Influences of Stress Ratio on the Fatigue Strength of Unidirectional Hybrid Composite GFRP/Al

Abstract

Effects of stress ratio on the off-axis fatigue strength of a unidirectional fiber-metal laminate GFRP/Al have been studied at room temperature. Tension-tension and tension-compression fatigue tests for three kinds of stress ratio: R=0.4, 0.1 and -0.2 are performed on GFRP/Al coupon specimens with different fiber orientations in the GFRP layers: θ=0, 5, 10, 15, 20, 30, 45, 60 and 90°. Also, a fatigue damage mechanics model is formulated which considers the effect of stress ratio. It is clearly observed that the fatigue strength is affected by stress ratio for every off-axis angle. Effects of the mean stress on the fatigue strength of GFRP/Al are examined on the basis of the constant fatigue life diagrams, and those diagrams are approximately described by the linear relationships of the Goodman type. Failure morphology of specimens indicates the fatigue failure of GFRP/Al is substantially governed by the fatigue strength of GFRP for the fiber orientations θ=0∼10°, while it is done by that of Al alloy for θ=30∼90°. It is demonstrated that the proposed fatigue damage mechanics model adequately describes the effect of stress ratio on the off-axis fatigue strengths of the unidirectional fiber-metal laminate.