INTRODUCTION TO LIQUID PENETRANT INSPECTION
LIQUID PENETRANT INSPECTION.
Penetrant is used to detect discontinuities, i.e., cracks, pits, etc., open to the surface on parts made of nonporous
materials. This method depends on the ability of the penetrant to enter into a surface discontinuity in the material to
which it is applied.
a. Due to its ability to inspect ferrous and nonferrous parts of all sizes and shapes, and its portability, the
liquid penetrant NDI method can be used at both depot and at field repair stations. For a specific
aircraft type, a technical manual on nondestructive inspection is used to define the method, technique,
equipment preparation, and precautions required to perform NDI on each component of the aircraft. A
separate manual is used for engines.
b. With wider use of the eddy current NDI method, liquid penetrant, long a primary NDI method, is now
becoming the secondary method for many applications. This is a result of the improved sensitivity of
new eddy current inspection techniques and the fact that eddy current does not require use and disposal
of potentially hazardous chemicals. For batch inspection of large areas, the penetrant method is still
preferred due to the shorter total process time when compared to eddy current.
This section contains introductory information for management, supervisors, and non-NDI personnel. The information
can also be used in the indoctrination training of beginning NDI personnel. The section provides background
information and outlines the basic penetrant process. It describes the purpose and some reasons for selecting the
penetrant inspection methods. Personnel qualification requirements for performing penetrant inspections are also
discussed. Penetrant test equipment is briefly discussed in general terms. The capabilities/advantages and
limitations/disadvantages of the process are also discussed.
a. Liquid penetrant inspection is one of the oldest of modern nondestructive inspection methods. It began
in the railroad maintenance shops in the late 1800s. Parts to be inspected were immersed in used
machine oil. After a suitable immersion time, the parts were withdrawn from the oil and the excess
surface oil wiped off with rags or wadding. The part surfaces would then be coated with powdered
chalk or a mixture of chalk suspended in alcohol (whiting). Oil trapped in cracks or flaws would bleed-
out causing a noticeable stain in the white chalk coating. This became known as the oil-and-whiting
b. The oil-and-whiting method was replaced by magnetic particle inspection on steel and ferrous parts in
1930. However, industries using non-ferromagnetic metals, especially aircraft manufacturers, needed a
more reliable and sophisticated tool than discolored machine oil and chalk. In 1941, fluorescent dye
materials were added to highly penetrating oil to make a penetrant material. Colored dyes, primarily
red, were produced a little later. Since then, a large number of penetrant systems or families have
evolved. These include developments in the following: various types and concentrations of dye
materials; types of penetrating oils and additives; materials and methods for removing the excess
surface penetrant; and various materials and forms of developing agents.