Discontinuity Detection, Evaluation and Acceptance Standards
Recognising discontinuities
Volumetric discontinuities - gas porosity, shrinkage, slag and inclusions - generally appear as dark, rounded or irregular spots whose darkness reflects lower attenuation. Planar discontinuities - cracks, lack of fusion, incomplete penetration - appear as sharp dark lines, often thin and aligned with the weld preparation. Tungsten inclusions appear bright because tungsten attenuates more than steel. Casting radiographs show gas porosity as round spots, shrinkage as jagged dendritic patterns and sand as mottled irregular images. Service-induced corrosion and fatigue cracking appear as thinning or fine lines.
Evaluation versus acceptance
Evaluation is a technical judgement of type, size, location, distribution and orientation. Acceptance is a code judgement against stated criteria. Relevant standards include ISO 5817 with quality levels B, C and D, ASME VIII UW-51 with ASME V Article 2, API 1104, and reference radiograph standards such as ASTM E446 and E186 with severity levels. The code edition and acceptance class must be stated in the report; the same indication may pass under one class and fail another.
Traps
Calling every dark spot porosity without checking shape and distribution; grading a discontinuity against the wrong thickness or quality class; forgetting that reference radiographs are comparative, not absolute; and treating a discontinuity as a defect when the acceptance criteria permit it.
Rule of thumb
Dark and rounded usually means a void; dark and linear with sharp ends suggests a crack or lack of fusion; bright means a dense inclusion such as tungsten.