Manufacturing Discontinuity Analysis

Learning outcomes

The learner should be able to classify discontinuities by process stage and morphology, build a cause-location-orientation hypothesis, avoid premature naming, distinguish relevant from non-relevant evidence and plan follow-up examination.

15.1 A structured classification

Manufacturing discontinuities can be grouped by when and how they form:

• Primary-metal and solidification: segregation, inclusions, gas porosity, shrinkage, hot tears and ingot pipe.
• Wrought processing: laminations, seams, laps, bursts, flakes, centreline cracking and rolled-in scale.
• Joining: incomplete fusion or penetration, porosity, slag, cracking, profile and dimensional conditions.
• Heat treatment: quench cracks, distortion, excessive hardness, soft zones, decarburisation and reheat cracking.
• Machining and finishing: grinding checks, tears, recast cracking, smeared laps, plating cracks and coating disbond.

Location and orientation often provide more diagnostic value than a generic shape. A linear indication at a weld toe, a planar response at plate mid-thickness and a rounded response in a casting hot spot have different prior probabilities.

15.2 Surface, subsurface and internal

Surface-breaking conditions interact with VT, PT and MT directly. Near-surface conditions may be detected by MT or ET depending on material and depth. Internal conditions commonly require UT or RT. "Subsurface" should be used with a defined depth context; a method's useful depth depends on frequency, material and geometry.

Planar conditions concentrate stress more severely when sharp and unfavourably oriented. Volumetric conditions remove section and can cluster. Neither label alone determines acceptance. Size, position, orientation, service stress and code criteria control evaluation.

15.3 From response to defensible interpretation

Use a five-part hypothesis:

1. Cause: Which process mechanism could create it?
2. Location: Does it lie where that mechanism acts?
3. Orientation: Does response movement or directional testing support the expected plane?
4. Morphology: Is it isolated, aligned, branched, rounded, diffuse or clustered?
5. Corroboration: Does another scan, view, method or production record agree?

Geometry, surface condition, permeability change, grain structure and technique artefacts can create non-relevant or false responses. The operator should challenge the initial label, especially when the consequence is high.

15.4 Mapping and reporting

Map indications in component coordinates with a declared datum. Record method, technique, surface, orientation, response, length or extent, depth where valid, and uncertainty. Separate observed data from the interpreted discontinuity type. If naming remains uncertain, report the evidence and limitation rather than selecting a dramatic name to make the report feel busy.

Practical checkpoint

Review ten anonymised indications from different products. For each, produce two plausible causes before choosing the most likely. Identify one test that would distinguish them. Mark whether acceptance can be evaluated from current evidence or needs further information.

Common errors and misconceptions

Several common errors recur in this topic: naming by screen shape alone; treating clustered porosity as one round pore; ignoring process records that contradict the first interpretation; reporting a defect before evaluation; and giving a precise depth from a technique not demonstrated for depth sizing.

Chapter summary

Discontinuity analysis is strongest when process mechanism, location, orientation, morphology and corroboration agree. Surface depth category guides method choice, while acceptance remains a separate controlled decision. Reports should preserve evidence and uncertainty.