Define the Failure Mode First: Why FFS Should Not Treat All Damage as Thinning

Why Failure Mode Definition Comes First

FFS assessment methods in API 579-1/ASME FFS-1, BS 7910, and related standards are organized by damage type and associated failure mode. Parts covering general metal loss assess against plastic collapse or burst. Parts covering crack-like flaws assess against fracture and fatigue crack growth. Parts covering creep assess against time-dependent deformation and rupture. Each part assumes a specific failure mode and uses physics appropriate to that mode.

Applying the wrong part — most commonly applying general metal loss assessment to damage that includes crack-like characteristics — can produce an unconservative result because the assessment physics no longer match the actual failure mechanism.

Common Failure Modes in Pressure Equipment

• Plastic collapse or burst — from general wall thinning reducing remaining strength below yield-based acceptance
• Fracture — from crack-like flaws where stress intensity reaches material fracture toughness
• Fatigue crack growth — from cyclic loading causing crack propagation to critical size
• Creep and creep rupture — from elevated-temperature operation over time
• Brittle fracture — from low toughness materials at low temperature with pre-existing flaws
• Environmental cracking — SCC, HIC, SOHIC, hydrogen embrittlement — driven by combined stress and environment
• Buckling and geometric instability — in shells, heads, or local thin regions under compressive loading

Signs That the Failure Mode May Not Be Thinning

The engineer should suspect a failure mode other than simple plastic collapse when: the inspection method identifies crack-like features alongside metal loss; the damage mechanism review identifies SCC, HIC, SOHIC, fatigue, or creep as credible; the damage is in a high-stress zone such as a nozzle, weld, or area with known residual stress; the component operates in a sour, hydrogen, or low-temperature service environment; or the damage was initiated by a process upset, overpressure, or thermal event.

How to Select the Correct FFS Method

The selection sequence should be: first, identify the damage mechanism and its interaction with material, operating conditions, and geometry; second, determine the credible failure mode or modes — there may be more than one; third, select the appropriate FFS part and level that addresses the identified failure mode; and fourth, identify the inspection data, material properties, and stress inputs required for the selected method.

When Multiple Failure Modes Are Credible

Some damage conditions may involve more than one failure mode. For example, corroded pipe with embedded laminations may need both general metal loss assessment and laminar flaw assessment. A cracked and corroded nozzle may need both general thinning and fracture assessment. In these cases, all credible failure modes must be assessed — the most critical result controls the decision.