Part Cleanliness and Particle Application (2)

Question 1

Can MPI be done on a surface that has some contaminants?

Answer 1

yes, but ideally the part should be free of oils, grease and water as they may cause a lack of particle mobility.

Question 2

Describe Dry Particle applications

Answer 2

1. By hand (dusting)
2. Shaker bottle
3. electrostatic gun or powder blower

Main objective = thin even dispersion on examination surface (“cloud” of particles).

Question 3

Advantages of the Dry Powder method

Answer 3

• portable
• locating subsurface (DC or HWRAC)
• less temperature restriction
• low cost
• easy use

Question 4

Disadvantages of the Dry Powder method

Answer 4

• less sensitive to fine surface leakage
• no automation
• difficult to cover surface quickly
• irregular shaped parts pose difficulty

Question 5

Describe the Wet Particle application

Answer 5

1. spray
2. stream/flow from bath
3. immersion

• Portable or stationary
• requires agitation to ensure particles don’t “settle”

Question 6

Advantages of Wet Particles

Answer 6

• sensitive to fine surface leakage
• highly mobile particles
• easy to cover surfaces
• fast
• re-use particle bath
• easier to reproduce results
• automation

Question 7

Disadvantages of Wet Particles

Answer 7

• not reliable for subsurface
• stationary form is expensive and requires a re-circulating system
• post cleaning
• temperature restrictions
• monitor particle concentration in solutions

Question 8

Describe the Continuous application method

Answer 8

Particles are applied simultaneously with the magnetization of the part under test.

• greatest amount of sensitivity
• don’t apply particles after the mag force has ceased it will “wash” away finer discontinuities.

Question 9

Describe the Residual application method

Answer 9

Particles are applied after the magnetizing force on the test specimen has ceased.

• relies on material retentivity and residual field
• particles remain on part for a while to allow particles to move to leakage fields
• excellent means of differentiating relevant and non-relevant defects.

Question 10

Colours of magnetic particles

Answer 10

• white
• black
• red
• yellow
• fluorescent

Question 11

Which colour of magnetic particle is considered the best?

Answer 11

Fluorescent under subdued or blackened conditions with a UV light
- almost impossible to miss
-

Question 12

Do pigments affect a particles magnetic properties?

Answer 12

yes a little, but it is better that you are able to visually see them vs. them having a higher magnetic force.

Question 13

What are “contrast aids”?

Answer 13

• white pigment suspended in a quick drying, colourless solvent
• chalk
• paints
• aluminum finishing

used to provide better contrast with the colour of magnetic particles.