Critical flaw size for structural steel: 1–2 inches

If you have a tough enough material, your fracture toughness tells you small cracks won't grow. When you do inspections on things, you're only looking for indications that are larger than an eighth of an inch, generally. Which means I could have a crack that's a sixteenth of an inch, or three thirty-seconds, and it would pass. The code says no cracks, but you could have pores or other slag inclusions or whatever that are smaller than that. The structural welding code will tell you you can have no more than one inch of undercut in twelve inches of weld. So it doesn't mean you can't have undercut, you can. But that undercut typically is not more than one or two millimeters, and someone already knows the critical flaw size, even in structural steels, is not ten or twenty inches like a submarine steel — it's more like an inch or two inches. So who cares about a two-millimeter flaw? Unless it's a bridge.