Types of penetrant (3)
- Fluorescent - type I
- Visible - type II
- Dual - type III
Fluorescent penetrant
- type I
- dyes that fluoresce when exposed to black light
- normally a yellow-green colour
Visible Dye penetrant
- Type II
- dye visible in white light
- Red
- simplest to use
Dual Sensitivity Dye penetrant
- white or black light
- white light = gross discontinuities
- black light = fine discontinuities
Sensitivity classifications for fluorescent penetrant
ASTM E1417 - Mil-I-25135 1/2 = very low 1 = low 2 = medium 3 = high 4 = ultra high
All penetrants must:
- penetrate fine openings
- remain in coarse openings
- not evaporate
- easy clean
- come out of openings
- brilliance
- colour permanence
- not react with material
- stable storage
What is Capillary action?
MOST important principle of penetrant testing
- fluid drawn into a small opening
Fluid properties that enhance capillary action (4)
- adhesion
- cohesion
- wetting ability
- surface tension
Define Adhesion
the attraction between two molecules of different makeup
- penetrant adhering to the surface
Define Cohesion
the attraction between molecules of the same make up
- penetrant “pulling” more dye
Define Wetting Ability
related to adhesion, ability to spread across a surface
- measured by “contact angle”
Define Surface Tension
related to cohesion, how attracted fluid is to itself
- LPI wants low surface tension so as to flow into discontinuities rather than “balling up”
Other penetrant properties: (8)
- viscosity
- specific gravity (density)
- volatility
- flash point
- inertness
- solubility
- solvent ability
- emulsifiability
Define Viscosity
how thick or runny a liquid is
- determines how long the “exiting” process will take
Define Specific Gravity (Density)
- fresh water = SG 1
- penetrants have <1
- oil floats on water
Define Volatility
ease at which a material evaporates
- high volatility chemicals are flammable
Define Flash Point
the lowest temperature at which a product will give off enough vapour to catch fire if a source of ignition is present
- low flash point = greater risk of fire
Define Inertness
tendency of a substance to remain stable under a wide range of conditions and not adversely react
Define Solubility
Ability to be removed from a surface
- removal solvents ease of removing penetrant
Define Solvent Ability
ability of a penetrant to hold dye to make it brilliant (visible)
Define Emulsifiability
ability to use a soap like substance and water to make a penetrant washable
