The Effect of Mean Stress on the Fatigue Behavior of Woven-Roving Glass Fiber-Reinforced Polyester Subjected to Torsional Moments

The effect of torsional mean stress on the fatigue behavior of glass fiber-reinforced polyester (GFRP) is studied by testing thin-walled, woven-roving tubular specimens with two fiber orientations, $[±45°]2s$ and $[0,90°]2s$⁠, at negative stress ratios $(R),R=−1,−0.75,−0.5,−0.25$⁠, 0. The $[±45°]2s$ specimens were found to have higher fatigue strength than the $[0,90°]2s$ specimens at all stress ratios. This is attributed to the difference in local stress components, the $[±45°]2s$ specimens being subjected to tension-compression local stress components, while the $[0,90°]2s$ specimens being subjected to pure local shear stress. For the studied stress ratios; the mean stress component had a detrimental effect on the amplitude component for the $[±45°]2s$ specimens; while it was ineffective for the $[0,90°]2s$ specimens in a certain region in the mean-amplitude diagram, region (1), then it had a detrimental effect in the rest of the diagram, region (2). The $S–N$ curves for positive stress ratios were extrapolated from those for negative stress ratios, which were found experimentally, for the $[0,90°]2s$ specimens. The positive stress ratio points, having the same local stress state as the negative ones, showed an acceptable behavior tending to decrease the amplitude component for the same life.