Field Geometry and Coverage Planning

Outcomes

After this chapter, the learner should be able to:

• distinguish longitudinal and circular fields;
• apply the right-hand rule to current-flow techniques;
• relate equipment orientation to discontinuity direction;
• design overlapping, two-direction coverage; and
• recognise dead zones, edge effects and geometry-driven field distortion.

4.1 Longitudinal fields

A longitudinal field follows the main axis of the magnetised region and produces external poles. It can be created by:

• a permanent U-shaped magnet;
• an AC or DC electromagnetic yoke;
• a rigid or flexible coil;
• magnetic flow between bench heads; or
• a suitable arrangement of adjacent conductors.

The term describes field direction, not equipment portability. A yoke placed across a weld can produce a local longitudinal field between its poles, while a coil produces a longitudinal field through its aperture.

A discontinuity is best detected when it lies across the field. With a yoke, a linear crack broadly parallel to the line joining the two poles is poorly oriented; a crack transverse to that line is favoured.

4.2 Circular fields

Current passing through a conductor creates a circular field around the current path. The conductor may be the component itself or a separate central conductor. Common techniques include:

• prods or contact clamps passing current through a local region;
• axial current flow through a solid component;
• a threading bar or cable through a bore; and
• induced current in a ring-shaped component.

Use the right-hand grip rule: point the right thumb in the conventional current direction; curled fingers show the circular field direction. For a straight component carrying axial current, a longitudinal crack interrupts the circular field and is favoured.

4.3 Two-direction examination

Unknown or multi-directional discontinuities require at least two effective magnetisations, usually approximately 90 degrees apart. Each shot must be planned so the field intersects a different discontinuity family. Rotating a yoke, repositioning a coil, or combining contact-head and coil shots are common approaches.

Two-direction coverage does not mean two arbitrary equipment placements. Each placement needs:

• sufficient field at the surface;
• a known effective area;
• overlap with adjacent placements;
• access to view the indication while it is preserved; and
• a documented relationship between field and expected discontinuity.

When a bench applies both circular and longitudinal fields sequentially, the circular shot is commonly made first and the longitudinal coil shot last. The final longitudinal residual field is generally easier to detect and remove. The approved technique remains controlling.

4.4 Coverage with a yoke

The effective area is within and near the poles, but it is not the entire rectangle suggested by the yoke outline. Sensitivity falls toward the edges and changes with pole spacing, leg angle, surface curvature and contact. A practical scan plan uses a grid:

1. set the permitted pole spacing;
2. mark or visualise overlapping placements;
3. energise only when both poles are stable on the component;
4. apply media during the specified field period;
5. inspect before moving; and
6. repeat with the yoke rotated about 90 degrees.

Do not drag an energised yoke over the wet surface. It smears indications, creates artefacts and turns a controlled test into magnetic finger painting.

4.5 Coils and long parts

A coil field is strongest near the inside wall and varies along its axis. A long component must therefore be examined in overlapping sections, commonly at intervals related to coil length. Low fill factor - a small component in a large coil - produces an inefficient magnetic circuit and requires a calculation or field demonstration appropriate to the procedure.

The component's major axis should align with the coil axis. Pole extenders may improve the effective L/D for a short part, but their material, contact and geometry must be appropriate. Adding steel nearby without controlling the magnetic path can distort rather than improve the field.

4.6 Hollow and irregular components

A central conductor is preferred when the inside surface of a hollow item must receive a circular field. For a centred conductor, the inner-surface current calculation uses the relevant inside perimeter; the outer-surface calculation uses the outside perimeter. An offset conductor creates local coverage, and the effective region must be demonstrated or measured. Large rings require multiple overlapping placements.

Corners of square sections, branch joints, keyways and abrupt diameter changes redistribute flux. The technician must avoid assuming equal field from a single current value over every face.

4.7 Practical: field-direction sketch

Before touching the equipment, draw three arrows on the technique sketch:

1. current direction, if current flows through a conductor;
2. resulting magnetic field direction at the test surface; and
3. discontinuity direction favoured by that field.

If the three arrows cannot be justified, the technique is not ready to use.

4.8 Common traps

• Confusing current direction with field direction.
• Saying a yoke "detects cracks between the legs" without stating orientation.
• Leaving narrow untested bands between placements.
• Treating coil aperture as uniformly sensitive.
• Using one head shot on a stepped shaft when current ratios exceed the permitted range.
• Assuming a central conductor tests only the outside surface.

Chapter summary

Field direction is the bridge between equipment and discontinuity detectability. Longitudinal and circular fields are complementary. A defensible scan plan shows how each placement creates a verified field, what orientation it favours and how adjacent areas overlap.