Please enable JavaScript.
Coggle requires JavaScript to display documents.
CHAPTER 3: Liquid Penetrant Inspection (LPI) / Penetrant Testing (PT) -…
CHAPTER 3: Liquid Penetrant Inspection (LPI) / Penetrant Testing (PT)
Basic Process
of PT
Remove Excess
Apply Developer
Apply Penetrant
Visual Inspection
Clean & Dry Component
Choices of
Penetrant Materials
Types of
Penetrants
Dye Penetrants: solution of red dye which reveal red indication on the white developer
Florescent Penetrants: solution containing fluorescence which emits green fluorescence indication when view in dark
Method
Water washable
Solvent Removable
Post emulsifier
Developer
In the
form of:
Dry Powder
Wet, Water Soluble
Wet, Water Suspendable
Wet, Non-Aqueous
What can be
Inspected via PT
Almost any material that has a relatively smooth, non-porous surface on which discontinuities or defects are suspected
What can NOT be
Inspected via PT
Components with rough surfaces, such as sand castings, that trap & hold penetrant
Porous ceramics
Wood & other fibrous materials
Plastic parts that absorb/react with the penetrant materials
Components with coatings that prevent penetrants from entering defects
How does
PT work?
In penetrant testing, a liquid with high surface wetting characteristics is applied to the surface of a component under test
The penetrant "penetrates" into surface breaking discontinuities via capillary action and other mechanisms
Excess penetrant is removed from the surface and a developer is applied to pull trapped penetrant back the surface
With good inspection technique, visual indications of any discontinuities present become apparent
Properties that affect
Penetrant Testing
Surface energy
Density/specific gravity
Removability
Color & fluorescence brightness
Viscosity
Dimensional threshold of fluorescence
Ultraviolet & thermal stability
Visible vs
Fluorescent PT
Inspection can be performed using visible/fluorescent penetrant materials
Visible PT is performed under white light while fluorescent PT must be performed in dark area. All are in the level 1 sensitivity range
Fluorescent PT is more sensitive than visible PT because the eye is more sensitive to the bright indication on a dark background
Sensitivity ranges from level 1 - level 4
Penetrant
Removal Method
Solvent removable penetrants are removed by wiping with a cloth dampened with solvent
Water washable penetrants are removed by using spray water & easiest to employ & most cost effective when inspecting large areas
Post-Emulsifiable penetrants are water-washable only after they have reacted with an emulsifier solutions. This system is used when washing the penetrant out of the defect is a concern
6 Steps of
Penetrant Testing
Penetrant Application
Excess Penetrant Removal
Pre-clean
Developer Application
Inspect/Evaluate
Post-clean
Dwell time
The penetrant solution must be allowed to "dwell" on the surface of the part to allow the penetrant time to fill any defects present
Vary according to penetrant type, temperature, material type & surface finish
Are usually recommended by the specifications
The times vary
depending on
Application
Penetrant materials
Used materials
Form of material
Type of defect
Minimum dwell time (5-60) minutes
Advantages of
Penetrant Testing
Relative ease of use
Can be use on wide range of material types
Large areas / large volumes of parts/materials can be inspected rapidly at low cost
Parts with complex geometries are routinely inspected
Indications are produced directly on the surface of the parts which provide a visual image of the discontinuity/flaws/cracks
Initial equipment investment is low cost
Aerosol spray cans can make the equipment very portable
Limitations of
Penetrant Testing
Only detects surface breaking defects
Requires relatively smooth, nonporous materials
Precleaning is critical due to contaminants as the contaminants can mask the defects
Requires multiple operations under controlled conditions
Chemical handling precautions necessary (toxicity, fire, waste)
Metal smearing from machining, grinding & other operations inhibits detection. Materials may need to be etched prior to inspection
Post cleaning is necessary to remove chemicals
Summary
PT is one of the most widely used NDT methods
Its popularity can be attributed to 2 main factors, which are its relative ease of use & its flexibility
However, PT Involves a number of processing steps that must be closely control to achieve optimal sensitivity