Clause 7: Fracture Assessment Calculator

BS 7910:2019 Clause 7 — FAD assessment with $K_I^P+K_I^S$, $\rho$ and reserve factors

BS 7910 Clause 7 — Fracture Assessment

This calculator follows the BS 7910:2019 Clause 7 procedure to decide whether a known flaw is acceptable.

Principle. The Failure Assessment Diagram (FAD) puts two competing failure modes on one chart. The vertical axis K_r measures how close the flaw is to brittle fracture (crack driving force divided by toughness); the horizontal axis L_r measures how close the section is to plastic collapse (reference stress divided by yield). The failure assessment line (FAL) is the safe boundary between them: if the assessment point (L_r,\ K_r) lies on or inside the FAL the flaw is acceptable, and if it lies outside it is not. Secondary (residual or thermal) stresses add to K_r only, never to plastic collapse, and collapse is judged on the flawed section — so remote gross-section collapse needs a separate check.

Steps.

  1. Enter the geometry (a, 2c, B, W), the primary stresses (P_m, P_b) and secondary stresses (Q_m, Q_b), and the material data (\sigma_Y, \sigma_U, E).
  2. Get the stress intensity factor from Annex M: the primary K_I^P and, where present, the secondary K_I^S.
  3. Get the reference stress \sigma_{ref} from Annex P, which sets the horizontal coordinate L_r=\sigma_{ref}/\sigma_Y.
  4. Set the fracture toughness K_{mat} — type it in directly, or get it from Charpy energy through Annex J.
  5. Build the vertical coordinate K_r=(K_I^P+K_I^S)/K_{mat}+\rho, where \rho corrects for the plasticity interaction between primary and secondary stress.
  6. Pick a FAD option to set the FAL, plot the point, and read the PASS/FAIL verdict with the toughness reserve factor F_{K_r} and load reserve factor F_{load}.
Verified building blocks (Annex M / P / J) combine into the assessment point (L_r,\ K_r), which is then judged on the FAD.

Advanced options are unlocked in the inputs below and explained in the field help:

Creep (Clause 9) is out of scope; fatigue crack growth (Clause 8) is the separate E2 calculator.

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