A tip-of-the-hat to reader Brenda who suggested that I look into porcelain-fused-metal crowns. I talked to my dentist and he was fine with the option.
It was funny, though. He kept trying to nudge me into ceramics. "They are three-times stronger, now..."
"Strong" is an ambiguous term
For example, Liberty ships in WWII were welded together out of first-generation High Strength, Low Alloy steel as a means to conserve steel for the war effort.
If you cut a sample of first-gen HSLA and put it in a tensile test machine, it might withstand 50,000 PSI compared to 30,000 or 36,000 for the more common structural steel in general use.
Several issues converged in the Liberty ships. Those steels had lower toughness than the more common steels. A crack, once started, grew rapidly.
Another issue is that the first-gen steels became very brittle at low temperatures which would not have been an issue if the convoys took more southerly routes to Europe and back but was a major issue in the North Atlantic.
A third issue is that the loading hatches had square corners rather than rounded-off corners. That was a concession to expedite manufacturing.
A fourth issue is they were all-welded construction. A crack in a plate on a riveted ship would grow to the edge of the plate and then stop. A crack on a welded structure can keep right on growing all the way around the ship (including below the water line!).
Many Liberty ships sank in the North Atlantic in WWII in part because a "stronger" steel was selected for the application.
In personal life
I had a client back in my working days who was "releasing" a tension strap. It swivelled on one end and had a "D"-ring rigidly affixed on the other. The client had looked through a catalog and picked the highest tensile strength steel listed and was having difficulty passing the "pull" test. If memory serves, the material the client committed to using had a tensile strength of 180,000 psi (six times garden variety steel).
It was a frustrating experience for both of us. The engineer was sure I was lying when I suggested that "the strongest" steel was the problem.
One issue was that the client specified a square-hole and a rivet to keep the "D"-ring from spinning. Square holes in brittle materials are a bad idea. There was no evidence of stretching in the force-deflection trace from the test machine. The fracture was sudden and catastrophic.
Their next effort was to MIG weld the D-ring to the strap. Unfortunately the very high tensile strength of the material was completely destroyed by the heat of the weld. It was not tough but lacked the tensile strength required.
Suggestions to use a tougher grade of steel in a thicker strap were rejected. The client had already committed to the very-thin strap and other engineers had snapped up the real-estate.
Our working relationship became toxic at that point. I think the client eventually got the part to pass by making the strap twice as thick and using a short-slot with round ends for the rivet hole.
Bottom line
"Three times as strong..." does not impress me unless the material has proven to be tolerant of imperfections in geometry or manufacturing execution.
The story I have on the Liberty ships was that welders were paid by linear foot…so they started slugging the welds in order to cut corners….
ReplyDeleteSome welders probably did.
DeleteBut later vintage Liberty ships and the later Victory series did not have the losses the earliest Liberties did. I don't think there was less pressure to produce as the war went on so I assume some of the welders were still tempted to slug welds.
That suggests that at least some of the issue was related to uncontrolled cracking that originated at the corners of the hatches.
One part of the "fix" was to use an "L" shaped piece of thicker steel with better low-temperature fracture toughness in the deck at the corners.
Hazy memory, but didn't LeTourneau invent the sub-arc welding process? Didn't they use that on those ships for the last half of the war, or thereabouts? I may be mis-remembering. I knew they put rods in the cracks and welded over them, but I never knew that was called slugging. Thanks for the edumacation.
DeleteWhen I used to write the specifications for drill pipe, material toughness factored high on the list of priorities. But toughness is a hard concept to get one's mind around. Engineers like the simplicity of using a number, such as ultimate yield strength. Material toughness, as a consequence, is under-sold.
ReplyDeleteSeeing a Charpy v-notch fracture toughness test in person is impressive, especially if you personally lift the pendulum striker up to the lock. That thing is heavy! Hearing the PING! when a sample breaks is impressive, but when you get a really tough hunk of steel and the tester goes THUNK and the pendulum stops is really a sight to see.
ReplyDeleteIt gives you a gut-level feeling for "work-hardening exponent"
Delete'Success' in one area doesn't guarantee success in ALL areas...
ReplyDelete