Magnetic Particle Testing (MT)

A method for detecting surface and near-surface defects in ferromagnetic materials using magnetic fields and fine particles.

Introduction Principles of MT Equipment & Materials Procedure Advantages & Limitations Applications Safety & Considerations Quiz

Introduction

Magnetic Particle Testing (MT) is a nondestructive examination method used to detect discontinuities on or just below the surface of ferromagnetic materials. It works by inducing a magnetic field in the part, and applying fine magnetic particles (dry or suspended), which accumulate near leakage fields caused by defects.

Principles of MT

• Magnetization: creating a magnetic field within the test object (longitudinal / circular / transverse magnetization).
• Leakage fields: flaws like cracks interrupt magnetic flux, producing stray lines of flux at the surface.
• Particle attraction: magnetic particles are attracted to leakage field and form visible indications.
• Types of particles: wet fluorescent, dry, non-fluorescent, etc.

Equipment & Materials

• Magnetizing equipment (yoke, coil, or permanent magnet)
• Magnetic particle (dry powder or wet suspension)
• Illumination or UV light (for fluorescent particles)
• Cleaning equipment and demagnetization tools

Procedure

1. Pre-clean the surface to remove dirt, oil, or other contaminants.
2. Magnetize the component (establish the magnetic flux path).
3. Apply magnetic particles evenly over the surface.
4. Inspect under appropriate lighting or UV light for visible particle indications.
5. Demagnetize and clean the component after inspection.

Advantages & Limitations

• Very sensitive for surface and near-surface defects in ferromagnetic materials.
• Fast and relatively simple processing.
• Cannot detect defects deep inside non-ferromagnetic materials.
• Reliant on proper magnetization and adequate particle type & lighting.

Applications

• Inspection of welds, castings, forgings for cracks and laps
• Surface inspection in manufacturing and in-service maintenance
• Inspection of shafts, gears, and large components

Safety & Considerations

• Use proper eye protection, especially under UV lighting.
• Adequate ventilation if using wet suspensions or solvents.
• Ensure no residual magnetism remains in the component after inspection.

Non-Destructive Testing (NDT) Quiz — 40 Questions

Magnetic Particle Testing (MT)

Q1: What type of materials can be inspected using MT?

A) Ferromagnetic materials
B) Non-ferrous metals
C) Plastics
D) Ceramic materials

Answer: A) Ferromagnetic materials

Q2: MT is primarily used to detect which type of defects?

A) Internal voids
B) Surface and near-surface cracks
C) Material hardness variations
D) Thickness differences

Answer: B) Surface and near-surface cracks

Q3: What is the role of the magnetic field in MT?

A) To clean the surface
B) Apply penetrant
C) Induce leakage fields at discontinuities
D) Measure thickness

Answer: C) Induce leakage fields at discontinuities

Q4: What type of particles are used in MT?

A) Non-magnetic powders
B) Colored dyes only
C) Plastic beads
D) Ferromagnetic particles, dry or suspended in liquid

Answer: D) Ferromagnetic particles, dry or suspended in liquid

Q5: Which MT method is best for complex shapes?

A) Yoke method
B) Wet method with bath or spray
C) Radiographic
D) Visual inspection

Answer: B) Wet method with bath or spray

Q6: Which magnetization technique is used for long components?

A) Circular magnetization
B) Longitudinal with a yoke
C) Longitudinal coil or solenoid
D) Eddy current

Answer: C) Longitudinal coil or solenoid

Q7: How are MT indications detected?

A) Visible accumulation of particles at discontinuities
B) Audible sound
C) Temperature change
D) Electrical resistance

Answer: A) Visible accumulation of particles at discontinuities

Q8: What is the purpose of demagnetization after MT?

A) Apply penetrant
B) Increase magnetic field
C) Increase particle adherence
D) Remove residual magnetism to avoid interference

Answer: D) Remove residual magnetism to avoid interference

Q9: Which MT inspection is best for detecting surface cracks?

A) Immersion
B) Wet continuous method with yoke
C) Radiographic
D) Visual only

Answer: B) Wet continuous method with yoke

Q10: How does surface condition affect MT?

A) No effect
B) Only color matters
C) Surface must be clean, smooth, and free from oil
D) Only surface temperature matters

Answer: C) Surface must be clean, smooth, and free from oil

Q11: Which particle color is commonly used for visible MT?

A) Red or black
B) Green
C) Blue
D) White

Answer: A) Red or black

Q12: Fluorescent MT requires which type of light for detection?

A) White light
B) Ultraviolet (UV) light
C) Infrared
D) Visible blue light

Answer: B) Ultraviolet (UV) light

Q13: What type of current is used in circular magnetization?

A) Alternating current only
B) Direct current only
C) AC or DC depending on the method
D) Pulse current

Answer: C) AC or DC depending on the method

Q14: What is the primary limitation of MT?

A) Cannot detect surface cracks
B) Expensive equipment
C) Only works on non-magnetic metals
D) Limited to ferromagnetic materials

Answer: D) Limited to ferromagnetic materials

Q15: How can MT sensitivity be improved?

A) Clean and polish the surface
B) Reduce magnetic field strength
C) Use smaller particles only
D) Increase part thickness

Answer: A) Clean and polish the surface

Q16: Which MT method is most suitable for flat surfaces?

A) Yoke method
B) Longitudinal coil method
C) Circular magnetization
D) Residual magnetism

Answer: B) Longitudinal coil method

Q17: What is the effect of oil or grease on MT?

A) Improves particle visibility
B) Has no effect
C) Reduces particle adhesion and sensitivity
D) Creates false indications

Answer: C) Reduces particle adhesion and sensitivity

Q18: Which MT technique uses a permanent magnet?

A) Yoke with AC
B) Wet method
C) Coil method
D) Yoke with DC or permanent magnet

Answer: D) Yoke with DC or permanent magnet

Q19: What is the purpose of a residual magnetism check?

A) Apply coating
B) Ensure demagnetization is complete
C) Apply penetrant
D) Measure flaw depth

Answer: B) Ensure demagnetization is complete

Q20: Which MT method is suitable for hollow parts?

A) Circular magnetization
B) Yoke only
C) Longitudinal coil method
D) Wet bath only

Answer: C) Longitudinal coil method

Q21: What is the main advantage of fluorescent MT over visible MT?

A) Higher sensitivity and easier detection under UV light
B) Works without magnetization
C) No surface preparation needed
D) Detects non-magnetic materials

Answer: A) Higher sensitivity and easier detection under UV light

Q22: Which factor determines the penetration depth of MT?

A) Particle color
B) Magnetic field strength and material thickness
C) Ambient light
D) Surface roughness only

Answer: B) Magnetic field strength and material thickness

Q23: Which MT inspection is best for detecting fine surface cracks?

A) Dry powder only
B) Yoke method with AC
C) Wet fluorescent particles
D) Visual inspection only

Answer: C) Wet fluorescent particles

Q24: Why is magnetization perpendicular to a crack preferred?

A) Easier to apply particles
B) Reduces false indications
C) Only works on flat surfaces
D) Produces maximum leakage field at the discontinuity

Answer: D) Produces maximum leakage field at the discontinuity

Q25: What is the role of AC current in MT?

A) Detects deep internal flaws
B) Improves surface crack detection
C) Reduces particle adhesion
D) Demagnetizes the part

Answer: B) Improves surface crack detection

Q26: Which type of discontinuity produces a weak MT indication?

A) Large surface crack
B) Strong planar crack
C) Small subsurface void
D) Corrosion pit

Answer: C) Small subsurface void

Q27: What is the main disadvantage of dry powder MT?

A) Requires UV light
B) Cannot detect surface cracks
C) Only for non-ferrous metals
D) Less sensitivity and particle adhesion compared to wet method

Answer: D) Less sensitivity and particle adhesion compared to wet method

Q28: How is AC Yoke method performed?

A) AC current is applied through a yoke to create a magnetic field
B) Part immersed in DC bath
C) Dry powder applied only
D) Eddy current induced

Answer: A) AC current is applied through a yoke to create a magnetic field

Q29: Which MT inspection is best for detecting subsurface flaws?

A) AC surface magnetization
B) DC or half-wave DC magnetization
C) Dry powder AC
D) Visual inspection

Answer: B) DC or half-wave DC magnetization

Q30: Which MT method uses a solenoid or coil around the part?

A) Yoke method
B) Wet bath method
C) Longitudinal coil method
D) AC surface method

Answer: C) Longitudinal coil method

Q31: How does particle size affect MT sensitivity?

A) Larger particles improve sensitivity
B) No effect
C) Only particle color matters
D) Smaller particles improve sensitivity for fine cracks

Answer: D) Smaller particles improve sensitivity for fine cracks

Q32: What is a typical use of magnetic particle inspection?

A) Detect surface cracks on shafts, welds, and castings
B) Measure thickness
C) Detect corrosion
D) Determine hardness

Answer: A) Detect surface cracks on shafts, welds, and castings

Q33: Which MT flaw orientation is easiest to detect?

A) Parallel to magnetic field
B) Perpendicular to magnetic field
C) At 45 degrees
D) Random orientation

Answer: B) Perpendicular to magnetic field

Q34: Why is a wet fluorescent method preferred for critical components?

A) Lower cost
B) Can be used on non-magnetic metals
C) Higher sensitivity and clear indication
D) Works without cleaning

Answer: C) Higher sensitivity and clear indication

Q35: How are MT results documented?

A) By measuring thickness
B) By visual observation only
C) By photographing only
D) By recording particle indications, location, and orientation

Answer: D) By recording particle indications, location, and orientation

Q36: Which MT equipment is portable for field inspections?

A) Hand-held yokes and coils
B) Immersion tanks
C) Large coil benches only
D) Radiography cameras

Answer: A) Hand-held yokes and coils

Q37: How is proper magnetization verified?

A) By UV light
B) By using a magnetic field indicator or test block
C) By couplant thickness
D) By visual surface finish

Answer: B) By using a magnetic field indicator or test block

Q38: What is the purpose of a yoke in MT?

A) Apply couplant
B) Measure crack depth
C) Create localized magnetic field
D) Remove surface rust

Answer: C) Create localized magnetic field

Q39: Why is inspection in low-light or dark conditions preferred for fluorescent MT?

A) Reduce power consumption
B) Avoid surface glare
C) Easier particle adhesion
D) UV light makes fluorescent indications visible

Answer: D) UV light makes fluorescent indications visible

Q40: How is MT sensitivity affected by magnetization direction?

A) Maximum when perpendicular to the flaw
B) Maximum when parallel
C) No effect
D) Only affects internal defects

Answer: A) Maximum when perpendicular to the flaw

Quick links

Introduction Principles of MT Equipment & Materials Procedure Advantages & Limitations Applications