Properties of Penetrant Material
Surface energy/Surface Wetting Capability
wetting ability -ability to freely wet the surface of the object being inspected
-If liquid molecules stronger attraction to molecules of solid surface than to each other wetting of surface occurs / spreading on surface
Surface Tension - Ability to hold surface molecule together
measure of the energy required to form a unit area of new surface at interface
To quantify a liquid's surface wetting characteristics - to measure the contact angle of a drop of liquid placed on the surface of an object
Liquids wet surfaces when the contact angle is less than 90 degrees
Capillarity
A driving force that draws fluid into narrowing opening (gravity)
Force = 2 prs LG cosq ( capillary force driving the penetrant into the crack is a function of the surface tension of the liquid-gas interface, the contact angle, and the size of the defect opening)
- Only have slight to negligible effect on the performance of a penetrant
- Increasing the specific gravity will increase the penetration speed.
When the gravitational pull is working against the capillary rise, the strength of the force is given by the following equation: Force = pr2hpg
Where:
r = radius of the crack opening
h = height of penetrant above its free surface
p = density of the penetrant
g = acceleration due to gravity
When the direction of capillary flow is in the same direction as the force of gravity, the added force driving the penetrant into the flaw is given by the formula shown below: Force = hAp
Where:
h = height of penetrant above its free surface
A = cross-sectional area of the opening
P = density of the penetrant
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Little effect on the ability of a penetrant material to enter a defect
Does have an effect on the speed at which the penetrant fills a defect.
Color and Fluorescent Brightness
Visible penetrant
Dye penetrant is usually vibrant red
Fluorescent penetrant
Contain one or more dyes that absorb electromagnetic radiation over a particular wavelength
Dye must provide good contrast against the developer. Contrast sensitivity : The higher the contrast, the easier objects are to see
Effect of color and fluorescence is not so straightforward
- Dyes fluoresce when exposed to UV radiation.
- One dye absorbing ultraviolet radiation to emit a band of radiation that makes dye glow.
Capillary Pressure = 2 s LG cosq)/ r
for cylindrical defect. Only apply when there is simultaneous contact of the penetrant along entire length of crack opening and a liquid front forms that is an equidistant from the surface.
A liquid penetrant will continue filled void until an opposing force balances the capillary pressure
Removability
without removing it from the flaw
must be removed from the sample surface as completely as possible to limit background fluorescence
Hence, adhesive forces of the penetrant must be weak enough that they can be broken by the removal methods used
Proper formulation of the penetrant materials needed to provides the correct balancing of adhesion and cohesive forces.
Formulation should not easily commingle and become diluted by the cleaning solution affect the concentration of the dye and reduce the dimensional threshold of fluorescence
Ultraviolet and Thermal Stability
Exposure to intense ultraviolet light and elevated temperatures - a negative effect on fluorescent penetrant indications.
At an elevated temperature, penetrants can experience heat degradation or "heat fade."
At excessive heat:
- evaporates the more volatile constituents which increases viscosity and adversely affects the rate of penetration.
- alters wash characteristics
- "boils off" chemicals that prevent separation and gelling of water soluble penetrants.
- kills the fluorescence of tracer dyes.
Heat causes the number of molecular collisions to increase, which results in more collision relaxation and less fluorescence but when the materials cool, the fluorescence will return
As at high temperature, penetrant solutions dry faster
Molecules become more closely packed in the dehydrated solution, collision relaxation increases and fluorescence decreases.
Generally, thermal damage occurs when fluorescent penetrant materials are heated above 71oC
When the temperature approaches 94oC, there is almost total attenuation of fluorescent brightness of the composition and sublimation of the fluorescent dyestuffs.
Lai Yoke Siew 134316
Lim Shu Sien 134319
Tan Hui Fang 134332
Density/Specific Gravity
Viscosity
Fluid's resistance to flow
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