Crack-Like Flaws Require Fracture Mechanics, Not Corrosion-Rate Logic

Why Crack-Like Flaws Are Different from Corrosion Thinning

Corrosion thinning progressively reduces wall thickness. Failure occurs when the remaining thickness can no longer withstand operating pressure — a process that can often be predicted from corrosion rate data using API 570 or FFS Level 1 methods. Crack-like flaws behave fundamentally differently. A sharp crack concentrates stress at its tip. Under applied loading, the stress intensity at the crack tip may reach the material's fracture toughness — at which point unstable crack extension may occur regardless of the remaining average wall thickness.

Examples of Crack-Like Flaws in Industrial Assets

• Stress Corrosion Cracking (SCC) — environmentally assisted cracking at tensile stress locations
• Hydrogen-Induced Cracking (HIC) — internal cracking driven by diffused hydrogen
• SOHIC (Stress-Oriented HIC) — cracking in high-stress zones in wet H2S environments
• Fatigue cracking — progressive crack growth under cyclic loading
• Lack of fusion and lack of penetration — planar weld discontinuities
• Hot cracks and solidification cracks — weld metal cracking during solidification
• Cold cracks / hydrogen-assisted cracking in HAZ or weld metal
• Service-induced weld-root cracking — from cyclic pressure or thermal loading

Fracture Mechanics Assessment

Fracture mechanics-based assessment — primarily using BS 7910 or API 579-1/ASME FFS-1 — evaluates whether a crack-like flaw will remain stable under the combination of applied stress, residual stress, and material fracture toughness. The Failure Assessment Diagram (FAD) approach considers both fracture-driven failure and plastic collapse, providing a combined assessment that is appropriate for materials and conditions where both failure modes are credible.

Key Inputs for Fracture Mechanics Assessment of Crack-Like Flaws

• Flaw dimensions — height, length, orientation, location, and aspect ratio from NDE with sizing uncertainty applied
• Applied stresses — from pressure, mechanical loading, thermal gradients, and bending
• Residual stresses — from welding, repair, or forming — often the dominant input
• Material fracture toughness — Kmat or Jmat from testing or derived from Charpy impact data
• Material strength — yield strength and tensile strength at temperature
• Fatigue crack growth data — if cyclic loading is relevant
• Environmental effects — sour service, hydrogen content, temperature

What Not to Do with Crack-Like Flaws

Do not calculate the remaining life of a crack-like flaw using a "corrosion rate" approach. Do not accept a flaw as safe because remaining wall thickness exceeds code minimum. Do not assume a flaw detected by PAUT is safe because it is small — size relative to toughness and stress, not size alone, determines fracture risk. Do not apply Level 1 FFS general metal loss methods to crack-like indications.