T.O. 33B-1-1
2-62
SECTION VI
DEVELOPERS
2.6
DEVELOPERS.
2.6.1
Summary.
This section covers the development process. Development follows penetrant application, dwell, and removal of excess
surface penetrant. The section contains basic, intermediate, and advanced information on the process theory, materials,
and application procedures. The first portion is introductory in nature, discussing the functions and required properties
of developers. The second portion is devoted to the mechanisms and theory of developers. The third portion covers
drying of the part after surface penetrant removal or application of a water base, aqueous, developer. The fourth
portion describes the materials and application procedures for dry, aqueous and nonaqueous developers. The section
concludes with a comparison of the various types of developers.
2.6.2
Fundamentals.
2.6.2.1
Functions Of A Developer.
The basic function of all developers is to improve the visibility of the entrapped penetrant indication. The improvement
in visibility is achieved through a number of mechanisms that include the following:
a. Assist in extracting the entrapped penetrant from discontinuities.
b. Spread or disperse the extracted penetrant laterally on the surface, thus increasing the apparent size of
the indication.
c. Improve the contrast between the indication and the background.
2.6.2.2
Self Development.
CAUTION
Self-development SHALL NOT be used in aircraft and engine maintenance
inspection where service-induced flaws must be detected.
Self-development is the formation of an indication without the application of a developer material. All penetrants are
capable of some degree of self-development since they will exude from a discontinuity and spread over the surface. The
critical factors are the size and volume of the discontinuities that must be detected. A relatively large volume of
entrapped penetrant is required, and self-development is not reliable in detecting small tight flaws.
2.6.2.3
Mechanisms of Developer Action.
2.6.2.3.1
Capillary Action.
Just as capillary action draws penetrant into a crack, it draws penetrant back out of a crack into the small spaces
between developer particles.
2.6.2.3.2
Adsorption versus Absorption.
Developer action involves adsorption and absorption. Adsorption refers to the collection of a liquid on the outer surface
of a particle due to adhesive forces. This action contributes to the developer particle build-up at a crack as the particles
adhere to the exuded penetrant. Absorption refers to the blotting action that occurs when a liquid merges into an
