Flexural strain assessment of multilayered double bias mat U section FRP beams under cyclic loading

The FRP (fiber-reinforced polymer) application in the field of civil engineering has been studied and discussed, and in brief period, FRP has become a smart material which is ready to replace concrete and steel. The FRP beam is made of E-glass type fiber using unsaturated polyester resin as the matrix material is proposed. This study introduces a novel multilayered E-glass FRP U-section beam designed to optimize performance under cyclic loading, addressing a gap in the Literature where I-beams and simpler laminates predominate. The composite consists of a collection of uniaxial mats with a 0° and biaxial mats with a ± 45° double bias angle. The tensioned section flange of U-profile has 17 mat layers [± 45°3/0°4/ ± 45°3/0°4/ ± 45°3], while the other sections have 9 double bias mat layers [± 45°]9. The beam depth varied for A, B, and C beams measuring 15 cm, 12.5 cm, and 10 cm, respectively. The span of the beam was set at 2000 mm, and the bending load was a three-point progression from 0 to 81kN. Unlike prior studies focusing on static or low-cycle fatigue, this research examines high-cycle fatigue at 20 Hz with a stress ratio of 0.6 and stress levels of 60%-80%, revealing good fatigue performance in the tensioned flange up to 106 cycles. The compressed flange failed due to local buckling, with failure stresses of 115 MPa (10 cm depth) to 90 MPa (15 cm depth), and deeper beams exhibited 18% higher average stress and 12% lower stress amplitude than shallower beams, enhancing fatigue resistance. Limited to specific loading and material conditions, future work could explore varied geometries, environmental effects, or alternative resins. By linking beam depth to fatigue resistance and introducing a tailored multilayered U-section design, this work provides quantitative insights for developing durable FRP structural components in cyclic loading applications, such as bridges and industrial frameworks.