Effects of R-ratio on the off-axis fatigue behavior of unidirectional hybrid GFRP/Al laminates at room temperature

Effects of stress ratio on the off-axis fatigue behavior of the unidirectional fiber-metal hybrid GFRP/Al laminate (GLARE-2) at room temperature are studied. Constant amplitude fatigue tests at three kinds of stress ratios R = 0.4, 0.1 and −0.2 are performed on coupon specimens with different fiber orientations θ = 0°, 5°, 10°, 15°, 20°, 30°, 45°, 60° and 90° in the GFRP layers. It is observed that the fatigue life of GLARE-2 is extended by the increase in stress ratio for every fiber orientation. A linear relationship of the Goodman type can approximately be fitted to the constant fatigue life data for GLARE-2, regardless of the fiber orientations. The fatigue fracture of GLARE-2 is characterized by the fatigue failure of the GFRP layers for θ = 0–15°, while it is brought about by the fatigue failure of the aluminum-alloy layers for θ = 30–90°. For each of the two groups, a master S–N relationship can be identified by means of a modified fatigue strength ratio. A fatigue damage mechanics model that considers the effect of stress ratio is developed using a modified theoretical fatigue strength ratio based on the Tsai–Hill static failure criterion. It is demonstrated that the fatigue model can successfully be used for description of the off-axis fatigue behavior of GLARE-2 over a range of stress ratios.