In this paper, fatigue life and crack initiation of aluminum-GFRP bonded lap joints are investigated under fourpoint bending loading. For this purpose, a new fixture was designed and constructed. Fatigue loads vs. number of cycles to failure (L-N) were obtained so that the influence of various load ratios on L-N were examined. Furthuremore, the load amplitude vs. number of cycle curves were investigated. In addition, the load amplitude vs. mean load curve was obtained at 100,000 cycles. The effect of different maximum fatigue loads and load ratios were investigated on crack initiation life by implementing the backface strain (BFS) measurement and using an optical microscope. From the results, it can be inferred that the life of the crack initiation phase is nearly equal to that of half of the total life for a maximum load of 50% of the static failure load, whereas it is negligible for maximum fatigue loads higher than 60% of the static failure load. It was found that for a load ratio of R=−1, the crack initiation phase is negligible. Moreover, It was found that BFS plots were ended to the L-N curve, and this curve could be considered as a limit for BFS plots. Finally, a combined diagram including L-N curve, BFS, initiation, and rapid failure curves can be used to predict the remaining life of a joint which is loaded up to a certain number of cycles.

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