Indication Interpretation

Learning outcomes

The learner should be able to classify false, non-relevant and relevant indications, describe linear and rounded morphology, use appearance time and location, recognise common process artefacts and develop a defensible interpretation.

9.1 Three relevance classes

A false indication is created by contamination or incorrect processing rather than a component feature. Examples include lint, developer cracks, fingerprints, water marks and penetrant transferred from gloves.

A non-relevant indication is a genuine response from an intended feature or geometry not subject to rejection as the discontinuity of concern. Threads, press fits, keyways and surface transitions may retain penetrant. Whether it is non-relevant must be demonstrated, not assumed.

A relevant indication is caused by a discontinuity that must be evaluated under the applicable criteria. Relevant does not automatically mean rejectable.

9.2 Morphology and arrangement

Linear indications have length substantially greater than width under the governing definition and are commonly associated with cracks, laps, seams or lack of fusion. Rounded indications have comparable length and width and may represent porosity or pits. A cluster, aligned group or diffuse area can be significant even when individual marks are small.

Indication shape is affected by opening shape, penetrant volume, removal, developer and development time. A subsurface branching crack may produce a simple surface line. A pore at an angle can bleed as an elongated mark. Morphology supports interpretation but does not prove it.

9.3 Location and time behaviour

Rapid strong bleed-out at a weld toe supports a different hypothesis from slow diffuse bleed-out over a rough casting. Relevant locations include weld toes and roots, sharp radii, holes, thread roots, formed edges, heat-treated section changes and service stress concentrations.

Observe the indication at controlled intervals. Repeated bleed-out after careful wipe-off may show a retained reservoir, but any wipe-off technique must be expressly permitted. Excessive wiping can destroy evidence and contaminate the area.

9.4 Interpretation workflow

1. Verify process validity and viewing conditions.
2. Mark the indication location without obscuring it.
3. Observe appearance, growth, colour/brightness and arrangement.
4. Compare with component geometry and process history.
5. Check for false-indication sources.
6. Use permitted magnification, clean viewing aids or complementary NDT.
7. Classify relevance, then measure for evaluation.
8. Preserve images with scale, datum and viewing details when required.

Practical checkpoint

Interpret a set of training samples containing cracks, pores, threaded geometry, lint and developer artefacts. For each, write the observed evidence first, then the likely source and a confidence statement. Do not see the answer key until the evidence statement is complete.

Common errors and misconceptions

Several common errors recur in this topic: calling any straight line a crack; declaring geometry non-relevant without demonstrating its source; evaluating a false indication instead of repeating an invalid process; measuring after excessive bleed-out has merged indications; and photographing without scale or location datum.

Chapter summary

Interpretation separates process artefacts, intended features and relevant discontinuities. Morphology, location, arrangement and time behaviour provide evidence, but none is conclusive alone. Process validity must be confirmed before evaluation.