PSA 2.4 — SIFs & Flexibility Factors: Markl's Gift to Piping
When pipe stops being straight — elbows, tees, branches — beam theory misses two things:
An elbow ovalises when bent — the cross-section flattens. Ovalisation makes it flexible (k > 1, often 5–20: elbows are the springs of your system) and simultaneously stressed at points beam theory never sees (i > 1).
Watch the mechanism do both jobs at once: drag the wall thinner and the bend visibly flattens while k and i climb together — artifacts/psa-sif-explorer.html
A. R. C. Markl (1950s) fatigue-tested components and defined:
i = (fatigue strength of a girth-butt-welded straight pipe) / (fatigue strength of the…
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