Naval Research Lab ratio analysis diagram

Framing reference. Tom invokes the NRL diagram (developed out of Liberty ship failures, 1950s–60s) as the parent framework before pivoting to Ashby plots.

When we look at trying to get both toughness and strength — this is the old ratio analysis diagram I put up earlier in the semester. This one is for steel, and it was developed out of the Liberty ships at the Naval Research Laboratory in the 50s and 60s. It plots toughness versus tensile or yield strength. This is called the technological limit. At low-strength steel you get very high toughness. As you go to higher yield strengths up to 300 ksi, toughness goes down and your critical flaw size gets smaller. These lines here — critical section thickness — show two and a half inches. You can build a pressure vessel out of a steel with properties anywhere along this line, two and a half inches thick, and it would leak before it breaks. You don't want a brittle fracture where it shatters like glass and sends shrapnel around in a pressure vessel or a submarine — which is also an externally pressurized pressure vessel. But you get down here to 300 ksi steels and your critical flaw size is a tenth of an inch.