Developer Selection and Use
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 |
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.