Inspection, Testing and Qualification of Welds
This domain tests method selection, standard acceptance levels and the geometry behind ultrasonic testing.
Methods and their limits
Visual testing (VT) finds surface profile, undercut, spatter and misalignment. Penetrant testing (PT) finds surface-breaking defects on any non-porous material. Magnetic particle testing (MT) finds surface and near-surface defects but only in ferromagnetic material. Radiography (RT) finds volumetric defects and is good for porosity and lack of penetration, but is poor for planar cracks aligned with the beam and carries radiation risk. Ultrasonic testing (UT) detects planar defects and is strong for thick sections, but depends on probe angle, surface condition and operator skill.
Acceptance levels
ISO 5817 defines quality levels B (stringent), C (intermediate) and D (moderate) for steel arc welds. Acceptance criteria for UT and RT are given in separate standards (e.g. ISO 11666 for UT). ISO 9606 covers welder qualification; ISO 15614 covers procedure qualification.
Worked example
A 20 mm thick butt weld is examined with a 70° shear-wave probe. Half-skip distance = 2 × t × tan 70° = 2 × 20 × 2.747 = 109.9 mm. Reasoning: the beam travels one full skip to the back wall and back to the probe; the half-skip is where the beam first reaches the far surface. This tells you where to index the probe and how much scanning length is needed to cover the root and cap.
Traps
Using PT on rough or painted surfaces. Applying MT to austenitic stainless or aluminium. Assuming RT detects all cracks. Mixing quality levels B/C/D with acceptance classes from a different standard.