Plastic Collapse

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). ...

This is the third of four problems on the welded A533B-1 plate. The first two both sat in the low load-ratio brittle-fracture regime and compared residual stress; this one shifts the battlefield — to the high load-ratio, large-plasticity regime. For the shared background and method behind all four problems, see the overview post [[bs7910-a533b-residual-stress-fad|Where does residual stress push the assessment point?]]. What this problem asks HLAW = High-$L_r$ + as-Welded: as-welded condition, assessment temperature raised to −30 ℃, load increased into the high load-ratio regime. The warmer temperature lifts the fracture toughness somewhat off the lower shelf ($K_{mat}=62\ \mathrm{MPa\cdot m^{0.5}}$), while the load is raised to a failure load of 5.10 MN. The residual stress stays the as-welded value ($K_I^S=46$). ...