EN 9934-1-2001 pdf download.Non-destructive testing – Magnetic particle testing – Part 1:General principles (ISO 9934-1 :2001).
Magnetizing techniques are described in the following Clauses.
NOTE More than one technique may be necessary to find discontinuities on all test surfaces and In all orientations. Demagnetization may be required where the residual field from the first magnetization cannot be overcome. Techniques other than those listed may be used povided they give adequate magnetization, In accordance with 8.1.
8.3.1 Current flow techniques
8.3.1.1 AxIal current flow
Current flow otters high sensitivity for detection of discontinuities parallel to the direction of the current.
Current passes through the component, which shall be in good electrical contact with the pads. A typical arrangement is shown in Figure 1. The current is assumed to be distributed evenly over the surface and shall be derived from the peripheral dimensions. An example of approximate formula for the current required to achieve a specified tangential field strength is given in annex A.
Care shall be taken to avoid damage to the component at the point of electrical contacts. Possible hazards include excessive heat, burning and arcing.
NOTE Certain contact materials such as copper or zinc may cause metallurgical damage to the component if arcing occurs. Lead contact pads may be used, but only in well ventilated conditions, because they may generate harmful vapours. Contact areas should be as clean and as large as practicable and of a material compatible with the component under test.
8.3.1.2 Prods; Current flow
Current is passed between hand-held or clamped contact prods as shown in Figure 2. providing an inspection of a small area of a larger surface. The prods are then moved in a prescribed pattern to cover the required total area. Examples of testing patterns are shown in Figure 2 and Figure 3. Approximate formulae for the current required to achieve a specified tangential field strength are given in annex A.
This technique otters the highest sensitivity for discontinuities elongated parallel to the direction of the current.
Particu’ar care shall be taken to avoid surface damage due to burning or contamination of the component by the prods, as for 8.3.1.1. The warning in this subclause concerning the use of lead prods should also be noted. Zinc plated or galvanised prods shall not be used. Arcing or excessive heating shall be regarded as a defect requiring a verdict on acceptability. If further testing is required on such affected areas, it shall be carried out using a different technique.
8.3.1.3 Induced current flow
Current is induced in a ring shaped component by making it, in effect, the secondary of a transformer, as shown in Figure 4. An example of an approximate formula for the induced current required to achieve a specified tangential field strength is given in annex A.
This method otters the highest sensitivity for discontinuities parallel to the direction of current flow. The example of approximate formula given in annex A for a central conductor is also applicable in this case. For a non-central conductor, the tangential field strength shall be verified by measurement.
8.3.2.2 Adjacent conductor(s)
One or more insulated current-carrying cables or bars are laid parallel to the surface of the component, adjacent to the area to be tested and supported a distance d above it, as shown in Figure 6 and Figure 7.
The adjacent conductor technique of magnetization requires the material being tested to be in close proximity to a current flowing in one direction. The return cable for the electric current shall be arranged to be as far removed from the testing zone as possible and, in all cases, this distance shall be greater than 10 d, where 2 d is the width of the tested area
The cable shall be moved over the component at intervals of less than 2 d to ensure that the inspection areas overlap. An example of an approximate formula for the current required to achieve a specified tangential field strength in the test zone is given in annex A.
8.3.2.3 Fixed installation
The component, or a portion of it, is placed in contact with the poles of an electromagnet, as shown in Figure 8.
8.3.2.4 Portable electromagnet (Yoke)
The poles of an a.c. electromagnet (yoke) are placed in contact with the component surface as shown in Figure 9.
The testing area shall not be greater than that defined by a circle inscribed between the pole pieces and shall
exclude the zone immediately adjacent to the poles An example of a suitable testing area is shown in Figure 9.
NOTE The magnetization requirements defined in 8.1, can only be met with a.c. electromagnets. D.c. electromagnets and permanent magnets may only be used by agreement at the Ume of enquiry and order.
8.3.2.5 Rigid coil
The component is placed within a current-carrying coil so that it is magnetized In the direction parallel to the axis of the coil, as shown in Figure 10. Highest sensitivity is achieved for discontinuities elongated perpendicular to the coil axis.
When using rigid coils of a helical form, the pitch of the helix shall be less than 25 % of the coil diameter.
NOTE For short components. where the length to diameter ratio is less than 5, It is recommended that magnetic extenders be used. The current required to achieve the necessary magnetization is thus reduced.
An example of an approximate formula is given in annex A for the current required to achieve a specified tangential field strength.EN 9934-1-2001 pdf download.