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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)
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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
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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.
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