When to Move Beyond Thickness: Selecting Advanced NDT Based on Damage Mechanism

When Thickness Measurement Is Appropriate

UT thickness measurement is appropriate when the dominant damage mechanism produces uniform or near-uniform wall thinning. It is well-suited for general corrosion, internal corrosion with relatively uniform distribution, and CUI where the objective is to confirm wall loss has not exceeded minimum required thickness at a known location. It is efficient, widely applicable, and provides quantitative remaining thickness data when coverage is adequate.

When Thickness Measurement May Not Be Enough

• Stress Corrosion Cracking (SCC) — produces crack-like flaws that may cause failure at remaining wall thicknesses that appear acceptable in UT thickness readings
• Hydrogen-Induced Cracking (HIC) — produces blistering and internal delaminations that may not be detectable as simple wall thinning
• Hydrogen-Assisted Cracking in welds (SOHIC, HACC) — crack-like features in weld metal or HAZ regions
• High-Temperature Hydrogen Attack (HTHA) — produces microstructural damage and internal cracking not detectable by standard UT
• Creep and creep cracking — microstructural damage may precede macro-cracking
• Fatigue cracking — surface or near-surface cracks that may not reduce measured wall thickness until advanced
• Localized pitting — may be missed if CML grid density is insufficient
• Weld defects and lack of fusion — planar flaws not represented by wall thickness readings

Damage Mechanism-Driven NDT Selection

For Crack-Like Flaws (SCC, HACC, fatigue)

PAUT, TOFD, or time-of-flight techniques provide better capability for crack detection, sizing, and characterization. Surface-breaking cracks may be detected by magnetic particle testing or penetrant testing. Method selection depends on crack orientation, access, material, and required detection/sizing confidence.

For HIC and Internal Delamination

Dedicated UT scanning procedures using appropriate frequencies and scan patterns for laminar/stepwise cracking detection. TOFD and PAUT may be used for larger-area coverage.

For HTHA

Advanced UT techniques with appropriate frequency selection; velocity ratio measurements; attenuation measurements; TOFD. Assessment should also consider operating conditions relative to Nelson curves per API 941.

For Localized Pitting

Dense UT scanning with appropriate probe size; corrosion mapping using automated or semi-automated PAUT or UT for better spatial resolution. Digital radiography can also provide useful profile information.

The Role of Engineering Judgement

No NDT method is universally appropriate for all damage mechanisms. The inspection team must understand what failure mode is credible, what the flaw morphology looks like, how the flaw initiates and progresses, what detection and sizing confidence is required, and how the inspection result will be used in an engineering decision. These questions must be answered before an NDT technique is selected — not after.