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ASNT NDT Level III study materials
ASNT NDT Level III study resources

ASNT NDT Level III: Selection of Techniques and Establishment of Acceptance Criteria

ASNT NDT Level III · Work through your exam, one topic at a time.

All guide sections

Selection of Techniques and Establishment of Acceptance Criteria

What you must know

Selection is a reasoned match between the expected discontinuity (type, orientation, location, size), the geometry and material, and the capability of the method: probability of detection, sizing accuracy, coverage, speed, access and cost.

Selection drivers:

  • Orientation - maximum response when the beam or flux is perpendicular to the flaw plane.
  • Location - surface vs subsurface, near-surface dead zones, one-sided access.
  • Material - conductivity, permeability, grain structure and attenuation.
  • Geometry - thickness, curvature, nozzle and weld configuration, cladding.
  • Documented requirement - the code, contract or specification may mandate the method.

Acceptance criteria come in two families:

  • Workmanship criteria - fixed limits on indication length, amplitude or density, reflecting good practice rather than a calculated margin.
  • Fitness-for-service / engineering critical assessment - allowable flaw size derived from fracture mechanics, applied stress and material toughness (as in API 579 or BS 7910).

Traps

  1. Assuming a code acceptance limit guarantees fitness for purpose - it does not, and vice versa.
  2. Comparing indications obtained by different techniques, frequencies or reference reflectors as if equivalent.
  3. Believing a tighter criterion is always better; it can produce excessive repair and rejection without any integrity benefit.
  4. Forgetting that a criterion is meaningless unless the technique, reference reflector and sizing method are stated with it.
  5. Ignoring sampling risk when 100 per cent coverage is not achieved.

Worked example

A thick-section nozzle-to-shell weld: through-thickness stress and inclusion stringers suggest planar, plate-parallel flaws, which are badly oriented for angle beams. Select straight-beam scanning plus a complementary surface method, and state acceptance against the contract criterion using the specified reference reflector.

Rule of thumb: if the flaw orientation is unknown, inspect from two directions roughly 90 degrees apart, and never set a criterion without naming the technique and sizing method.