b. Film-type developers. Permanent records can also be made using strippable coating developers.
c. Laser scanning with a variety of computer driven, data retrieval systems are being developed for the
inspection of large critical parts.
d. Digital computerized pattern-recognition systems are being developed to record flaws as variations to a
recognized pattern. Some types of pattern recognition systems are able to discriminate between linear,
crack-like indications and rounded, non-critical indications.
SPECIAL PURPOSE MATERIAL
SPECIAL PURPOSE MATERIALS.
The materials described in this section are not covered by the military specification
on penetrant materials.
There are a number of penetrant materials that are different than the materials described in the previous sections.
These materials are formulated for special applications and purposes. This section describes these special purpose
materials and discusses the reasons for their use. Application procedures are not covered. The procedures vary widely
between materials and manufacturers. Each of the manufacturers provides detailed application procedures for the
particular material when it is procured.
Oxygen Compatible Penetrants.
Oxygen has a high degree of chemical reactivity. It will explosively react or combine with a large number of materials.
This includes traces or residues from normal penetrant inspection materials. There are special cleaning procedures to
be used on parts and components that will be contacting gaseous or liquid oxygen. Simple disassembled parts can be
penetrant inspected and sent to the cleaning shop for complete removal of residual inspection materials in the usual
manner. Difficulties are encountered with assembled parts (on or off of aircraft) and complex shaped parts containing
crevices, recessed areas, or faying surfaces where inspection materials become trapped and are not easily removed by
cleaning. Such items should be inspected by another nondestructive test method or special penetrant materials used,
which do not react with oxygen. There are liquid oxygen (LOX) compatible materials available by special order.
These materials are mainly intended for use on space vehicles and can be used on aircraft when required.
Requirements for Lox Compatible Materials.
Testing for LOX compatible materials involves dropping a weight on the material in a liquid oxygen environment. If
the material is not compatible (i.e., will readily burn in an oxygen rich atmosphere), it will cause an audible explosion,
a visible flash in a darkened room, discolor the impact surface, or leave evidence of charring. There are two ways of
avoiding a LOX reaction from penetrant materials:
a. Completely remove all conventional inspection material residues.
b. Use only process materials that are inert in an oxygen environment.