Introduction to Non-Destructive Testing Techniques - Magnetic Particle Testing

Magnetic Particle Testing – Comprehensive Study Notes

Basic Principles of Magnetic Particle Testing

• Magnetic Particle Testing (MPT) uses magnetic fields and small magnetic particles (e.g., iron filings) to detect flaws in ferromagnetic components.
• Inspectable material requirement: ferromagnetic materials that can be magnetized (e.g., iron, nickel, cobalt, and their alloys).
• Applicability: casts, forgings, weldments; industries include structural steel, automotive, petrochemical, power generation, aerospace; underwater inspection possible for offshore structures and pipelines.
• Core concept: MPT combines magnetic flux leakage testing with visual (magnetic) indications. The magnetic field exits/enters the material at poles (north and south). A crack or defect creates a small air gap which causes flux leakage (the field spreads in air where magnetic support is weaker).
• When iron particles are sprinkled on a magnetized part, they cluster at the flux leakage fields, including edges of cracks, forming visible indications that reveal the flaw.
• First step in MPT: magnetize the component. Defects near/at the surface create leakage fields detectable by particle indications.

Ferromagnetic Materials and Field Response

• Materials response to magnetic fields is governed by magnetic moments arising from electron motion, external-field effects, and electron spin.
• In most atoms, electrons are paired with opposite spins, yielding no net magnetic moment. Materials with unpaired electrons can exhibit net magnetic response.
• Material classes by magnetic response:
  • Diamagnetic: weak, negative susceptibility; repelled by magnetic fields; no permanent net moment when external field removed; examples: copper, silver, gold.
  • Paramagnetic: small, positive susceptibility; slightly attracted; some unpaired electrons; no permanent moment after field removal; examples: magnesium, molybdenum, lithium.
  • Ferromagnetic: large, positive susceptibility; strongly attracted to magnetic fields and can retain magnetization after external field is removed; strong magnetic properties due to magnetic domains where moments align in large numbers; examples: iron, nickel, cobalt.
• Ferromagnetic materials are commonly inspected with MPT due to their high permeability and ability to hold a magnetization state.

Magnetic Field Fundamentals

• Magnetic field concepts:

  • Magnetic field lines (lines of force) exit/enter materials at magnetic poles; poles exist in pairs (dipole).
  • A bar magnet is a dipole with a north and a south pole; flux leakage occurs at edges/cracks due to air-gap effects.

• The magnetic field is produced by moving charges; electrons produce magnetic fields as they move; a current-carrying conductor creates a magnetic field.

• Faraday's Law of Magnetic Induction describes how external fields affect magnetic moments within materials.

• Field sources include permanent magnets, electromagnets, and current-carrying conductors; right-hand rule (clasp rule) determines field direction around a conductor:

  • If you grasp a conductor with the right hand, thumb in current direction, fingers curl in the direction of the magnetic field.

• SI unit concepts used in magnetism:

  • Magnetic field strength H: H ext{ is measured in } ext{A/m}. 1 A/m corresponds to the center of a circular conductor with diameter 1 m carrying a steady current of 1 A.
  • Flux density (magnetic induction) B: B ext{ is measured in Tesla (T)}. 1 T = 1 N/(A·m) in the given context; the flux density relates to the magnetic field in a material.
  • Magnetic flux ɸ: oldsymbol{\Phi} ext{ (Weber, Wb)}. Total flux is the flux density over an area; flux units compact as Weber.
  • Magnetization M: M ext{ in A/m}. It measures the magnetic dipole moment per unit volume.
  • Permeability μ: \mu = \frac{B}{H}. This describes the ease with which a material can establish a magnetic flux in response to an applied field; the slope of the B–H curve at a point.

• Relationship among B, H, and μ: B = \mu H where \mu is the material's permeability. Some texts also use relative permeability μr or absolute μ.

• Hysteresis loop (B–H loop): shows how B responds to changing H; provides key material properties (retentivity, coercivity, permeability, etc.).

• Units (summary):

  • Field strength: H in A/m.
  • Flux density: B in Tesla.
  • Flux: \Phi in Weber.
  • Magnetization: M in A/m.
  • Permeability: \mu (dimensionless in relative form μr; absolute μ has units of H/m).

The Hysteresis Loop and Magnetic Properties

• A hysteresis loop demonstrates the relationship between induced flux density (B) and magnetizing force (H).
• Key points on the loop:
  • Saturation (point a): almost all domains aligned; beyond this, increasing H yields little increase in B.
  • Retentivity (B at point b): residual flux density when H is reduced to zero; material’s ability to retain magnetization.
  • Coercivity (H at point c): amount of reverse magnetic field needed to bring flux to zero.
  • Re-saturation (points d, e, f): loop behavior in reverse direction; some residual magnetism remains; loop path does not retrace to the origin due to remanence.
• Quantities derived from hysteresis:
  1) Retentivity: residual flux density after saturation, i.e., the value of B when H returns to zero on the reverse path.
  2) Residual magnetism: residual flux density when the magnetizing force is zero; equal to retentivity at saturation, otherwise may differ.
  3) Coercive force (coercivity): the field strength needed to demagnetize to zero flux.
  4) Permeability: ease of flux establishment, defined as the slope of the B–H curve, i.e., \mu = \Dfrac{dB}{dH} at a given point.
• Relative versus absolute permeability:
  • The permeability value given in literature is often the maximum permeability, typically where the tangent from the origin touches the B–H curve.
• Material loop differences:
  • Materials with wider loops tend to have lower permeability, higher retentivity, higher coercivity, higher reluctance, and higher residual magnetism.
  • Materials with narrower loops tend to have higher permeability, lower retentivity, lower coercivity, lower reluctance, and lower residual magnetism.

Permeability and Material Characteristics

• Permeability μ describes how readily a material supports the formation of a magnetic field within itself:
  • High carbon content in steel tends to reduce permeability and increase remanent magnetism; i.e., high-carbon components may retain more flux and be less penetrable by the defect-detecting field.
• Practical implication: materials with different carbon contents and alloying show different residual magnetism and flux leakage behavior, affecting sensitivity and indication clarity in MPT.

Field Orientation and Flaw Detectability

• Detectability depends on crack orientation relative to magnetic lines of force:
  • Longitudinal field: magnetic lines run parallel to the part’s long axis; produced by coils/solenoids or permanent magnets/electromagnets in longitudinal setup.
  • Circular (circumferential) field: lines run around the part; produced by current through the component or by current in a surrounding conductor; field is strongest at the surface along the circumference.
• Best defect detection occurs when lines of force intersect the defect at 45 to 90 degrees, creating a conspicuous flux leakage.
• Practically, parts are magnetized in two directions at right angles to each other to detect defects oriented variously:
  • If the field is parallel to the defect, flux leakage is minimal and the indication may be weak or absent.
• Example: for a component where current passes along the end-to-end direction, a circular field is produced; longitudinal defects aligned with the current may still be detected, but transverse defects may be more detectable with a longitudinal magnetization.

Methods of Magnetization (Direct vs Indirect)

• Direct magnetization (induction): current is passed directly through the component, generating a circular magnetic field around the path of the current.
  • Methods include:
  • Clamping the component between two electrical contacts in a dedicated fixture; DC or AC can be used; magnetization strength proportional to current;
  • Using clamps or prods placed on the component to allow current to flow from contact to contact.
  • Important cautions: ensure good electrical contact to avoid arcing and overheating; residual field remains after the current is stopped (DC more penetrating in ferromagnetic materials).
• Indirect magnetization (external field): uses external magnetic fields to magnetize the component:
  • Permanent magnets for low-cost, simple setups but with limited field control.
  • Electromagnetic yokes (adjustable) for stronger, controllable fields; current through the yoke’s soft iron core creates a strong local field between north and south poles.
  • Central conductor method: current through a central conductor induces a circular field within/around cylindrical components.
  • Coil/solenoid: a long component can be magnetized by placing it in the center of a coil to create a longitudinal field;