MPI

• Magnetic Particle Inspection (MPI)


  Principles of MPI, basic physics Of magnetism, permeability, flux density, cohesive force, magnetizing force, retentivity, residual magnetism, Methods of magnetization, magnetization techniques such as headshot technique, cold shot technique, central conductor testing, magnetization using products using yokes, direct and indirect method of magnetization, continuous testing of MPI, residual technique of MPI, system sensitivity, checking devices in MPI- Interpretation of MPI, indications, advantage and limitation of MPI
• MPI is governed by the laws of magnetism and is therefore restricted to the inspection of materials that can support magnetic flux lines, metals can be classified as ferromagnetic, paramagnetic, or diamagnetic

  
• Ferromagnetic metals


  Strongly attracted to a magnet and can become easily magnetized (e.g. iron, nickel, cobalt)
• Paramagnetic materials


  Very weakly attracted by magnetic forces of attraction and cannot be magnetized (e.g. Aluminum)
• Diamagnetic materials


  Have a weak, negative susceptibility to magnetic fields, slightly repelled by a magnetic field and do not retain the magnetic properties when the external field is removed (e.g. copper, silver, gold)
• The only requirement for inspection through MPI is that the components being inspected must be made of a ferromagnetic material (materials that can be magnetized) such as iron, nickel, cobalt, or some of their alloys
• Industries that use MPI


  • Structural steel
  • Automotive
  • Petrochemical
  • Power generation
  • Aerospace
• Underwater inspection is another area where magnetic particle inspection may be used to test items such as offshore structures and underwater pipelines
• Magnetism


  The ability of matter to attract other matter to itself
• Magnetic field lines


  • Form complete loops
  • Do not cross
  • Follow the path of least resistance
  • All have the same strength
  • Have direction such that they cause poles to attract or repel

  
• Ferromagnetic materials


  Materials with a significant Iron, nickel, or cobalt content that can be magnetized, made up of many regions in which the magnetic fields of atoms are aligned (called magnetic domains)
• How Magnetic Particle Inspection Works


  1. Ferromagnetic test specimen is magnetized with a strong magnetic field

  2. Finely milled iron particles coated with a dye pigment are applied to the test specimen

  3. Particles are attracted to leakage fields and will cluster to form an indication directly over the discontinuity

  
• Basic Procedure-MPI


  1. Component pre-cleaning

  2. Introduction Of magnetic field

  3. Application of magnetic media

  4. Interpretation of magnetic particle indications
• Pre-cleaning


  Essential for the particles to have an unimpeded path for migration to both strong and weak leakage fields, the part's surface should be clean and dry before inspection
• Methods of introducing magnetic field


  • Using a permanent magnet or an electromagnet that contacts the test piece
  • Flowing an electrical current through the specimen
  • Flowing an electrical current through a coil of wire around the part or through a central conductor running near the part
• Wet versus Dry Magnetic Particle Application


  Dry method is more portable, wet method is generally more sensitive since the liquid carrier gives the magnetic particles additional mobility
• Magnetic Particles/Media


  Made of ferromagnetic materials, usually combinations of iron and iron oxides, with high permeability and low retentivity, particles having high permeability are easily attracted to and magnetized by the low-level leakage fields at discontinuities, low retentivity is required to prevent the particles from being permanently magnetized

  
• Dry Magnetic Particles


  • Come in a variety of colors, a color that produces a high level of contrast against the background should be used
• Wet Magnetic Particles


  • Typically supplied as visible or fluorescent, visible particles are viewed under normal white light and fluorescent particles are under black light

  
• Dry Magnetic Particles


  Common, relatively cheap, generally applied to rougher surfaces, painted or sprayed onto surfaces
• Wet Magnetic Particles


  More expensive, accurate, elongated aligns well with magnetic fields, rounded-move freely across a surface, removal type (water, solvent, or emulsifier)

  
• Types of Magnetizations


  • Longitudinal magnetic field (magnetic lines of force run parallel to the long axis of the part)
  • Circular magnetic field (magnetic lines of force run circumferentially around the perimeter of a part)

  
• Being able to magnetize the part in two directions is important because the best detection of defects occurs when the lines of magnetic force are established at right angles to the longest dimension of the defect

  
• Methods of Magnetization


  • Direct Induction (current passed directly through the component)
  • Indirect Induction (using a strong external magnetic field to establish a magnetic field within the component)
• Direct Magnetization (Direct Induction)


  1. Clamping the component between two electrical contacts (Head Shot Method)

  2. Using Clamps or Prods
• Indirect Magnetization (Indirect Induction)


  1. Use of permanent magnets

  2. Electromagnets (Using Yoke)

  3. Central Conductor Magnetization

  4. Use of coils and solenoids
• Types of Magnetizing Current


  • Direct Current (DC)
  • Alternating Current (AC)
  • Rectified Alternating Current
• Direct Current (DC)


  Flows continuously in one direction at a constant voltage, very desirable when inspecting for subsurface defects because it generates a magnetic field that penetrates deeper into the material

  
• Alternating Current (AC)


  Reverses in direction at a rate of 50 or 60 cycles per second, when used to induce a magnetic field in ferromagnetic materials, the magnetic field will be limited to a thin layer at the surface of the component due to the "skin effect"

  
• Rectified Alternating Current


  AC can be converted to current which is very much like DC through the process of rectification, using rectifiers the reversing AC can be converted to a one directional current
• Skin effect


  When AC is used to induce a magnetic field in ferromagnetic materials, the magnetic field will be limited to a thin layer at the surface of the component

  
• It is recommended that AC be used only when the inspection is limited to surface defects
• Rectified Alternating Current


  AC can be converted to current which is very much like DC through the process of rectification
• Half Wave Rectified Alternating Current (HWAC)


  1. Single-phase alternating current is passed through a rectifier

  2. The reverse half of each cycle is blocked out

  3. A one-directional, pulsating current is produced

  4. The current rises from zero to a maximum and then returns to zero

  5. No current flows when the reverse cycle is blocked out
• Half Wave Rectified Alternating Current (HWAC)


  • The pulsation of the HWAC helps magnetic particle indications form by vibrating the particles and giving them added mobility
  • HWAC is most often used to power electromagnetic yokes

  
• Full Wave Rectified Alternating Current (FWAC) (Single Phase)


  1. Full wave rectification inverts the negative current to a positive current rather than blocking it out

  2. This produces a pulsating DC with no interval between the pulses

  3. Filtering is usually performed to soften the sharp polarity switching in the rectified current
• Full Wave Rectified Alternating Current (FWAC) (Single Phase)


  • Particle mobility is not as good as half-wave AC due to the reduction in pulsation
  • The depth of the subsurface magnetic field is improved

  
• Three Phase Full Wave Rectified Alternating Current


  • Three-phase current is often used to power industrial equipment because it has more favorable power transmission and line loading characteristics
  • When rectified and filtered, the resulting current very closely resembles direct current
• Stationary magnetic particle equipment wired with three-phase AC will usually have the ability to magnetize with AC or DC (three-phase full wave rectified), providing the inspector with the advantages of each current form
• Purposes of MPI equipment


  • Provide sufficient power of the right type
  • Provide suitable contact and coils for proper magnetization
  • Provide means of applying the magnetic particles
  • Provide well-lighted space for careful examination of the part of indication