Downloaded from https://www.everyspec.com AMCP 706-196 Leakage FluxF -X -. .. ---- . .. .. . -........ ... _.. . -_ __ _ -_ __.-.... - - _ - - I - I - -- I - - i I (A) Surface Flaw (B) Intemral Flaw Figure 8-2. Effect of Flow of Magnetic Flux Lines' the defect, hut the particles are larger than The method depends on the type of flux the defect and are filtered out and left be- lines desired. Passing a current through a hind on the surface. These particles then give specimen generates circular flux lines around an indication of a flaw. Fluorescent particles the current path. A coil around a specimen can be tused to provide more contrast, and magnets produce longitudinal magneti- zation.
Both types of flux lines may be Liquid penetrant inspection is covered in needed because the extent to which it flaw detail in Refs, 3 and 7. perturbs flux lines depends on its orientation with respect to the direction of the lines. For example, a crack perpendicular to flux 8-6 MAGNETICS lines perturbs them whereas a crack parallel W to the lines may not. Thus, both flux This method is based on the fact that orientations may be necessary to detect all flaws in a magnetic material have magnetic flaws in a material, properties different from those of the material itself.
Once a magnetic field is in- Several methods can be used to detect the duced in a material, any flaw will perturb or perturbations caused by flaws and defects. distort the field because the flaw has a differ- The simplest is to pass a compass over the ant magnetic permeability and, thus, a differ- magnetized surface, The compass needle ent reluctance, than the material. The flaw is will align with the overall field except hi the located by measuring these perturbations. vicinity of a flaw. Although this method is Fig.
8-2 shows how flaws affect magnetic flux crude and insensitive, the same principle gives lines, good results when extended to distributing ir(,n filings - either dry or in a liquid su!i- A magnetic field can be set up in a magnetic pension - over the surface of interest. These material by: filings sometimes are coated with a fluorescent I material that produces a more visible pattern. 1. Passing a current through all or a portion 'i 1 ic filings line up with the induced magr"'tlc of the specimen field, except in the area of flaws or discojitinu- ities in the material. Another detection 2.
Passing a current through a coil sur- method is to pass a current-carrying search rounding or in .ontact with the Npecirnen coil over the surface. When the coil moves through a perturbation, a voltage is generated 3. Permanent magnets. between the coil and the inspected materbl. 8-8 4W