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#11
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control failure
Stuart & Kathryn Fields wrote: Don: Be sure to share what you find. I think that I'm only scratching the surface of some stuff. One thing I found and had never seen before was an equation relating the increase in stress due to a crack. It scares the hell out of me. I'm afraid to use a metal fork in my salad. The equation basically says that the max stress is 2 times the load divided by the area times the square root of the crack length divided by the radius of curvature of the end of the crack!!! If the radius of curvature was equal to the crack length, the max stress is already twice what you would calculate using the applied load and the element cross section. Now put a reasonably sharp crack and see what happens....as the radius approaches 0.001 times the length of the crack......???? That is because the load is not equally spread across the part, but is concentrated at the end of the crack. Think about a piece of metal bar in tension with a crack halfway across it. The cross sectional area that has already seperated cannot bear any load at all as it has already failed. The end of the crack is taking a lot of load because the crack pulls apart when it is under tension. Solution: Don't use cracked parts!! (Well duh) Don W. |
#12
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control failure
Don: Reading in a text book on Fatigue design for aluminum structures,
there is at least one design technique that assumes that there are cracks inherent in the material to start with that are not readily detectable by a visual inspection. The design philosophy progresses from there. This whole area of study is explaining why the time life specs on parts can be very meaningful. It also can allude to the excessive safety factors that can be used in determining time life specs. It looks like I've got a lot more studying to do in this area. -- Stuart Fields Experimental Helo magazine P. O. Box 1585 Inyokern, CA 93527 (760) 377-4478 ph (760) 408-9747 publication cell "Don W" wrote in message ... Stuart & Kathryn Fields wrote: Don: Be sure to share what you find. I think that I'm only scratching the surface of some stuff. One thing I found and had never seen before was an equation relating the increase in stress due to a crack. It scares the hell out of me. I'm afraid to use a metal fork in my salad. The equation basically says that the max stress is 2 times the load divided by the area times the square root of the crack length divided by the radius of curvature of the end of the crack!!! If the radius of curvature was equal to the crack length, the max stress is already twice what you would calculate using the applied load and the element cross section. Now put a reasonably sharp crack and see what happens....as the radius approaches 0.001 times the length of the crack......???? That is because the load is not equally spread across the part, but is concentrated at the end of the crack. Think about a piece of metal bar in tension with a crack halfway across it. The cross sectional area that has already seperated cannot bear any load at all as it has already failed. The end of the crack is taking a lot of load because the crack pulls apart when it is under tension. Solution: Don't use cracked parts!! (Well duh) Don W. |
#13
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control failure
Stuart & Kathryn Fields wrote:
Don: Reading in a text book on Fatigue design for aluminum structures, there is at least one design technique that assumes that there are cracks inherent in the material to start with that are not readily detectable by a visual inspection. The design philosophy progresses from there. This whole area of study is explaining why the time life specs on parts can be very meaningful. It also can allude to the excessive safety factors that can be used in determining time life specs. It looks like I've got a lot more studying to do in this area. Hi Stuart, If you've ever looked at aluminum under a high power microscope you have seen that it is not a smooth material at all, but consists of "grains" of material more or less mashed together. If it is alloyed with copper, you can see the copper grains around the edges of the aluminum grains. Fatigue of material is an interesting subject. Do you remember the problems introduced at the drive shaft on the Rotorway 162F when people started changing out the chain drives for belt drives? The chain drives needed an oil bath which required oil seals, and a housing all the way around the chain. Since the only way to change the bottom seal when it started leaking is to remove the housing, and that requires removing the chain and drive sprockets it seemed that replacing the chain sprockets and chain with a gilmer belt and tensioner was a good idea--at least until the drive shaft from the motor started failing right on the other side of the support bearing. The problem was/is that the drive shaft is supported by only one bearing below the drive pulley, and the gilmer belt must be kept in much more tension than a drive belt. Consequently the end of the drive shaft is bent ever so slightly by the belt tension. This would be okay except that when it rotates, the direction of the bend keeps changing. The effect is like bending a piece of metal back and forth 50 times per second. Even the fairly thick drive shaft failed in a hundred hours or so given that treatment. The failures puzzled everyone at first, and they called it "fretting fatigue"--I guess because the designers were fretting over it ;-) Don W. |
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