▁expected ▁that ▁tur bine ▁bl ades ▁go ▁through ▁~ 1 0 ^ 9 ▁loading ▁cycles . ▁Wind ▁is ▁another ▁source ▁of ▁rot or ▁bla de ▁loading . ▁ ▁L ift ▁causes ▁b ending ▁in ▁the ▁flat wise ▁direction ▁( out ▁of ▁rot or ▁plane ) ▁while ▁air flow ▁around ▁the ▁bla de ▁cause ▁edge wise ▁b ending ▁( in ▁the ▁rot or ▁plane ). ▁ ▁F laps ▁b ending ▁involves ▁t ension ▁on ▁the ▁pressure ▁( up wind ) ▁side ▁and ▁compression ▁on ▁the ▁su ction ▁( down wind ) ▁side . ▁▁ ▁Edge wise ▁b ending ▁involves ▁t ension ▁on ▁the ▁leading ▁edge ▁and ▁compression ▁on ▁the ▁trailing ▁edge . ▁ ▁Wind ▁loads ▁are ▁cy clic al ▁because ▁of ▁natural ▁vari ability ▁in ▁wind ▁speed ▁and ▁wind ▁she ar ▁( high er ▁spe eds ▁at ▁top ▁of ▁rotation ). ▁ ▁Fail ure ▁in ▁ult imate ▁loading ▁of ▁wind - t urb ine ▁rot or ▁bl ades ▁exposed ▁to ▁wind ▁and ▁gravity ▁loading ▁is ▁a ▁failure ▁mode ▁that ▁needs ▁to ▁be ▁considered ▁when ▁the ▁rot or ▁bl ades ▁are ▁designed . ▁The ▁wind ▁speed ▁that ▁causes ▁b ending ▁of ▁the ▁rot or ▁bl ades ▁exhib its ▁a ▁natural ▁vari ability , ▁and ▁so ▁does ▁the ▁stress ▁response ▁in ▁the ▁rot or ▁bl ades . ▁Also , ▁the ▁resistance ▁of ▁the ▁rot or ▁bl ades , ▁in ▁terms ▁of ▁their ▁tens ile ▁strength s , ▁exhib its ▁a ▁natural ▁vari ability . ▁ ▁In ▁light ▁of ▁these ▁failure ▁modes ▁and ▁increasing ly ▁larger ▁bla