Pressure vessels, pipelines, and structures develop flaws during service — cracks, corrosion, wall thinning. When an inspection finds such a flaw, engineers face one key question:

Can this equipment keep running? If so, for how long?

This is exactly what Fitness-for-Service (FFS) assessment answers.

The Limit of Traditional Rules

Traditional design codes (such as ASME or GB 150) are written for new, flaw-free equipment. When a flaw is found, the codes often say: “out of tolerance — repair or retire.”

But in practice, not every flaw that breaks a manufacturing tolerance will cause a failure. An overly conservative call creates two problems:

  1. Unnecessary shutdowns — lost production time and repair costs;
  2. False confidence — the repair itself may introduce new flaws.

FFS assessment replaces rule-of-thumb conservatism with fracture mechanics and materials science, giving engineers a documented, defensible verdict on whether a flawed component is still safe.

The Core Tool: The Failure Assessment Diagram (FAD)

The most widely used FFS tool is the Failure Assessment Diagram (FAD).

The FAD has two axes, each measuring a different failure mode:

  • Vertical axis $K_r$ (fracture ratio): crack driving force divided by material fracture toughness — measures the risk of brittle fracture;
  • Horizontal axis $L_r$ (load ratio): applied load divided by the limit load — measures the risk of plastic collapse.

Plot the assessment point $(L_r, K_r)$ for the flawed structure on the diagram:

  • Point inside the Failure Assessment Line (FAL): flaw is acceptable — the component can keep running;
  • Point on or outside the FAL: flaw is not acceptable — repair or retire.
$$ K_r = \frac{K_I^P + K_I^S}{K_{mat}} + \rho \leq f(L_r) $$

Here $K_I^P$ is the primary stress intensity factor, $K_I^S$ is the secondary stress contribution (e.g. residual stress), $K_{mat}$ is the material fracture toughness, and $\rho$ is the plasticity correction term.

Main Assessment Standards

Three standards are widely used around the world for FFS:

Standard Typical use
BS 7910 Welded structures, pressure vessels (UK / international)
API 579-1 / ASME FFS-1 Refinery and process pressure equipment (USA / global)
ASME XI Nuclear power plant pressure boundaries (USA nuclear)

All three share the same FAD framework in principle, but differ in option levels, formula details, and scope.

What MechCalc Is Building

MechCalc turns these assessment methods into open, step-by-step online calculators:

  • Every calculation step is shown with the standard clause and formula reference;
  • Full PDF calculation reports can be exported;
  • Three-language interface (Chinese / English / German) for international engineering teams.

This blog covers the theory, standard interpretation, and engineering examples behind the tools — so engineers don’t just use a tool, they understand the physics behind it.