Carbon Fork Fatigue Failure. Real world experience?
08-21-14, 12:05 PM
#76
Senior Member
Long ago, maybe during the 1970s before carbon fiber was widely used, I had a conversation with a physicist who worked for Grumman. At that time Grumman was investigating the use of cabon fiber as a structural material. The main fact that impressed hm was that the material seemed to have infinite fatigue resistance. You could bend the material forever with no loss of strength unlike metals which will eventually fracture.
As a shop monkey who has worked extensively with composite laminates, doing work for Boeing and MacDonald Douglas, and knowing the care devoted to shop procedures in such high end work, I don't feel the same care is used lowly bike forks consistantly. The fork on my bike, with 15,000 miles is carbon and it is fine but I check for hidden delamination frequently. I think this is called trust but verify.
As a shop monkey who has worked extensively with composite laminates, doing work for Boeing and MacDonald Douglas, and knowing the care devoted to shop procedures in such high end work, I don't feel the same care is used lowly bike forks consistantly. The fork on my bike, with 15,000 miles is carbon and it is fine but I check for hidden delamination frequently. I think this is called trust but verify.
08-21-14, 04:27 PM
#77
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Carbon laminates are not a single material. There are numerous carbon formulations being used. Then, the carbon itself is 50% to about 65% of the total laminate, the remainder being epoxy resin. There are scores of resins formulated by chemical engineers and many catalysts that could be used in the process. Each catalyst can alter the properties of the final cured resin and the overall laminate. In our shop, we had a small lab where we tested several different epoxy resins and catalysts each week. We received samples from manufacturers for testing continuously. With careful engineering and shop practices, all worked reliably but it all required paying close attention at each stage.
Most of our work was research and development. Our one production job was the building of inlet ducts for the old U2 spy plane for NASA which was still used as a photography platform at that time. Every aspect of the manufacturing process was carefully monitored for each part produced.
Most of our work was research and development. Our one production job was the building of inlet ducts for the old U2 spy plane for NASA which was still used as a photography platform at that time. Every aspect of the manufacturing process was carefully monitored for each part produced.
08-21-14, 05:00 PM
#78
Senior Member
I need to clarify.
Fatigue is defined as repeated application of high stress. Everything is susceptible to fatigue - aluminum, steel, even carbon. However, the key is the definition of "high stress". Fatigue occurs when applied stress is comparable to ultimate strength of the material (that is, stress that would cause immediate failure), and life expectancy grows exponentially with the decrease in stress. For carbon fiber, life expectancy is essentially infinite if stress is less than ~60% of ultimate strength. For steel, it's 50%, and aluminum has a bad rep because its effective fatigue limit is as low as 30-40%. (I say "effective" because aluminum does not have a true fatigue limit, but, by 30%, life expectancy is into tens of millions of cycles. If you hit an aluminum object once per second with the stress of 30% of ultimate strength, it will last several months of continuous abuse before it fails.)
None of this is particularly relevant to bicycle forks, because they are built with a lot of excess strength in them. They are built to be stiff, and that automatically makes them far stronger than you'd possibly need. You can ride your bike off a curb and your fork will not explode. Therefore, the stress that occurs when you jump a curb is less than the ultimate strength of the fork material. And normal everyday stresses of riding over rough pavement are much smaller than that stress. Yield strength of aluminum and carbon fiber are on the order of 50 to 70 _thousand_ pounds per square inch. Cross section area of a road fork is right on the order of 1 square inch, which means that you need to generate 50 thousand pounds of force to break the fork and some nontrivial fraction of that amount to cause fatigue. And even if you do, spokes in the wheel will be the first ones to go, since they can't handle nearly as much stress.
Fatigue is defined as repeated application of high stress. Everything is susceptible to fatigue - aluminum, steel, even carbon. However, the key is the definition of "high stress". Fatigue occurs when applied stress is comparable to ultimate strength of the material (that is, stress that would cause immediate failure), and life expectancy grows exponentially with the decrease in stress. For carbon fiber, life expectancy is essentially infinite if stress is less than ~60% of ultimate strength. For steel, it's 50%, and aluminum has a bad rep because its effective fatigue limit is as low as 30-40%. (I say "effective" because aluminum does not have a true fatigue limit, but, by 30%, life expectancy is into tens of millions of cycles. If you hit an aluminum object once per second with the stress of 30% of ultimate strength, it will last several months of continuous abuse before it fails.)
None of this is particularly relevant to bicycle forks, because they are built with a lot of excess strength in them. They are built to be stiff, and that automatically makes them far stronger than you'd possibly need. You can ride your bike off a curb and your fork will not explode. Therefore, the stress that occurs when you jump a curb is less than the ultimate strength of the fork material. And normal everyday stresses of riding over rough pavement are much smaller than that stress. Yield strength of aluminum and carbon fiber are on the order of 50 to 70 _thousand_ pounds per square inch. Cross section area of a road fork is right on the order of 1 square inch, which means that you need to generate 50 thousand pounds of force to break the fork and some nontrivial fraction of that amount to cause fatigue. And even if you do, spokes in the wheel will be the first ones to go, since they can't handle nearly as much stress.
08-21-14, 05:04 PM
#79
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Nope. Alloy frames will last forever under normal use. Alloy bike frames are unlikely to ever meet fatigue levels in normal every day riding. Its a tough and durable material. Airplanes are still built of it because no other material delivers such an incredible lightness to strength ratio.
08-21-14, 05:11 PM
#80
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