Critical flaw size in HY-100 vs. aluminum

If I've got a 60 ksi weld metal in steel, I can design it to 20 ksi — a factor of three or 3.3. If I look at the code for the base metal, it's half of that. I can actually have the base metal stressed to 35 ksi or so. In aluminum I might have 25 ksi as a yield, not an ultimate, and I may only be allowed seven ksi as my design strength. Because aluminum is less forgiving. Why is aluminum less forgiving? It's not as tough as steel. If I went back to that plot I showed you in the very beginning, steel is the toughest material — has the greatest combination of strength and toughness and cost of any material. That's why we use one and a half billion tons. Aluminum we use 45 million tons a year. But aluminum doesn't have the same type of toughness. So aluminum is not as tolerant of large defects. I told you the critical flaw size for ripping a sheet of paper — if it was in an HY-100 steel, it'd be two feet in most cases. But in aluminum it might be two inches.