Types of
Developers
There are several types of developers in common use in
liquid penetrant testing: (1) dry powder, (2) water soluble, (3) water
suspendible and (4) nonaqueous (solvent
suspendible).
The basic component of all developers is manufactured
as a powder and performs its basic function as a power on the surface of a part
being inspected. The developers by the listed types according to the condition
of the condition of the developer at the time of application to a part during
the liquid penetrant testing process.
The second and third listed types are
often referred to as aqueous wet or simply aqueous and the fourth as nonaqueous
wet because they are wet when applied.
Actions of
developers
Actions of developers in forming liquid penetrant indications
Developers action appears to be a combination of solvent affect, adsorption. (Adsorption implies collection by adhesion to a surface. Absorption refers to assimilation of liquid into the bulk of an absorbent material.)
The developer powder exerts a combination of adsorption and absorption effects on liquid penetrant residues, drawing entrapped liquid penetrant to the test part surface as the liquid penetrant dispersed in the developer powder.
It forms
test indications that can be readily observed by the inspector.in the case of
the nonaqueous wet and special film type developers, solvent action has been
shown to play a part in promoting the withdrawal action and enhancing the
liquid penetrant test indications.
Application of developer coatings of excessive
thickness can cover, obscure or extinguish the fluorescent or visible color
discontinuity the fluorescent or visible color discontinuity indications by the
thick overlying layer of porous powder developer.
A normal effect of developer
layers of appropriate thickness is the lateral diffusion or bleeding of liquid
penetrant from discontinuity entrapments.
This lateral migration of liquid
penetrant through the developer coating may cause small adjacent discontinuity
indications to eventually become lost within the spreading indications from
large discontinuity entrapments.
For this reason, it is often desirable for
test operators to examine the test parts soon after developer application and
periodically thereafter throughout the recommended development time, to observe the initial appearance and growth
of discontinuity indications.