Formability of complex composite structures with ribs made of long carbon-fiber-reinforced prepregs

A long fiber prepreg sheet (LFPS) composed of long carbon fibers with lengths of 50 mm and a fast-curing epoxy resin was used to fabricate complex-shaped structures with ribs using thermal compression molding. The formability of the composite structures was evaluated for various compression pressures (10, 30, and 60 MPa) and times (5 and 10 min) at a fixed curing temperature (155 °C). Using a single layer of LFPS, the highest geometric accuracy of the composite structure with ribs was 99.64%, and the highest average fiber volume fraction of the rib part was 29.82% under 30 MPa for 10 min. Furthermore, the highest flexural strength and modulus of the ribs were 253 MPa and 18 GPa, respectively, under the condition of 30 MPa and 10 min. These values increased to 366 MPa and 26 GPa, respectively, when two layers (a fiber volume fraction of 38.15%) were applied. Among the various thermoforming conditions, an applied pressure of 30 MPa and a pressing time of 10 min at 155 °C were the most appropriate to achieve relatively excellent net-shape molding with improved mechanical properties.