Shim Type/Foil Strips (Type I And Type Ii Indicator Strips)

These are small brass or silver finish strips containing artificial discontinuities (3 slots) in Mu metal; Mu metal is a highly permeable material containing nickel, manganese and iron. The brass finish strips are commonly used in general engineering applications and indicate low field strengths. The silver finish strips will show higher field strengths and are used in aerospace industries or areas that require a more critical examination.

In either case, if the field is strong enough, three linear indications will show on the strip; they should be rotated on the specimen surface to define field direction which will be at right angles to the indications.

A drawback with the foil strips is that they do not show what field you have in the test material, only in the air just above the test surface. Consequently, indications from a field would show even if the material was non-ferromagnetic. It is also possible for any residual magnetism in the part to cause an indication.

Fig. 8.2 - Type I Indicator Strip.

With reference to the brass finish strip (type I), if no indications are apparent the field is too weak for inspection and it is below 3 amps A.C. r.m.s./mm. Three indications show that the field strength is greater than 5 amps A.C. r.m.s./mm.

With reference to the silver finish strip (type II), if no indications are apparent the field is below 5 amps A.C. r.m.s./mm. Three indications show that the field strength is greater than 15 amps A.C. r.m.s./mm and the field is suitable for critical inspection.

Some manufacturers produce both Type I and Type II with a brass finish. Some other manufacturers differentiate between the two different types with the letters ‘A’ and ‘G’.

Note: The above figures are based on a mild steel bar and only meet the BS 6072 requirements.