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Abstract
Studying the fatigue behavior represents one of the most important targets fir any new material before being used. This is because; fatigue behavior cannot be predicted and the fatigue failure cannot be expected as in the case of static yielding. This thesis studies the fatigue behavior of wove-roving glass fiber reinforced polyester (GFRP) under combined bending and torsional moments, in-phase, with different fluctuating stresses. Fatigue tests were conducted on two fiber orientations, [0,9012 and [±4512, thin-walled tubular specimens with different ratios of the tiexural stress(A) to the (ors()nal shear stress
(B) , these ratios were A/B 0.5, 1, 2. And to study the effect of mean Stress of fatigue mm .stress behavior, specimens were fatigue tested at different stress ratios (R ), R = - 1 max .slress -0.75 , -0.5 , -0.25 , 0 for [0,9012 specimens with A1B2 and R -1, -0.75 , -0.5 , -0.25 for [45)2 specimens with AJB= 1.
The results showed that the [±4512 woven-roving GFRP is more fatigue resistant in torsion than the [0,9012; as being subjected to tension and compression local stress components, while the later being subjected to pure local shear stress. And the [0,9012 woven-roving GFRP is more fatigue resistant in bending than the [±4512; this because the [0, 9012 specimens are subjected to only tension stress in fiber direction, while the [±4512 specimens are subjected to tension in both fiber directions with local shear stress components. The validity of the SWT parameter was examined and it showed good vat idity for both fiber orientations, tending to plot a single curve when plotted versus the number of cycles to failure, far different negative stress ratios. Also, the validity of the modified fatigue strength ratio was examined for the present case and 1-for similar specimens under bending moments only and under torsional moments only, which were tested before. Using the power formula L’ a2 N b2 has proved its suitability