CHAPTER 2 SECTION I INTRODUCTION TO LIQUID PENETRANT INSPECTION

Process Control in the Darkroom CONT. - TM-1-1500-335-23_103

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 2-1 CHAPTER 2 SECTION I INTRODUCTION TO LIQUID PENETRANT INSPECTION 2 LIQUID PENETRANT INSPECTION. 2.1 INTRODUCTION. 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. 2.1.1 GENERAL. 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. 2.1.2 Summary. 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. 2.1.3 Background. 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 After a suitable immersion time, the parts were withdrawn from the oil and the excess surface oil wiped off with rags or wadding.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 method. 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.

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