According to the application of tapered sandwich composites in wind turbine blade, it is important to study the influence of design parameters affecting on the strength, durability, and failure modes of these structures. In this way, the present research aims at investigating the influence of ramp-down area geometry of tapered sandwich composite on the static strength, fatigue response, and failure mode under four-point bending load. For this purpose, slopes of 11° (reference), 18°, and 45° were considered as geometrical factors, and distance of loading roller to tapered area and mid-span were considered as four-point loading factors. The extended finite element method (XFEM) was implemented to simulate crack growth behavior using the virtual crack closure technique (VCCT) in combination with the maximum tangential stress theory. Results showed that strength of tapered sandwich composites with slope of 18° in the ramp-down zone was 7.4% higher than that of the reference specimen, while, those having 1:1 slope resulted in 16.5% lower strength compared to reference specimen. It was found that distance of loading roller to tapered area and mid-span distance can significantly affect the failure mode and strength. Examination of crack propagation using experimental tests and XFEM revealed that, generally, the crack initiated under the loading roller, propagated into the foam core, changed its growth direction toward the tapered zone, and finally, led to debonding of the facesheets. Under high fatigue load levels, it was found that tapered design with slope angle of 18° increased the fatigue life more than twice of the life of reference specimen group.

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