Most of the stress on a connecting rod is compressive, right?
- Thread starter Howard
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Because steel is, unlike concrete, strong both when it is compressed and stretched. You can not make steel that is weak when stretched and strong when compressed.
OK this explanation might not make sense to you.
OK this explanation might not make sense to you.
Because they are stronger under compression also and if you have ever seen an engine that blew due to a connecting rod that failed you would understand the reason for spending the money for the extra insurance on a high performance engine.
The "tensile" stress refers to the "yield stress" (usually the 0.2% offset stress) found during a tensile test. Most theories of plasticity assume that materials yield at a certain effective deviatoric stress--and since that's computed in terms of squares, its positive regardless of tension or compression. (I would hope I would know this..... or my advisor would kill me
)More practically, steel yields at the same stress in tension and compression. You can try this in a "tensile tester" (most will do compression)... you just have to do it carefully. Tension tests are easy--the metal fails, the jaws pull apart. But compression tests can lead to problems--when the material yields, the jaws can move rapidly, crashing them together (and Instrons aren't cheap). But you can show that the yield stress in tension and compression are quite close.
Oh, and a random fact--there IS significant tension experienced by a connecting rod (usually when the piston is drawing air/fuel mixture into the cylinder). Note also that just because the center of the connecting rod is in compression, it does not mean that the areas around the pins are in compression. Most connecting rod failures (that I've seen anyway) aren't due to yield, but due to fracture. And fracture is usually driven by high tensile stresses.
Saw a beautiful red twin turbo rx-7 on the way home from work tonight. Looks like a beautiful baby Viper to me.
more of the stress is actually on the wrist pin,and the big and little ends,{where the bearings go}
what would you suggest? cast iron rods?
what would you suggest? cast iron rods?
No, what I meant was, why not high-compressive and not high-tensile. But I guess you people have somewhat remedied my ignorance.
Originally posted by: HokieESM
The "tensile" stress refers to the "yield stress" (usually the 0.2% offset stress) found during a tensile test. Most theories of plasticity assume that materials yield at a certain effective deviatoric stress--and since that's computed in terms of squares, its positive regardless of tension or compression. (I would hope I would know this..... or my advisor would kill me
More practically, steel yields at the same stress in tension and compression. You can try this in a "tensile tester" (most will do compression)... you just have to do it carefully. Tension tests are easy--the metal fails, the jaws pull apart. But compression tests can lead to problems--when the material yields, the jaws can move rapidly, crashing them together (and Instrons aren't cheap). But you can show that the yield stress in tension and compression are quite close.
Oh, and a random fact--there IS significant tension experienced by a connecting rod (usually when the piston is drawing air/fuel mixture into the cylinder). Note also that just because the center of the connecting rod is in compression, it does not mean that the areas around the pins are in compression. Most connecting rod failures (that I've seen anyway) aren't due to yield, but due to fracture. And fracture is usually driven by high tensile stresses.
Thats a hell of an answer!
Definately.
Just imagine the whole 4 cycles in slow motion. When the compression stroke ignites, the connecting rod is slammed into the top side of the crankpin. That's why you'll always see wear on the top and bottom of the connecting rod's big end first; the squeeze shears the oil film and causes wear.
The connecting rod has some pretty intense forces working on it. They're generally the weakest link in an engine, and the first thing that goes in a failure.
JC
Diamond Member
- Feb 1, 2000
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Originally posted by: HokieESM
The "tensile" stress refers to the "yield stress" (usually the 0.2% offset stress) found during a tensile test. Most theories of plasticity assume that materials yield at a certain effective deviatoric stress--and since that's computed in terms of squares, its positive regardless of tension or compression. (I would hope I would know this..... or my advisor would kill me
More practically, steel yields at the same stress in tension and compression. You can try this in a "tensile tester" (most will do compression)... you just have to do it carefully. Tension tests are easy--the metal fails, the jaws pull apart. But compression tests can lead to problems--when the material yields, the jaws can move rapidly, crashing them together (and Instrons aren't cheap). But you can show that the yield stress in tension and compression are quite close.
Oh, and a random fact--there IS significant tension experienced by a connecting rod (usually when the piston is drawing air/fuel mixture into the cylinder). Note also that just because the center of the connecting rod is in compression, it does not mean that the areas around the pins are in compression. Most connecting rod failures (that I've seen anyway) aren't due to yield, but due to fracture. And fracture is usually driven by high tensile stresses.
I'm thinking most of the tension forces are when the rods are trying to keep the pistons from flying out of the block at TDC at high RPM.
In any case, the rods surely endure a harsh environment, even in a mild engine.
JC
Originally posted by: JC
Originally posted by: HokieESM
The "tensile" stress refers to the "yield stress" (usually the 0.2% offset stress) found during a tensile test. Most theories of plasticity assume that materials yield at a certain effective deviatoric stress--and since that's computed in terms of squares, its positive regardless of tension or compression. (I would hope I would know this..... or my advisor would kill me
More practically, steel yields at the same stress in tension and compression. You can try this in a "tensile tester" (most will do compression)... you just have to do it carefully. Tension tests are easy--the metal fails, the jaws pull apart. But compression tests can lead to problems--when the material yields, the jaws can move rapidly, crashing them together (and Instrons aren't cheap). But you can show that the yield stress in tension and compression are quite close.
Oh, and a random fact--there IS significant tension experienced by a connecting rod (usually when the piston is drawing air/fuel mixture into the cylinder). Note also that just because the center of the connecting rod is in compression, it does not mean that the areas around the pins are in compression. Most connecting rod failures (that I've seen anyway) aren't due to yield, but due to fracture. And fracture is usually driven by high tensile stresses.
I'm thinking most of the tension forces are when the rods are trying to keep the pistons from flying out of the block at TDC at high RPM.
In any case, the rods surely endure a harsh environment, even in a mild engine.JC
The rod bolts would be your weak link in that situation. Plenty of super strong aftermarket bolts are available to prevent this. I personally like the ARP Wave Loc. They are what's in my engine.
JC
Diamond Member
- Feb 1, 2000
- 5,836
- 64
- 91
Originally posted by: Pacfanweb
Originally posted by: JC
Originally posted by: HokieESM
The "tensile" stress refers to the "yield stress" (usually the 0.2% offset stress) found during a tensile test. Most theories of plasticity assume that materials yield at a certain effective deviatoric stress--and since that's computed in terms of squares, its positive regardless of tension or compression. (I would hope I would know this..... or my advisor would kill me
More practically, steel yields at the same stress in tension and compression. You can try this in a "tensile tester" (most will do compression)... you just have to do it carefully. Tension tests are easy--the metal fails, the jaws pull apart. But compression tests can lead to problems--when the material yields, the jaws can move rapidly, crashing them together (and Instrons aren't cheap). But you can show that the yield stress in tension and compression are quite close.
Oh, and a random fact--there IS significant tension experienced by a connecting rod (usually when the piston is drawing air/fuel mixture into the cylinder). Note also that just because the center of the connecting rod is in compression, it does not mean that the areas around the pins are in compression. Most connecting rod failures (that I've seen anyway) aren't due to yield, but due to fracture. And fracture is usually driven by high tensile stresses.
I'm thinking most of the tension forces are when the rods are trying to keep the pistons from flying out of the block at TDC at high RPM.
In any case, the rods surely endure a harsh environment, even in a mild engine.JC
The rod bolts would be your weak link in that situation. Plenty of super strong aftermarket bolts are available to prevent this. I personally like the ARP Wave Loc. They are what's in my engine.
D'oh! Right you are....
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