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Properties of Penetrant Materials (Surface Energy/Surface Wetting…
Properties of Penetrant Materials
Surface Energy/Surface Wetting Capability
Surface Tension
Ability to hold surface molecule together due to cohesive forces between molecules of a liquid
Wetting Ability
Ability of penetrant to cover surface of part
Wetting occurs
When adhesive forces between liquid and second substances>cohesive forces between liquid molecules
Liquid Penetrant Testing
Surface energy of solid-gas interface>combined surface energies of liquid-gas & solid-liquid interfaces
Surface energy of solid-gas interface>surface energy of solid-liquid interface
Density/Specific Gravity
no direct effect on the performance of penetrant
commercial penetrant has specific gravity <1
made up of organic materials of low specific gravities
water contamination sinks to the bottom of penetrant tanks
Viscosity
Definition
Fluid's resistance to flow
Example
Water flow easily, has a lower viscosity than liquids such as ketchup
Effects
Little effect on ability of penetrant material to enter a defect
does not have an effect on the speed at which the penetrant fills defect
Excessive viscosity
Long dwell times
Low viscosity
Reduce dwell times but makes the penetrant prone to over washing
Ultraviolet & Thermal Stability
High intensity UV light
Lose brightness of fluorescent materials
Elevated Temperature
Heat degradation/'heat fade'
Excessive Heat
Evaporates more volatile constituents, increases viscosity, decreases penetration rate
Alters wash characteristics
Kills fluorescence of tracer dyes
Removabillity
Remove the penetrant without remove it from the flaw
removed from the sample surface as completely as to limit background fluorescence
Good surface wetting
adhesive forces must be stronger than the cohesive forces
Proper formulation of the penetrant materials provides the correct balancing of these forces
Dilution of the penetrant materials
affect the concentration of the dye
Reduce the dimensional threshold of fluorescence
Color & Threshold of Fluorescence
designed to fluoresce as close as possible to the eyes' peak response.
heat fade
dyes in flourescent-dye penetrants loses brightness at elevated temperatures
thermal stability required
good contrast sensitivity
high contrast enables objects to be easily seen