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Introduction to Penetrant Testing

Introduction
History
Improving Detection
—Visual Acuity
—Contrast Sensitivity
—Eye's Response to Light

Principles
Steps for Liquid PI
Common Uses for PI
Pros and Cons of PI

PT Materials
Penetrant Testing Matl's
Penetrants
—Surface Energy
—Specific Gravity
—Viscosity
—Color and Fluorescence
   —Why things Fluoresce
—Dimensional Threshold
—Stability of Penetrants
—Removability
Emulsifiers
Developers

Methods & Techniques
Preparation
—Cleaning Methods
—Metal Smear
Technique Selection
Application Technique
Penetrant Removal
Selecting Developer

Quality & Process Control
Temperature
Penetrant
Dwell
Emulsifier
Wash
Drying
Developer
Lighting
System Performance Check

Other Considerations
Defect Nature
Health & Safety

References

Quizzes
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Viscosity

Viscosity describes a fluid's resistance to flow. Liquids such as water that flow easily, have a lower viscosity than do liquids such as ketchup. Viscosity has little effect on the ability of a penetrant material to enter a defect but it does have an effect on the speed at which the penetrant fills a defect. The equations for the fill times of a  cylindrical void and an elliptical void are shown below:

Cylindrical Void
Fill time = (2l2m)/ rcosq sLG

Elliptical Void
Fill time = [(2l2m) / sLGcosq] * [a2+b2 / (a+b)ab]

Where: l = defect depth
m = viscosity
r = radius of the crack opening
s LG = liquid-gas surface tension
q = contact angle
a = flaw width
b = flaw length

From these equations, it can be seen that fill time is directly proportional to penetrant viscosity. While it has no real bearing on this discussion, it should be noted that the two equations do not take into account entrapped gas that could be present in a closed end capillary.

Reference:

Deutsch, S. A, Preliminary Study of the Fluid Mechanics of Liquid Penetrant Testing, Journal of Research of the National Bureau of Standards, Vol. 84, No. 4, July - August 1979, pp. 287-291.