Abstract: The study investigated the effect of porosity on the flexural stiffness of thick composites used in wind turbines. Thick composite laminate test panels with porosities of 0.64%, 2.04%, and 5.42% were fabricated using vacuum infusion molding. Four-point bending specimens were prepared according to ISO 14125 standard, and cyclic bending fatigue tests up to 1 000 000 cycles were conducted. The flexural modulus of the specimens before fatigue testing and after 200 000,600 000, 800 000, and 1 000 000 cycles was calculated using strain gauge and deflection methods, respectively. Experimental results revealed that for high-porosity (5.42%) specimens, a significant discrepancy (61% higher modulus via strain method) existed between the two measurement approaches. This discrepancy arises because strain gauges only capture surface strain and fail to reflect the global load-bearing capacity of structures with internal defects such as interlaminar delamination. Porosity significantly influenced the degradation of flexural modulus in thick structures after fatigue. Low-porosity (0.64%) specimens exhibited a mere 3.7% modulus reduction from 52.18 GPa to 50.27 GPa after 1 000 000 cycles, whereas high-porosity (5.42%) specimens experienced a 38.4% modulus drop from 50.65 GPa to 31.21 GPa after 200 000 cycles, further declining to 28.90 GPa after 1 00 0 000 cycles. Finally, a porosity-dependent flexural modulus degradation model was established based on experimental data, providing critical support for mechanical simulation of thick composite shell structures under bending loads.

Key words: thick composite, fatigue cycles, porosity, flexural stiffness