Inspection, Testing and NDE: Method Selection and Timing
What to master
Know the capabilities and limits of visual, penetrant, magnetic particle, ultrasonic, radiographic and phased-array inspection. VT is the first line and requires access, lighting and surface condition. PT detects surface-breaking discontinuities in non-porous materials. MT detects surface and near-surface flaws in ferromagnetic materials. RT reveals volumetric flaws and is sensitive to density and thickness changes; UT detects planar flaws and requires a couplant and trained interpretation. Understand sensitivity, coverage, scan overlap, calibration and the significance of indication versus defect. Timing matters: hydrogen-assisted cracks may not appear immediately, and PWHT can change the inspection window.
Traps
A common trap is using PT or MT to find internal lack of fusion. Another is treating an indication as a defect without applying acceptance criteria. RT is poor at detecting tight planar cracks oriented parallel to the beam, while UT is poor at detecting scattered porosity without a suitable scan. Do not compare NDE methods by 'accuracy' without defining the flaw type. Also, surface preparation and temperature can invalidate PT/MT results.
Worked example
For a 10 mm butt weld inspected with a 70° angle beam, the first leg path is T / cos 70° = 29.2 mm, and the surface skip distance is 2T tan 70° = 54.9 mm. These values set the scan index and help confirm that the probe covers the root and sidewall regions. The inspector should mark the index offset, verify calibration on a reference block, and record the angle, frequency and sensitivity.
Quality Assurance, Auditing and Inspection Programme Management
What to master
Distinguish quality control from quality assurance: QC is inspection and testing of the product; QA is the planned system that makes quality predictable. Know the quality plan, inspection and test plan (ITP), hold points, witness points, review points and surveillance. Understand document control, traceability, nonconformance reports (NCR), corrective action requests (CAR), root-cause analysis and verification of closure. Audits should be independent, evidence-based and planned; ISO 19011 provides audit principles, and ISO 3834 provides welding quality requirements. Key performance indicators should drive improvement, not just count defects.
Traps
A frequent error is releasing a hold point without the required signature or record. Another is closing an NCR by rework alone without investigating cause, so the problem recurs. Do not confuse a witness point with a hold point: at a witness point the work may proceed if the notified party does not attend, unless the contract says otherwise. Audit independence matters - the person responsible for the work should not audit their own area. Also, calibration certificates and personnel certificates must be checked for scope and validity, not just presence.
Worked example
An ITP lists root-pass inspection as a hold point. Production cannot proceed to fill passes until the inspector signs the release. If the same ITP lists final visual inspection as a witness point, the contractor must notify the client, but if the client does not attend, the contractor may proceed and record the absence. The rule of thumb: a hold point stops work; a witness point only requires notification.