Discontinuities, Interpretation and Reporting

Outcomes

After this chapter, the learner should be able to:

• relate product history to likely discontinuities;
• distinguish manufacturing and service-induced conditions;
• interpret indication morphology with appropriate caution;
• apply a structured evaluation sequence;
• create a complete examination record and report; and
• state coverage and acceptance decisions accurately.

12.1 Product technology directs the search

Discontinuities are not distributed randomly. Material processing creates predictable orientations and locations. A technique should be designed around those expectations while retaining enough second-direction coverage to reveal unexpected conditions.

MT cannot confirm the full depth of a crack from particle length or brightness. It also cannot reliably distinguish two mechanisms with identical surface orientation without supporting history or another examination.

12.2 Welding examples

Weld inspection begins with process knowledge. High-restraint joints can develop delayed hydrogen-assisted cracking. Crater regions may contain shrinkage cracking. Lack of fusion is planar and can be detectable only where it reaches or approaches a test surface and is favourably oriented. Rounded particle patterns may arise from surface porosity, but spatter, undercut and weld profile also produce non-relevant collection.

Examine each accessible surface with field directions that cross expected longitudinal and transverse cracking. The weld toe is both a likely crack location and a geometry transition, so repeatability, orientation response, cleaning and visual correlation are important.

12.3 Forging, casting and wrought-product examples

Forging laps can be tight, linear and aligned with material flow. Bursts may be internal but become MT-relevant if they approach a surface. Casting hot tears are irregular linear conditions at restrained or changing sections. Rolled seams tend to extend along the product axis. Grinding cracks may form a fine network or repeated transverse lines.

The Level 2 technician should avoid reporting a specific manufacturing label merely because a textbook photograph looks similar. Report observable location, length, orientation and classification, then add the likely origin only when supported by component history.

12.4 Interpretation sequence

Use a repeatable sequence:

1. Observe: note pattern, direction, sharpness, distribution and background.
2. Validate the process: confirm field direction, media, lighting and timing.
3. Eliminate false causes: clean and repeat without losing the original record.
4. Evaluate non-relevant causes: compare geometry, permeability and field boundaries.
5. Classify: relevant/non-relevant/false and linear/non-linear as required.
6. Characterise: measure location, length, separation and grouping according to the product rule.
7. Evaluate: apply acceptance criteria only when authorised.
8. Record: preserve raw observations, limitations and disposition.

If an indication cannot be confidently discounted as false or non-relevant, treat it as requiring further evaluation and recording.

12.5 Measurement and sizing

Particle indications are larger than the discontinuity opening. Measure the indication according to the governing rule and state what was measured. Use a scale in photographs, control camera angle and retain component orientation. For grouped indications, record individual and overall distribution dimensions if the acceptance rule requires them.

Do not infer depth from brightness. A strong surface crack, a larger shallow buried feature and local over-magnetisation can all create bright patterns for different reasons.

12.6 Examination record

The contemporaneous record should enable another qualified person to understand what was done. Include as applicable:

• job, client, location and component identity;
• drawing, weld or area reference;
• material and product form;
• stage of manufacture/service and surface condition;
• procedure, technique sheet and acceptance document;
• equipment and instrument identifiers;
• magnetising method, waveform, current basis, settings, pole/contact spacing and coil data;
• media, carrier, batch and contrast coating;
• equipment/media/system check results;
• viewing measurements;
• field directions, placement coverage and inaccessible areas;
• raw indication observations, calculations, sketches and photographs;
• repairs, retests and deviations; and
• technician identity, qualification and date/time.

12.7 Final report

The report is a controlled summary of the examination and result. It should identify the organisation and location, component, controlling documents, technique, equipment, media, surface preparation, viewing conditions, date, personnel, limitations and results. If residual field is controlled, record the measured final value and location. State whether reported indications comply with the named criteria; do not use an unqualified "pass" when areas were inaccessible.

Separate observed fact from interpretation:

• Observation: "Fine linear fluorescent indication, 14 mm long, at the 3 o'clock weld toe, oriented circumferentially."
• Interpretation: "Relevant linear indication, repeatable in the field direction that crosses the line."
• Evaluation: "Not acceptable to clause X of the named product standard."

12.8 Recording methods

Written notes, scaled sketches, photographs and authorised transfer techniques can all be used. Transparent tape lifts and magnetic-rubber replicas are specialised records whose distortion and traceability must be understood. A photograph should include identity, orientation and scale, not just a dramatic green line floating in darkness.

12.9 Common traps

• Reporting "crack" when the evidence supports only "linear relevant indication."
• Applying an acceptance rule from memory or from another product sector.
• Omitting field direction from an indication photograph.
• Measuring an indication after particles have run or dried.
• Hiding inaccessible areas behind a general "100% inspected" statement.
• Transcribing a clean final report while discarding contradictory raw notes.

Chapter summary

Product history predicts likely discontinuities, but field evidence determines what was actually observed. Interpretation separates false, non-relevant and relevant patterns. A defensible record connects component identity, technique, controls, raw observations, evaluation and limitations.