Reference Stress

A fracture assessment watches two things at once — how close to brittle fracture, and how close to plastic collapse. Annex M (the stress intensity factor) covers the first and gives the vertical axis $K_r$ of the Failure Assessment Diagram (FAD); this guide’s reference stress $\sigma_{ref}$ (Annex P) covers the second and gives the horizontal axis $L_r$. Together they complete the Clause 7 fracture assessment . Prologue: the horizontal axis measures “how close to plastic collapse” A cracked structure can fail along two paths: brittle fracture, where the crack-tip driving force exceeds the toughness, and plastic collapse, where the cracked section as a whole yields and loses its capacity. The first is measured by $K_I$ (vertical axis); the second relies on the reference stress $\sigma_{ref}$ (horizontal axis). ...

The fracture assessment of a cracked structure watches two competing failure modes. This guide uses a single Failure Assessment Diagram (FAD) to weave together the assessment principle, the calculation chain, and the BS 7910 assessment steps into one clear storyline. By the end you should be able to see how an assessment point is computed and how it is judged acceptable or not. Prologue: after a crack is found When an in-service pressure vessel, pipe, or welded structure is inspected, crack-like flaws are often found. The question the engineer must answer is not “can this crack be used” — a crack is not something you “use” — but rather: can the structure that contains this flaw still operate safely under the current loads? ...