RSTRENG effective-area calculator

How the RSTRENG effective-area method works

Real corrosion is rarely a neat rectangle. It is usually a shallow patch with a few deeper pits. B31G and Modified B31G assume the deepest pit runs the full length of the flaw, which throws away metal that is still there. The effective-area method behind Kiefner & Vieth's RSTRENG program uses the actual profile instead.

1. Project the deepest reading at each axial position onto one line: the river-bottom profile.
2. For every pair of points (start i, end j), integrate the metal-loss area A between them with the trapezoid rule, and take A₀ = L·t.
3. Compute the failure stress for that subsection with the Modified B31G Folias factor and flow stress.
4. The subsection with the lowest failure pressure is the critical subsection. Its pressure is the answer.

DigSheet evaluates n(n−1)/2 subsections. That is about 20,000 for a 200-point laser profile, which takes milliseconds in a browser. The chart highlights the critical subsection, and the work table compares the result with Modified B31G on the same flaw.

Getting a good profile

• Use a fine, even grid where you can (0.5 in / 12 mm or finer). Coarse spacing can miss short deep pits.
• Apply your interaction rules first, so separate pits are combined or kept apart consistently.
• Use the same wall thickness basis (nominal vs. measured) that your procedure requires.

When not to use it

• Cracks, crack-like flaws, gouges, laminations, or metal loss in a dent.
• Selective seam corrosion or grooving at ERW/flash-welded seams.
• Significant axial or bending loads (the methods assume internal-pressure hoop stress).
• Metal loss deeper than 80% of wall: repair or replace regardless of length.
• Low-toughness or vintage pipe where fracture may control before plastic collapse.

FAQ