|Originally posted by DigitalGriffin
ASTM manual of materials. Warning: It will give you back problems if you try to lift it. It will also set you back five benjamins. :-)
I believe what you are referring to may be a strain hardening that occurs in the actual manufacturing of the raw fiber. I'll check ASTM and ASM and see what they have to say. However, once in the fiber or composite form these materials, including Kevlar, don't have the elastic/plastic regions corresponding to typical ductile metal behavior. Take a look at some published stress/strain curves for typical composites (Kevlar/epoxy, S2/epoxy, E-glass/vinylester, carbon/epoxy, etc.). There isn't a plastic region. There is no strain hardening effect. It is much closer in behavior to a brittle material such as some grades of cast aluminum, or a ceramic. Composites typically don't have a yield strength, they only have an ultimate strength. Exceed that and it's broken.
|The author who wrote this book I believe wrote other books on the break in, lifetime, and characteristics of materials that are subject to mechanical stress...You might be able to contact him if you want to know more.
I've taken advanced fatigue and fracture courses. Let me tell you... that is one area of solid mechanics that still has a lot of work to be done. You really get the feeling learning such material that all the equations are just pseudo-emperical instead of derived from first principes (that isn't always
the case in F&F, but it is sometimes, often a mix of the two) - they work, in some cases, usually with some degree of accuracy, but there's always that nagging thought that the smartest people in the field still
don't really konw what is happening. Just the statistical aspect alone is enough to be discouraging. That being said, I highly suggest taking such courses, as in the future it will be more and more important for engineers to have such knowledge.
OK, now that I've covered that tangent thought, I should point out that fatigue in metals doesn't cause a change in modulus. It might eventually cause plastic deformation which results in a stiffness change in a given member, but that is a geometric effect and not a material effect.
|BTW: What's a matter? Don't believe in metal fatique/work hardening? I tell you what, why don't we stick you in an aircraft frame that has been subjected to loading and vibration for 100,000 hours. Do you feel safe that the characteristics of that frame is the same?!?!?
Perhaps you should briefly revisit those texts you have handy. As I clearly stated previously, work hardening produces a change in material strength
, not elastic modulus. And the characteristics of an airplane that you should be worried about are the strength and crack lengths, not material stiffness. That hasn't changed, no matter how many hours are on the clock. I'd be worried to be sure, but not for the reason you are assuming.
Oh, and I'd list my credentials but don't see the need, though you can rest assured that I'm not speaking from my as* either. ;)