Due to the increasing usage of bonded joints in engineering structures and the fact that they sustain cyclic loads during their working life, it is crucial to estimate fatigue life with acceptable accuracy and efficiency in industrial applications. The main objective of this paper is to evaluate the validity of fatigue life prediction models for metal-to-composite bonded joints under different maximum fatigue loads and load ratios in bending. For this end, fatigue life cycles of aluminum-GFRP bonded single lap joints subjected to four-point bending were estimated using strain-life criteria and finite element modeling. The parameters of the strain-life criteria of Manson-Hirschberg, Morrow, and Smith-Watson-Topper (SWT) were determined using numerical stress and strain results and experimental fatigue life data. Afterward, the influences of different maximum fatigue loads of 40%, 50%, 60%, and 70% of the average static failure load (ASFL), and load ratios (R, ratio of minimum fatigue load to maximum one) of R = 0.5, 0, and −1 were studied. Results revealed that the SWT criterion achieves a more accurate estimation and gives a more minor error for 60% and 70% of ASFL, compared to other fatigue loads. Moreover, among all criteria, the Manson-Hirschberg and Morrow criteria do not provide an accurate life prediction for 40% and 50% of ASFL. For loads of 60% and 70% of ASFL, the SWT criterion yields a more accurate estimation for R = −1 in comparison to R = 0.

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