MMET 402 EXAM 1 QUESTIONS AND
ANSWERS ALL CORRECT
Why do cracks propagate from the surface usually - Answer- Machining and other
manufacturing processes cause thousands of tiny cracks on the surface making it more
likely to start from the surface
What is the difference between production and in-service inspection - Answer-
Production inspections are conducted during or/and at the end of a manufacturing
process for a part, while in-service inspections are during routine maintenance and
usually only have to worry about cracks
What kind of materials work best with MT testing - Answer- Ferromagnetic materials
When magnetized, what happens to the magnetic domains - Answer- They become
aligned in the same direction causing lines of magnetic flux (a magnetic field)
What is a magnetic domain - Answer- Small regions with an overall spin orientation,
look like snake scales before magnetizing
What is the right hand rule - Answer- Fingers wrap around and point in direction of
current flow, thumb points up towards magnetic field
What is H and B - Answer- H: Applied magnetic field from a coil
B: Induced magnetic field in a material being magnetized by H
What types of magnets produce what kinds of hysteresis loops (B/H graphs) - Answer-
Paramagnetic and Diamagnetic: Straight Line (B/H=1)
Ferromagnetic: Hysteresis Loop (B/H>1)
Nonmagnetic: No response
Where is the residual magnetic field, coercive force, and saturation on hysteresis loop -
Answer- Residual magnetic field: B when H=0
Coercive force: H when B=0
Saturation: Max or min H and B
What happens to the domains during a magnetizing-demagnetizing hysteresis cycle -
Answer- Demagnetizing on top and bottom, Magnetizing top right or bottom left,
, Hard vs Soft magnets - Answer- Hard: Large coercive field, ideal for permanent
magnets
Soft: Small loop area, small energy loss per cycle, good for AC transformers
How is magnetic permeability and what does it measure - Answer- u=B/H (induced flux
density/applied magnetic field strength)
Measures the ease a B field can be induced by an external field H
What is the relative permeability of common magnets - Answer- Nickel: 250
Cobalt: 600
Iron: 5000
Diamagnetic: 0.9995
Paramagnetic: 1.01
What are the properties of each magnetic material classification - Answer-
Ferromagnetic: Most useful, produces a much stronger reinforcing field parallel to
applied field
Paramagnetic materials: Can produce a slight reinforcing field parallel to applied field
Diamagnetic: Will produce a slight opposing field to applied field
What is the strongest ferromagnetic material - Answer- NdFeB: Neodymium Iron Boron
rare earth magnet
What is magnetic flux leakage - Answer- When a discontinuity near the surface or on
the surface causes the magnetic flux lines to warp around it and leak above the surface,
attracting magnetic particles
At what angle should the magnetic field be from the crack to detect it - Answer- atleast
45 degrees but ideally 90
What are the two inspection methods for MT testing - Answer- Continuous: Magnetic
particles are applied while current is still flowing during a magnetization process, used
extensively for field inspection with dry particles
Residual: Magnetic particles are applied after current has ceased, good for materials
with good residual magnetic fields, can be done in batches
Both use wet or dry particles
What are the 4 magnetization techniques - Answer- Current Flow (Direct): Magnetizing
a part by passing current directly through it creating a circular magnetic field (headshot)
Coil (Indirect): Magnetize a part consistently through a magnetic field from a large
electromagnet (coil shot)
Central Conductor (Indirect): A copper rod used to circularly magnetize ring-like parts,
able to magnetize multiple parts without passing current through it
What are the guidelines for headshots - Answer- 1000 A of current per inch of diameter
and 0.25in^2 of contact area to prevent overheating
ANSWERS ALL CORRECT
Why do cracks propagate from the surface usually - Answer- Machining and other
manufacturing processes cause thousands of tiny cracks on the surface making it more
likely to start from the surface
What is the difference between production and in-service inspection - Answer-
Production inspections are conducted during or/and at the end of a manufacturing
process for a part, while in-service inspections are during routine maintenance and
usually only have to worry about cracks
What kind of materials work best with MT testing - Answer- Ferromagnetic materials
When magnetized, what happens to the magnetic domains - Answer- They become
aligned in the same direction causing lines of magnetic flux (a magnetic field)
What is a magnetic domain - Answer- Small regions with an overall spin orientation,
look like snake scales before magnetizing
What is the right hand rule - Answer- Fingers wrap around and point in direction of
current flow, thumb points up towards magnetic field
What is H and B - Answer- H: Applied magnetic field from a coil
B: Induced magnetic field in a material being magnetized by H
What types of magnets produce what kinds of hysteresis loops (B/H graphs) - Answer-
Paramagnetic and Diamagnetic: Straight Line (B/H=1)
Ferromagnetic: Hysteresis Loop (B/H>1)
Nonmagnetic: No response
Where is the residual magnetic field, coercive force, and saturation on hysteresis loop -
Answer- Residual magnetic field: B when H=0
Coercive force: H when B=0
Saturation: Max or min H and B
What happens to the domains during a magnetizing-demagnetizing hysteresis cycle -
Answer- Demagnetizing on top and bottom, Magnetizing top right or bottom left,
, Hard vs Soft magnets - Answer- Hard: Large coercive field, ideal for permanent
magnets
Soft: Small loop area, small energy loss per cycle, good for AC transformers
How is magnetic permeability and what does it measure - Answer- u=B/H (induced flux
density/applied magnetic field strength)
Measures the ease a B field can be induced by an external field H
What is the relative permeability of common magnets - Answer- Nickel: 250
Cobalt: 600
Iron: 5000
Diamagnetic: 0.9995
Paramagnetic: 1.01
What are the properties of each magnetic material classification - Answer-
Ferromagnetic: Most useful, produces a much stronger reinforcing field parallel to
applied field
Paramagnetic materials: Can produce a slight reinforcing field parallel to applied field
Diamagnetic: Will produce a slight opposing field to applied field
What is the strongest ferromagnetic material - Answer- NdFeB: Neodymium Iron Boron
rare earth magnet
What is magnetic flux leakage - Answer- When a discontinuity near the surface or on
the surface causes the magnetic flux lines to warp around it and leak above the surface,
attracting magnetic particles
At what angle should the magnetic field be from the crack to detect it - Answer- atleast
45 degrees but ideally 90
What are the two inspection methods for MT testing - Answer- Continuous: Magnetic
particles are applied while current is still flowing during a magnetization process, used
extensively for field inspection with dry particles
Residual: Magnetic particles are applied after current has ceased, good for materials
with good residual magnetic fields, can be done in batches
Both use wet or dry particles
What are the 4 magnetization techniques - Answer- Current Flow (Direct): Magnetizing
a part by passing current directly through it creating a circular magnetic field (headshot)
Coil (Indirect): Magnetize a part consistently through a magnetic field from a large
electromagnet (coil shot)
Central Conductor (Indirect): A copper rod used to circularly magnetize ring-like parts,
able to magnetize multiple parts without passing current through it
What are the guidelines for headshots - Answer- 1000 A of current per inch of diameter
and 0.25in^2 of contact area to prevent overheating
