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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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Use and Selection of a Developer

The use of developer is almost always recommended. One study reported that the output from a fluorescent penetrant could be multiplied by up to seven times when a suitable powder developer was used. Another study showed that the use of developer can have a dramatic effect on the probability of detection (POD) of an inspection. When a Haynes Alloy 188, flat panel specimen with a low-cycle fatigue crack was inspected without a developer, a 90 % POD was never reached with crack lengths as long as 19 mm (0.75 inch). The operator detected only 86 of 284 cracks and had 70 false-calls. When a developer was used, a 90 % POD was reached at 2 mm (0.077 inch), with the inspector identifying 277 of 311 cracks with no false-calls. However, some authors have reported that in special situations, the use of a developer may actually reduce sensitivity. These situations primarily occur when large, well defined defects are being inspected on a surface that contains many nonrelevant indications that cause excessive bleedout.

Type of Developer Used and Method of Application

Nonaqueous developers are generally recognized as the most sensitive when properly applied. There is less agreement on the performance of dry and aqueous wet developers, but the aqueous developers are usually considered more sensitive. Aqueous wet developers form a finer matrix of particles that is more in contact with the part surface. However, if the thickness of the coating becomes too great, defects can be masked. Also, aqueous wet developers can cause leaching and blurring of indications when used with water-washable penetrants. The relative sensitivities of developers and application techniques as ranked in Volume II of the Nondestructive Testing Handbook are shown in the table below. There is general industry agreement with this table, but some industry experts feel that water suspendable developers are more sensitive than water-soluble developers.

Sensitivity ranking of developers per the Nondestructive Testing Handbook.
Sensitivity Ranking (highest to lowest) Developer Form Application Technique.

Ranking
1
2
3
4
5
6
7
8
9
10
Developer Form
Nonaqueous, Wet Solvent
Plastic Film
Water-Soluble
Water-Suspendable
Water-Soluble
Water-Suspendable
Dry
Dry
Dry
Dry
Method of Application
Spray
Spray
Spray
Spray
Immersion
Immersion
Dust Cloud (Electrostatic)
Fluidized Bed
Dust Cloud (Air Agitation)
Immersion (Dip)

The following table lists the main advantages and disadvantages of the various developer types.

Developer
Advantages
Disadvantages
Dry

Indications tend to remain brighter and more distinct over time

Easily to apply

Does not form contrast background so cannot be used with visible systems

Difficult to assure entire part surface has been coated

Soluble

Ease of coating entire part

White coating for good contrast can be produced which work well for both visible and fluorescent systems

Coating is translucent and provides poor contrast (not recommended for visual systems)

Indications for water washable systems are dim and blurred

Suspendable

Ease of coating entire part

Indications are bright and sharp

White coating for good contrast can be produced which work well for both visible and fluorescent systems

Indications weaken and become diffused after time
Nonaqueous

Very portable

Easy to apply to readily accessible surfaces

White coating for good contrast can be produced which work well for both visible and fluorescent systems

Indications show-up rapidly and are well defined

Provides highest sensitivity

Difficult to apply evenly to all surfaces

More difficult to clean part after inspection

To review a summary of some of the research that has been done on developer usage and performance, take this link.

Research on Developer Use

References:

- Brittain, P. I., The Amplifying Action of Developer Powders, QUALTEST 3 Conference, Cincinnati OH, Oct 1984.

- Rummel, W. D., Probability of Detection as a Quantitative Measure of Nondestructive Testing End-To-End Process Capabilities, Materials Evaluation, January 1998, pp. 35.

- Nondestructive Testing Handbook, Vol. 2, Liquid Penetrant Tests, Robert McMaster, et al., American Society for Nondestructive Testing, 1982, pp. 283-319.