux leakage around discontinuities and the background color of the test surface.
The amount of flux leakage and the contrast of indication are dependent upon the large difference in magnetic permeability between the discontinuity and the parent metal, the orientation of planar discontinuities to the magnetic field, and the density of flux near the test surface. Magnetic permeability is the ratio of flux density (B) to the applied magnetic field (H). A crack in a magnetized steel plate is an air gap which breaks the magnetic circuit. The permeability of air is unity and for mild steel is eight hundred. For a given value of H, the flux density in the air gap is 1/800th of the flux density in the parent plate. The magnetic flux will therefore leak from the surface, creating poles which will attract the magnetic particles.
Equipments Used in Magnetic Particle Testing
The equipments used in magnetic particle testing include magnetizing apparatus, directing medium, and accessories. Magnetizing apparatus includes electro and permanent magnets that produce longitudinal magnetic fields, prods and flexible cables that produce circular magnetic fields, and heavy transformers. Directing medium includes magnetic particles used as dry powders or as suspensions in paraffin or water, with colors such as red, black, or fluorescent. Accessories include flux indicators, flux meters, demagnetizers, sediment flasks, and black lights.
Advantages of Magnetic Particle Testing:
• Simple to use
• Can detect surface and sub-surface defects
Disadvantages of Magnetic Particle Testing:
• Can only be used on ferromagnetic materials
• High magnetizing currents may cause arcing or burning
• Demagnetization may be required in many cases