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yet another residual stress redux
points that appear clear:
1. yielding can reduce residual stress. [see point i. below.] points that are not: i. that further yielding can increase residual stress. as one local region yields and decreases, the next in line can increase. and this is time dependent. ii. that residual stress is the cause of spoke fatigue. residual stress profiles for bent spokes do not match with those observed for fatigue initiation, i.e. it should be that fatigue initiates at regions of high tensile residual stress. instead, initiation is seen to initiate at a points of low tensile or even compressive residual stress. iii. that points with higher local residual stress are not bearing more of any applied stress than the rest of the material. [that's particularly interesting!] iv. that stainless spoke steel does not have strain aging region of its stress/strain graph so it does not strain below its yield point - as would mild steel. v. that allusions to local "micron" yielding betray ignorance of dislocations, their local strain fields, and the effects of high dislocation densities in highly cold worked materials - like spoke wire. bottom line: wheel builds benefit from the process of "stress relief" because it seats everything properly, keeps spokes from going slack and correspondingly keeps wheels true in service. but there ain't nothing magical about it, certainly not regarding fatigue. the best way to avoid spoke fatigue is to use straight pull spokes. next best thing is to use quality material in spoke manufacture. bringing up third place is ensuring spokes don't go slack in service thereby increasing their bending cycle. |
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