Definition of Terms

FIGURE 3-41 SEQUENCE OF STEEL PROCESSING STAGES, INDICATING THE PRINCIPLE OPERATION AND THE DEFECTS MOST LIKELY TO BE FOUND IN THE MATERIAL AFTER EACH PROCESS

Figure 3-43. Broad Indication of Subsurface Discontinuity in a Weld

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 3-91 3.7.2 Definition of Terms. The magnetic particle inspector must understand the distinctions between discontinuity, indication and defect. 3.7.2.1 Discontinuity. A discontinuity is an interruption in the normal physical structure or properties of a part. Discontinuities may be cracks, laps in the metal, folds, seams, inclusions, porosity, and similar conditions. A discontinuity may be very fine or it may be quite large. A discontinuity may or may not be a defect; that is it may or may not affect the intended use of the product or part. A discontinuity, which would be a defect in one part, may be entirely harmless in another part designed for a different service. 3.7.2.2 Indication. An indication is an accumulation of magnetic particles being held by a magnetic leakage field to the surface of a part. The indication may be caused by a discontinuity or it may be caused by some other condition that produces a leakage field or it may be caused by mechanically held particle accumulation. 3.7.2.3 Defect. A defect is a discontinuity that interferes with the intended use of a part. 3.7.3 Basic Steps of Inspection. Magnetic particle inspection can be divided into these three basic steps: a. Producing an indication on a part. b. Interpreting the indication. c. Evaluating the indication. 3.7.3.1 Producing an Indication. In order to produce a proper indication on a part, it is necessary to have some knowledge of the principles of magnetism, the materials used in inspection, and the technique employed. Since these subjects have been covered in previous sections of this manual, observance of the procedural steps therein should insure that a proper indication is produced. 3.7.3.2 Interpreting the Indication. After the indication is created, it is necessary to interpret that indication. Interpretation is the determination of what caused that indication. Knowledge of metal processing is often invaluable in identifying the cause of an indication. 3.7.3.2.1 Indications caused by a discontinuity at the surface of the part are characterized by particles that are tightly held to the surface by a relatively strong magnetic leakage field. The particle accumulation has well defined edges and there is a noticeable "build-up" of the particles. This build-up consists of a slight mound or pile of particles, which on deep surface cracks is sometimes high enough above the surface of the part to cast a shadow. If such an indication is wiped off the discontinuity can usually be seen. 3.7.3.2.2 Indications caused by a discontinuity below the surface are characterized by a broad and fuzzy looking accumulation of particles. The particles in such an indication are less tightly held to the surface because the leakage field is weaker. 3.7.3.2.3 The difference in appearance between indications of surface and subsurface discontinuities is clearly shown in Figure 3- 42 and Figure 3-43. Notice the sharpness and definition of the accumulation of magnetic particles in Figure 3-42. The pattern in Figure 3-43 is much broader than that in Figure 3-43 and is quite typical of the indications formed over subsurface discontinuities.

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